System (IS-IS) routing protocol. Many texts assume the reader
has knowledge of other link-state protocols such as Open Shortest
Path First (OSPF). We don’t make that assumption, and discuss IS-IS from the ground up.
To start, we take a big-picture view of what the protocol provides; this includes basic design
of IS-IS networks and network addressing. Our generic coverage of link-state protocols helps
set the stage for how IS-IS works. We then discuss specific details about IS-IS states, IS-IS adjacencies,
and the Designated Intermediate System (DIS) election on broadcast links. After that, we
take a look at various configuration, verification, and troubleshooting commands. Finally, we
briefly compare IS-IS to OSPF.
Let’s begin with an overview of the IS-IS protocol.
Overview of IS-IS
The International Standards Organization (ISO) calls a router an
intermediate system
. A host
is referred to as an
end system
by the ISO. Since routers connect hosts in the IP world, intermediate
systems connect end systems in an ISO network. IS-IS was originally designed to support
the Connectionless Network Protocol (CLNP) and was later adapted to support IP reachability.
Both the IP and CLNP information is carried within the payload of the IS-IS routing updates.
The Juniper Networks implementation of IS-IS supports only IP routing, so we focus on this
aspect of the protocol for the remainder of this chapter.
The Juniper Networks implementation of IS-IS is fully interoperable with other
vendor implementations that utilize both the CLNP and IP protocol stacks.
Throughout our discussion, we use the single sample network shown in Figure 7.1.
Four routers make up this entire IS-IS network. An Ethernet segment interconnects Cabernet,
Merlot, and Shiraz. These routers are configured within IS-IS area 47.0005.80.8300. Riesling
is connected via point-to-point links to Cabernet and Merlot. Riesling is in a different IS-IS area,
49.0001. We discuss the significance of the area values and connectivity of the routers in the
“Addressing” section later in this chapter.
Overview of IS-IS
277
FIGURE 7 . 1
IS-IS sample network
Link-State Review
Before we start our discussion of the IS-IS particulars, a brief review of link-state protocol concepts
is in order. Once a link-state router starts operating on a network link, information associated
with its logical networks is added to its
link-state database
by the local router.
Hello
messages
are then sent by the router on all operational links to determine whether other routers
are using the same protocol. If additional routers are located, both attempt to form an
adjacency
with each other. The routers use this adjacency to advertise summary database information to
each other. This is not the actual database information but is truly a summary of the data. Each
router checks this summary list to verify that it has the most up-to-date information. Should one
of the routers require an information update, it sends a request to its neighbor for a link-state
update. The update includes the actual data contained in the link-state database. This exchange
process continues until both routers have identical link-state databases.
This common view of the link-state database forms the basis of the network topology. Each
router uses the
Dijkstra Algorithm
to process the database information into a path from the
local router to each remote destination. Every router uses the same algorithm to process its database;
therefore, each router must have consistent information to get proper results. This consistent
database concept is a central tenet of link-state protocols and allows the protocols to
ensure a loop-free topology. Each router then makes consistent forwarding decisions for user
data packets. In this state—a sort of network nirvana—no routing loops exist in the network.
Ensuring the advertisement and consistency of link-state updates as well as propagating these
updates quickly remains the only barrier to preventing loops.
Area: 49.0001
Area: 47.0005.80.8300
Cabernet Merlot
Riesling
Shiraz
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IS-IS Levels
Let’s start examining some further details of how IS-IS transmits its information to other routers
in the network. We’ve previously stated that each link-state router must maintain a consistent
link-state database. More specifically, each database within an
IS-IS level
must be identical. The
ISO committee uses the term
level
to represent an arbitrary boundary or grouping of routers.
Since the database in each level is the same, that level becomes the farthest distance that a linkstate
update can propagate. It follows that the Dijkstra Algorithm is then calculated by the local
router using the information in the database within a specific level. IS-IS routers exchange linkstate
information with each other based on their level configuration—either Level 1 or Level 2.
Level 2
Two IS-IS routers form an adjacency and share database information when both ends of their
common link are configured for Level 2. Let’s take a look at Figure 7.2. All of the interfaces on
Riesling are within the defined Level 2 area. In addition, both Cabernet and Merlot have an
interface within that same Level 2 area. The dotted line represents the shared topology knowledge
within the Level 2 link-state databases on the routers. The area values are different on the
routers (49.0001 and 47.0005.80.8300), but the only requirement for a Level 2 adjacency is
that each end of the link reside within Level 2.
Level 1
The requirements for a Level 1 adjacency are a bit different. Two IS-IS routers form an adjacency
when each end of the common network link is configured for Level 1 and the IS-IS area value of
each router is identical. Figure 7.3 displays a Level 1 area. All interfaces on Shiraz and a single
interface on Cabernet and Merlot reside within Level 1. Cabernet, Merlot, and Shiraz all share an
IS-IS area value of 47.0005.80.8300. This common area value allows adjacencies to form and
updates to be exchanged. As before, all interfaces bounded by the dotted line exchange link-state
updates.
When the common link between Cabernet and Riesling is configured for Level 1, no IS-IS
adjacency forms since the area values of the two routers are different. Only a Level 2 adjacency
can form between these two routers.
Overview of IS-IS
279
FIGURE 7 . 2
IS-IS Level 2 coverage
FIGURE 7 . 3
IS-IS Level 1 coverage
Both Cabernet and Merlot have two link-state databases. One database contains
the Level 1 data while the other contains the Level 2 information. The Dijkstra
Algorithm is calculated within each level database.
Area: 49.0001
Area: 47.0005.80.8300
Level 1 Only
Level 2 Only Level 2 Only
Cabernet Merlot
Riesling
Shiraz
Area: 49.0001
Area: 47.0005.80.8300
Level 1 Only
Level 2 Only Level 2 Only
Cabernet Merlot
Riesling
Shiraz
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Design Considerations
The design of a level topology depends on network scalability and personal preference. For a
small network (fewer than 100 or 200 routers), you might decide to place all routers within the
same level. For a larger network (hundreds of routers), you might decide to use multiple levels.
The core/backbone routers comprise one level (Level 2) while smaller sets of routers are in several
other levels (Level 1). These smaller sets sometimes exist in different physical locations.
Level 1 and Level 2 Operation
The ISO committee designed the level hierarchy for large network topologies requiring multiple
levels. Level 1 routers contain IP routes for their specific level and maintain a default
route (0.0.0.0 /0) toward a backbone network. Level 2 routers are devices that serve as the
backbone routers. Level 2 routers have complete routing knowledge of the entire network. An
individual IS-IS router can be one of the following:
Level 1 router (L1)
Level 2 router (L2)
Level 1 and Level 2 router (L1/L2)—JUNOS software default
Level 2 routers share route knowledge with each other about all areas of the network. In a
hierarchical network design, at least one router is both an L1 and an L2 router. Each router
maintains a complete link-state database for each level configured. An L2 router connected to
another L2 router in a different area sets the attached bit in its L1 updates. An L1 router that
receives an update with the attached bit set assumes that the L2 router has reachability to the
remainder of the network. The L1 router installs a 0.0.0.0 /0 default route locally that points
Good Network Design?
In examining Figures 7.2 and 7.3, you might wonder if we had a reason for placing the IS-IS
levels in specific places. The short answer is “sort of.” We used some general rules, but level
placement in a live network depends on a number of factors. Let’s talk about what the figures
represent.
Cabernet, Shiraz, and Merlot share an area address and reside in the same administrative domain.
This is probably because a common Ethernet network interconnects them all. Cabernet and Merlot
each connect to Riesling over wide area network (WAN) links. This leads to the possibility that the
opposite ends of the links may be within different areas. In fact, Figure 7.2 shows this to be the case.
We’ve discussed here some general comments about how routing domains may be interconnected.
An Ethernet network between two or more routers doesn’t mean that they will
reside in the same administrative domain and IS-IS level. Likewise, a WAN link between two
routers doesn’t ensure that the routers are in different administrative domains.
Overview of IS-IS
281
to the L2 router as a next hop. Since all Level 1 routers have explicit knowledge of routes within
their area, the default route is used only to reach routes outside the Level 1 area.
An Example of a Multilevel Network
Let’s explore the operation of a multilevel IS-IS network in greater detail. Look at the network
in Figure 7.4.
Suppose an ISP in Europe has routers in multiple countries with major concentrations in the
metropolitan areas of London and Rome. The routers within London share an area address
of 49.0002 and are configured for Level 1. Likewise, the routers within Rome share an area
address of 49.0001 and are also configured for Level 1. The remaining routers have different
area addresses (49.0003 and 49.0004), but they are all configured for Level 2. This configuration
imposes a logical hierarchy to the network.
FIGURE 7 . 4
IS-IS level hierarchy
A router in London reaches a route in the Rome metropolitan area through the Level 2 backbone
area. The London router forwards all inter-area traffic to the London L1/L2 router using its
local default route. Recall that the L1/L2 router prompts this default route through the advertisement
of the attached bit. The Level 2 backbone routers have complete link-state knowledge of all
routes in the network. The London L1/L2 router forwards the user traffic from the London L1
router across the backbone to the Rome L1/L2 router. This router then forwards the traffic to the
Rome L1 router.
Area: 49.0004
Area: 49.0002
Paris Munich
Geneva
Rome
Madrid
London
Stockholm
Level 2
Level 1
Level 1
Area: 49.0003
Area: 49.0001
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Summary of IS-IS Levels
To summarize, the level boundaries determine the extent of propagation for link-state updates.
All routers within a level maintain a complete link-state database of all other routers in the same
level. Each router then uses the Dijkstra Algorithm to efficiently determine the shortest path
from the local router to all routes in the link-state database.
Addressing
We’ve been referencing IS-IS area values up to this point. These area values are encoded in the
IS-IS address of the router called the
Network Entity Title (NET)
. Let’s now explore the details
of IS-IS addressing. IS-IS uses the standard Network Service Access Point (NSAP) addressing as
defined in ITU X.213. The size of the NSAP address varies from 8 to 20 bytes in length. There
are three major parts to the address structure: area, system ID, and N-selector. The format of
the NET is shown in Figure 7.5.
The first part of the address indicates the IS-IS area value. This field begins with the
Authority
and Format Indicator (AFI)
, is followed by the Initial Domain Identifier, and finishes with the
Domain-Specific Part (DSP)
. The AFI byte indicates the governing body that administers the
address space and assigns addresses. Networks often use 0x49 as their AFI, which represents the
private NSAP address space. The NSAP private addresses are analogous to the private IP address
space defined in RFC 1918. Your network requires a registered address only when Connectionless
Network Protocol (CLNP) routing is desired with another network. The JUNOS software default
does not route CLNP packets, so using private NSAP area addresses is perfectly fine.
NSAP Addressing
There are two major forms of registered NSAP addresses. The British Standards Institute administers
the International Code Designator (ICD) address space. Each country has an address registration
authority that administers the Data Country Code (DCC) address space. Each registered
address space begins with a different value: 0x47 for ICD and 0x39 for DCC. Within the United
States, you can order your own NSAP address (mine is 0x47.0005.80.8300). The Initial Domain
Identifier (IDI) follows the AFI. The remaining area field indicates the DSP. In total, the combination
of the AFI, IDI, and the DSP provides the complete area address.
Now that the alphabet soup is out of the way, let’s talk about what this really means. First, think
about your assigned IP address space. A registration authority decided that you should use certain
bits to represent your network. The remainder of the address space is yours to subnet. In
ISO-speak, your assigned address space is the combination of AFI and IDI numbers. You can
subnet your network using the remainder of the area address, the DSP, as you see fit.
Overview of IS-IS
283
FIGURE 7 . 5
Network Entity Title format
The field containing the
system ID
appears immediately after the Area field. The system ID
uniquely identifies the router to the network. You can think of it as the host portion of the
address. You are free to place any value in this field, but there are some common practices.
The first is to use the Media Access Control (MAC) address of a broadcast interface as the
system ID. This method guarantees uniqueness but carries with it the problem of user readability.
A second method helps administrators more easily read the system ID. This approach
uses an IP address assigned to the router (typically the router ID) to represent the ID value.
You pad the address with leading zeros to provide 12 characters. As an example, assume our
loopback address is 172.16.10.1 /32. We pad each dotted decimal value so that the address
now reads 172.016.010.001 /32. The JUNOS software always uses a length of 6 bytes for
the System ID field, which is also 12 characters long (in hexadecimal notation). Our padded
IP address now fits neatly into the System ID field and provides us with an easy way to identify
an IS-IS router in our network.
The last portion of the NET address is the
N-selector (SEL)
byte. The selector is used to distinguish
different data services operating on the same router. A Juniper Networks router sends
updates with a selector value of either 0x00 or some nonzero value. The 0x00 value is advertised
in updates that represent the router itself, its links, and its neighbors. This type of update is
always advertised into the network. A nonzero value is sent in updates for which the local router
is acting as a pseudonode on a broadcast network. We discuss pseudonodes in the “Protocol
Data Units” section later in this chapter. In following our analogy to IP addressing, the selector
byte is similar to the function of the TCP/UDP port number in that it represents different logical
processes.
Now that we understand what the pieces are, let’s discuss how the JUNOS software comprehends
the assigned NET address. You want to start reading the address from the right-hand
side. The first byte is the selector, the next 6 bytes are the system ID, and the rest of the address
is the area. It helps to interpret the address in this manner since the area value can range from
1 to 13 bytes in length.
System ID SEL
AFI DSP 0x00
Area
1–13 bytes
1 byte 1–12 bytes 6 bytes 1 byte
IDI
Initial Domain Part
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Protocol Specifics
To this point, we’ve talked about link-state protocols, IS-IS levels, and addressing. Let’s now
begin discussing more specific details of the protocols. We look at the various IS-IS adjacency
states first, followed by a discussion of how a router sends network data in a link-state update.
We then explore the election process for the Designated Intermediate System (DIS) and finish
with a look at the Protocol Data Units (PDUs) used by IS-IS in its operation.
IS-IS Adjacency States
If you recall our generic link-state discussion at the beginning of this chapter, two routers must
first agree to exchange information before actually exchanging that data. This agreement to communicate
is called an
adjacency
. The method for forming an adjacency is simple: two connected
routers exchange IS-IS Hello messages. There are six possible states for an IS-IS adjacency:
New
This state is seen when the IS-IS adjacency process is just beginning. Start events could
include router boot-up or initial configuration.
One-Way
Your IS-IS router transitions to this state after sending an IS-IS Hello PDU. In addition,
any received hellos do not contain the local router’s address as a neighbor.
Initializing
When a local router sees itself in a neighbor’s hello, it transitions to this state.
This state shows that bidirectional communications are established.
Up
This is a fully functioning state for IS-IS. An adjacency relationship is formed and the databases
have been exchanged.
Down
This represents a nonfunctioning adjacency. An IS-IS router moves to this state for one
of several reasons, including area mismatches, expiration of the hold time, and authentication
failures.
Reject
Upon an authentication failure, an IS-IS router will transition between this state and
the
Down
state.
General IS-IS Information Exchange
Now that our routers (Router A and Router B) have agreed to communicate, they then start
exchanging information. Each router starts sending its partner a complete list of the information
in its link-state database. The data exchanged at this time is the number of each link-state
PDU in the database. This number is very similar to the table of contents for a book. If you are
missing a chapter, you ask for that chapter number. Likewise, if Router A does not have a copy
of a particular link-state PDU that Router B advertised, it asks for the missing information.
Additionally, Router A might find that Router B has more updated information in its database,
so Router A asks for the latest data. In both cases, Router B sends the complete data set related
to the requested PDU. In this manner, both Router A and Router B generate complete copies of
the link-state database. Recall that this is a critical concept for a link-state protocol like IS-IS.
This process is represented in Figure 7.6.
Protocol Specifics
285
FIGURE 7 . 6
IS-IS startup sequence
Router A and Router B are forming an adjacency and exchanging the information in their
databases. The specific steps of this process are:
1.
IS-IS Hello messages are exchanged to form an adjacency.
2.
Each router sends a Complete Sequence Number PDU (CSNP) to its peer. These contain a
complete summary listing of the link-state database, including sequence numbers and the
age of each data segment.
3.
Router B determines that it is missing information from its database and sends a Partial
Sequence Number PDU (PSNP) to Router A.
4.
Router A responds to this request with a link-state PDU (LSP) containing the requested
information.
5.
Router B issues either a PSNP (on a point-to-point link) or a CSNP (on a broadcast link)
to inform Router A that the advertised link-state PDU was received. This acknowledgement
is a critical step because it guarantees the reliable flooding of database information to all
routers in the network.
Router B issues a CSNP on a broadcast link only when it’s the Designated Intermediate
System for that link. We discuss the election of the DIS in the “Designated
Intermediate System” section later in this chapter.
Protocol Data Units
We’ll now talk about the details of each IS-IS
Protocol Data Unit (PDU)
. We’ve been discussing
some of the PDUs already, but this section contains an exhaustive look at each type.
Hello
Initializing
Time = t0
Time = tn
New (for new adjacencies only)
Up
CSNP
State
Routers determine missing LSPs
Issue PSNP request for missing LSP
Send requested LSP
Issue PSNP reply (on P2P link), or
Issue CSNP periodically on Broadcast link.
Router A Router B
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FIGURE 7 . 7
IS-IS common PDU header
Each PDU shares a common header, illustrated in Figure 7.7. The header consists of the following
fields:
Protocol ID (1 octet)
This field is set to a constant value of 0x83 and designates that the
higher-level data belongs to IS-IS.
Header Length (1 octet)
This field indicates the total length, in octets, of the IS-IS headers. It
includes both the common IS-IS header and any PDU-specific headers that follow.
Version/Protocol ID Extension (1 octet)
This field is set to a constant value of 0x01. The
IS-IS specification defines this field as an extension area for the Protocol ID data. The JUNOS
software does not implement this function.
ID Length (1 octet) This field is used to inform other systems of the system ID length. For
backward compatibility, the default length of 6 bytes is represented with the constant value of
0x00. The JUNOS software does not use a larger ID size, so this field is set to a constant value
of 0x00.
PDU Type (1 octet) This field designates the PDU carried after the common header. The first
3 bits are set to 0. The remaining bit combinations include:
Level 1 LAN Hello (15)
Level 2 LAN Hello (16)
Point-to-Point Hello (17)
Level 1 link-state PDU (18)
Level 2 link-state PDU (20)
Level 1 Complete SNP (24)
Level 2 Complete SNP (25)
Level 1 Partial SNP (26)
Level 2 Partial SNP (27)
Version (1 octet) This field is set to a constant value of 0x01, the current IS-IS version.
Reserved (1 octet) This field is set to a constant value of 0x00 and is ignored on receipt.
32 bits
8
Maximum Area
Addresses
ID Length
8
Reserved
Version
8
Version
Header Length
8
PDU Type
Protocol ID
Protocol Specifics 287
Maximum Area Addresses (1 octet) This field is set to a constant value of 0x00. It informs
other systems how many area addresses are supported by the local router. A value of 0 means
that no more than three area addresses are assigned to this router.
Details of each PDU type follow the common header. The information within the PDUs is
encoded in a format called a triple (Type, Length, Value). IS-IS makes extensive uses of this format
(often abbreviated as TLV) to convey information within its messages.
IS-IS LAN Hello PDU
We’ve previously stated that IS-IS routers exchange IS-IS Hello (IIH) PDUs to establish an adjacency.
While the purpose of the Hello PDU is the same, there are three different formats the router
can use. One is for point-to-point links, and the two others are for broadcast links—one each for
Level 1 and Level 2. Recall from the “IS-IS Levels” section earlier that L1 routers must share the
same area address to form an adjacency, while L2 routers do not have this limitation. The separate
LAN Hello PDUs simply tell the receiving router to check or ignore this information.
L1 LAN Hello PDUs are multicast to the “All L1 ISs” address of 01:80:c2:00:00:14. L2 routers
share a separate multicast address “All L2 ISs” of 01:80:c2:00:00:15. Both LAN Hello PDUs
share a common packet format, as shown in Figure 7.8.
The IS-IS LAN Hello PDU consists of the following fields:
Circuit Type (1 octet) The first 6 bits are set to 0. The remaining bits designate the level at
which the interface is operating: L1 (0x01), L2 (0x02), or L1/L2 (0x03). PDUs with a value of
0x00 in this field are ignored.
Source ID (6 octets) This field designates the sender of the IIH. The field is set to the 6-byte
system ID of the sending router.
Why Use a TLV Encoding Scheme?
The Type, Length, Value (TLV) format might at first glance appear to be unnecessary overhead.
After all, each small piece of transmitted data is encoded in this format, resulting in larger transmissions
between routers. It turns out, though, that this disadvantage is outweighed by the
usefulness of the TLV format. TLVs allow the protocol to extend its capabilities and functionality
very easily. For example, as new data formats were defined to support Traffic Engineering
over Multiprotocol Label Switching, only a new TLV structure—not an entirely new PDU format—
had to be defined. In addition, an IS-IS router ignores TLVs it does not support and uses the TLVs
it does understand. Protocols based on message types alone do not have this luxury. The message
type is either accepted or it is not. So although a TLV format adds more overhead to a specific
data transmission, it makes the use of the protocol simpler in the long run.
288 Chapter 7 Intermediate System to Intermediate System (IS-IS)
Holding Time (2 octets) The value in this field represents the amount of time each neighboring
router should wait before terminating the adjacency after the last received IS-IS Hello PDU
from this neighbor.
PDU Length (2 octets) The value in this field represents the total length of the IS-IS Hello
PDU. The field is set to a constant value of 1492 bytes (0x05D4).
Priority (1 octet) The first bit is set to 0. The remainder of the byte designates the value used
for the election of the DIS. The default value for the JUNOS software is 64.
LAN ID (7 octets) This field designates the ID of the current DIS on the broadcast circuit. The
field is set to the 6-byte system ID and 1-byte circuit ID of the DIS.
TLVs (Variable) This field contains information about the sending router, including the area
address, neighbor ID, authentication, and interface addressing.
We discuss circuit ID values in the “show isis interface” section later in this
chapter.
FIGURE 7 . 8 IS-IS Hello PDU (broadcast links)
If you refer back to Figure 7.1, Shiraz is advertising an IS-IS LAN Hello on its fe-0/0/0.0
interface:
May 2 22:50:54 Sending L1 LAN IIH on fe-0/0/0.0
May 2 22:50:54 max area 0, circuit type l1
May 2 22:50:54 neighbor 0:90:69:64:90:1f
May 2 22:50:54 neighbor 0:90:69:99:9c:0
May 2 22:50:54 No change in DR
May 2 22:50:54 hold time 9, priority 64, circuit id Shiraz.02
May 2 22:50:54 speaks IP
May 2 22:50:54 speaks IPv6
May 2 22:50:54 IP address 10.0.8.1
May 2 22:50:54 area address 47.0005.8083.00 (6)
8 8 8
Length Priority
Holding Time
(continued)
Source ID (continued) Holding Time
LAN ID
LAN ID (continued) TLVs
TLVs (continued)
Source ID
8
Circuit Type
32 bits
Protocol Specifics 289
Relevant portions of the output have been highlighted. You see the Circuit Type, Circuit ID,
Hold Time, and Priority fields. Shiraz is advertising a hold time of 27 seconds. This is the default
value for the JUNOS software on LAN interfaces unless the local router is the DIS. Hello PDUs
are advertised every (hold time / 3) seconds, so the default Hello timer is 9 seconds.
Shiraz is also advertising a local DIS priority of 64, the JUNOS software default. This is the
first tiebreaker for the election of the DIS, which we explain in the next section.
Designated Intermediate System
The concept of a Designated Intermediate System (DIS) is an important one when you’re learning
about IS-IS and link-state protocols. It helps to reduce the amount of data in the link-state
database and aid in the processing of the shortest path first (SPF) calculation. We’re examining
its functionality here since a DIS is elected only on a broadcast-capable link.
Broadcast links in a network pose a special issue for link-state protocols. Using the example
described earlier in this chapter in the “General IS-IS Information Exchange” section, each IS-IS
router on the link forms an adjacency with every other router and advertises that information into
the network. This requires information advertisements on the order of N*(N–1), where N is the
number of routers on the link. Many texts refer to this amount of data as O(N2) updates. This
adds unnecessary information and overhead to the protocol because each router is advertising the
exact same information.
You can mitigate this situation by introducing a pseudonode that represents the broadcast link
to the rest of the network. The pseudonode will advertise the neighbor relationships of all routers
in its database update; the actual routers advertise a relationship with only the pseudonode.
Let’s examine Figure 7.9. Without a pseudonode on this network, Shiraz advertises a relationship
with Merlot, Riesling, and Cabernet. All other routers follow this same procedure. This
O(N2) advertisement grows the database size exponentially as the number of routers on the broadcast
link grows. When a pseudonode is introduced on the link, all routers only advertise a relationship
to that node. The database size now grows on O(N) as the number of routers grows.
FIGURE 7 . 9 Designated Intermediate System updates
Advertised adjacencies
without pseudonode O(N2)
Advertised qdjacencies
with pseudonode O(N)
Cabernet
Riesling
Merlot
Shiraz
Cabernet
Riesling
Merlot
Shiraz
290 Chapter 7 Intermediate System to Intermediate System (IS-IS)
Within IS-IS, the elected pseudonode is called the DIS. The election process is very deterministic
since the router with the best (highest) criteria is always the DIS. The first criterion checked
is the advertised DIS priority of the router. The priority range is between 0 and 127, with a
JUNOS software default of 64. When two or more nodes share priority values, the second criterion
checked is the MAC address of the advertising router.
The Hello and hold-time timer values are changed for elected DIS routers. The 27-second
hold time is reduced to 9 seconds. The Hello timer is still (hold time / 3), which results in a Hello
PDU every 3 seconds. These quicker intervals allow the non-DIS routers to notice the loss of the
DIS in a timely manner and elect a new DIS.
IS-IS Point-to-Point Hello PDU
IS-IS adjacencies on point-to-point links are also formed through the advertisement of Hello
PDUs. On broadcast links, separate PDUs have been defined for Level 1 and Level 2. Only a single
Hello PDU is defined for point-to-point links. The desire to be adjacent with a neighbor at L1, L2,
or L1/L2 is encoded in the Circuit Type field within the PDU itself. The format of the PDU is
shown in Figure 7.10.
FIGURE 7 . 1 0 IS-IS Hello PDU (point-to-point links)
The IS-IS point-to-point Hello PDU consists of the following fields:
Circuit Type (1 octet) The first 6 bits are set to 0. The remaining bits designate the level at
which the interface is operating: L1 (0x01), L2 (0x02), or L1/L2 (0x03). PDUs with a value of
0x00 in this field are ignored.
Source ID (6 octets) This field designates the sender of the IIH. It is set to the 6-byte system ID
of the sending router.
Holding Time (2 octets) The value in this field represents the amount of time each neighboring
router should wait before terminating the adjacency after the last received IS-IS Hello PDU.
PDU Length (2 octets) The total length of the IS-IS Hello PDU is encoded in this field. The
field is set to a constant value of 1492 bytes (0x05D4).
Circuit ID (1 octet) This field designates the specific ID of the local router’s interface. All
point-to-point interfaces share a value of 0x01 within the JUNOS software.
TLVs (Variable) This field contains information about the sending router, including the area
address, authentication, and interface addressing.
8 8 8
Holding Time Length Circuit ID
(continued)
Source ID (continued) Holding Time
TLVs
Source ID
8
Circuit Type
32 bits
Protocol Specifics 291
We discuss circuit ID values in the “show isis interface” section later in this
chapter.
In our example, Riesling is advertising an IS-IS point-to-point Hello on its e3-0/2/0.101
interface. It wants to form only a Level 2 adjacency with Cabernet, its neighboring router:
May 2 22:52:12 Sending PTP IIH on e3-0/2/0.101
May 2 22:52:12 max area 0, circuit type l2
May 2 22:52:12 ptp adjacency tlv length 15
May 2 22:52:12 neighbor state up
May 2 22:52:12 our extended local ciruit id 9
May 2 22:52:12 neighbor sysid Cabernet
May 2 22:52:12 neighbor extended local circuit id 5
May 2 22:52:12 speaks IP
May 2 22:52:12 speaks IPv6
May 2 22:52:12 IP address 192.168.1.1
May 2 22:52:12 area address 49.0001 (3)
IS-IS Hello PDUs and Data-Link MTUs
You may notice that both the LAN and point-to-point Hello PDUs have preset lengths assigned to
them. This arises from the fact that an IS-IS router does not resize any PDU to match the maximum
transmission unit (MTU) on an interface. Therefore, each interface must support the transmission
of the maximum IS-IS PDU of 1492 bytes. To enforce this requirement, the IS-IS Hello
PDUs are padded to this maximum value. If the hello gets to the neighboring router, the connecting
interface supports the maximum PDU size. Should the hello not be received by the
neighboring router, no adjacency forms and this link is not used by IS-IS.
A point-to-point interface assumes a payload size of 1500 bytes but subtracts the transmission
overhead of the High-Level Data Link Control (HDLC) broadcast frame (1 byte), an unnumbered
information control field (1 byte), and the PPP Protocol ID field (2 bytes). This leaves 1496 bytes for
IS-IS to operate within, 4 bytes more than the size of the Hello PDU for a point-to-point interface.
292 Chapter 7 Intermediate System to Intermediate System (IS-IS)
For further reading, refer to Handbook of Computer Communications Standards,
William Stallings (Macmillan, 1990), pp. 76-87.
Complete Sequence Number PDU
The Complete Sequence Number PDU (CSNP) contains a complete listing of the link-state
PDUs in the link-state database of the local router. The CSNP provides an identifier, a lifetime,
a sequence number, and a checksum for each piece of information in the database. A CSNP is
sent periodically on both broadcast and point-to-point links to maintain database correctness.
In addition, CSNPs are advertised between two neighbors during the formation of an adjacency.
As with the IS-IS LAN Hello PDUs, there are separate CSNPs for Level 1 and Level 2 used
on all media types. Level 1 PDUs are multicast to the “All L1 ISs” address of 01:80:c2:00:00:14.
Level 2 PDUs are multicast to the “All L2 ISs” address of 01:80:c2:00:00:15. Figure 7.11 shows
the format of the CSNP.
Broadcast links also begin with a 1500-byte payload field but have different overhead requirements.
Juniper Networks and other router vendors use the IEEE 802.2 Logical Link Control (LLC)
encoding for IS-IS packets on broadcast interfaces. The 802.2 LLC format assumes 3 bytes of
data, one each for the destination service access point (DSAP), the source service access point
(SSAP), and the control field. This leaves 1497 bytes available while the Hello PDU is using only
1492 bytes. (The 5 bytes of difference are left to account for the option that a vendor might use
an Ethernet SNAP header for IS-IS. This would use an additional 5 bytes of user payload, leaving
IS-IS with only 1492 bytes available to it.) Therefore, the maximum PDU size of a Hello PDU for
a broadcast link is set to 1492 to account for this possibility.
The following information is not specific to IS-IS but involves more detail in the Ethernet encapsulation
techniques used in networking. If you want to focus only on IS-IS, return to the chapter
text at this point. For you true network nerds out there, please read on.
The IEEE 802.2 committee defined three methods for using the LLC in an Ethernet network.
Type 1, unacknowledged connectionless service, uses the data-link layer as a stream of data.
There is no inherent connection established to transmit the data reliably. Type 2 is a connectionoriented
mode service that allows for a connection establishment, some data transfer, and a disconnect
sequence. This is very much like the functions of TCP for Ethernet. Finally, Type 3 is for
acknowledged connectionless service where the receiving side sends messages to the sender to
verify its receipt.
While Types 2 and 3 are valuable in a network using IBM’s System Network Architecture
(SNA), modern implementations of Ethernet use only Type 1 LLC encoding. Higher layers of
the protocol stack assume responsibility for connections between systems. The Ethernet network
should send the data in an unsequenced fashion. A Type 1 packet uses a control field
value of 0x03 in the LLC header. When added to the default DSAP and SSAP values of 0xFE,
the entire 802.2 LLC header for an IS-IS broadcast packet is 0xFE-FE-03.
Protocol Specifics 293
FIGURE 7 . 1 1 IS-IS Complete Sequence Number PDU
The fields of the CSNP include:
Length (2 octets) The total length of the CSNP, in octets, is encoded in this field.
Source ID (7 octets) This field designates the sender of the CSNP. It is set to the 6-byte system
ID and 1-byte circuit ID (0x00) of the sending router.
Start LSP ID (8 octets) This field is set to a constant value of 0x0000.0000.0000.00-00. It
designates the smallest possible LSP ID value.
End LSP ID (8 octets) This field is set to a constant value of 0xFFFF.FFFF.FFFF.FF-FF. It designates
the largest possible LSP ID value.
TLVs (Variable) This field contains the summary database information from the local router.
Here, Cabernet has received a CSNP from Riesling on its e3-0/2/0.101 interface:
May 2 22:49:51 Received L2 CSN, source Riesling, interface e3-0/2/0.101
May 2 22:49:51 LSP range 0000.0000.0000.00-00 to ffff.ffff.ffff.ff-ff
May 2 22:49:51 packet length 83
May 2 22:49:51 LSP Riesling.00-00 lifetime 916
May 2 22:49:51 sequence 0x42 checksum 0x60a7
May 2 22:49:51 Matched database, matching sequence numbers
May 2 22:49:51 LSP Merlot.00-00 lifetime 1160
May 2 22:49:51 sequence 0x3c checksum 0xb88d
May 2 22:49:51 Matched database, matching sequence numbers
May 2 22:49:51 LSP Cabernet.00-00 lifetime 801
May 2 22:49:51 sequence 0x3d checksum 0xc376
May 2 22:49:51 Matched database, matching sequence numbers
8 8 8 8
End LSP ID TLVs
(continued)
End LSP ID (continued)
Start LSP ID End LSP ID
(continued)
Start LSP ID (continued)
Source ID Start LSP ID
(continued)
Source ID (continued)
Length Source ID
32 bits
294 Chapter 7 Intermediate System to Intermediate System (IS-IS)
Each segment of Riesling’s database contains the LSP ID, a sequence number, a lifetime value,
and a checksum. The combination of these data segments uniquely identifies each LSP in the network.
As Cabernet receives the CSNP, it checks the database entries against its own local linkstate
database. If some advertised information is missing, Cabernet requests the specific LSP
details using a Partial Sequence Number PDU.
At this point, Cabernet’s local database matches the advertised LSP information. The
JUNOS software designates a match with the Matched database, matching sequence
numbers message.
Partial Sequence Number PDU
An IS-IS router uses the Partial Sequence Number PDU (PSNP) to request LSP information
from a neighbor. The PSNP is also used to explicitly acknowledge the receipt of a received LSP
on a point-to-point link. On a broadcast link, CSNPs are used as implicit acknowledgments.
The PSNP has both a Level 1 and Level 2 variety, like the CSNP and IIH PDUs. On broadcast
networks, Level 1 PSNPs are multicast to the “All L1 ISs” address of 01:80:c2:00:00:14 and
Level 2 PSNPs are multicast to the “All L2 ISs” address of 01:80:c2:00:00:15. The format of the
PSNP is shown in Figure 7.12.
FIGURE 7 . 1 2 IS-IS Partial Sequence Number PDU
A Partial Sequence Number PDU includes the following fields:
Length (2 octets) The total length of the PSNP, in octets, is encoded in this field.
Source ID (7 octets) This field designates the sender of the PSNP. It is set to the 6-byte system
ID and 1-byte circuit ID (0x00) of the sending router.
TLVs (Variable) This field contains the requested database information or the LSP being
acknowledged.
Here, Cabernet has received another CSNP from Riesling on its e3-0/2/0.101 interface:
May 9 15:22:21 Received L2 CSN, source Riesling, interface e3-0/2/0.101
May 9 15:22:21 LSP range 0000.0000.0000.00-00 to ffff.ffff.ffff.ff-ff
May 9 15:22:21 packet length 83
8 8 8
Source ID
(continued)
Source ID (continued)
TLVs
Source ID
8
Length
32 bits
Protocol Specifics 295
May 9 15:22:21 LSP Riesling.00-00 lifetime 1194
May 9 15:22:21 sequence 0x336 checksum 0x9a76
May 9 15:22:21 Missing LSP, requesting
May 9 15:22:21 Sending L2 PSN on interface e3-0/2/0.101
May 9 15:22:21 LSP Riesling.00-00 lifetime 1192
May 9 15:22:21 sequence 0 checksum 0x9a76
May 9 15:22:26 Received L2 LSP Riesling.00-00, interface e3-0/2/0.101
May 9 15:22:26 from Riesling
May 9 15:22:26 sequence 0x336, checksum 0x9a76, lifetime 1188
May 9 15:22:26 New LSP, adding to database
May 9 15:22:26 Sending L2 PSN on interface e3-0/2/0.101
May 9 15:22:26 LSP Riesling.00-00 lifetime 1186
May 9 15:22:26 sequence 0x336 checksum 0x9a76
As Cabernet compares the CSNP to its local database, it determines that the Riesling.00-00
LSP is missing. Cabernet issues a PSNP for the missing LSP, which Riesling returns in a link-state
PDU (which we describe in the next section). The received LSP of Riesling.00-00 is installed in
Cabernet’s database and an acknowledgement PSNP is returned to Riesling.
Link-State PDU
Thus far, we’ve been talking about the link-state database from numerous perspectives. IS-IS
routers have formed adjacencies and compared their databases. Complete and Partial Sequence
Number PDUs have been sent between routers to synchronize the databases. We’ve failed to discuss
the actual database information to this point. Let’s now tackle this subject.
A link-state PDU (LSP) contains information about each router in the network and its connected
interfaces. Metric and IS-IS neighbor information is also included. Figure 7.13 shows the
format of the link-state PDU.
FIGURE 7 . 1 3 IS-IS link-state PDU
8 8 8 8
Checksum Attributes TLVs
Sequence Number
LSP ID (continued)
LSP ID
Length Remaining Lifetime
32 bits
296 Chapter 7 Intermediate System to Intermediate System (IS-IS)
A link-state PDU includes the following fields:
Length (2 octets) The total length of the LSP is encoded in this field.
Remaining Lifetime (2 octets) This field lists the amount of time, in seconds, each router
should consider the LSP active. The JUNOS software default lifetime value is 1200 seconds.
LSP ID (8 octets) This field uniquely identifies the LSP throughout the network. The value is
a combination of the system ID (6 bytes), circuit ID (1 byte), and LSP Number value.
Sequence Number (4 octets) This field is set to the current version number of the LSP. The initial
number is 0x01 and is incremented each time the originating router updates the LSP.
Checksum (2 octets) This field contains the checksum value of the PDU fields after the
Remaining Lifetime.
Attributes (1 octet) This field contains multiple settings related to the state of the local router.
The specific bit positions are:
Bit 7 Partition bit. Set to 0 and not supported by the JUNOS software.
Bit 6 Attached bit for error metric. Set to 0 and not supported by the JUNOS software.
Bit 5 Attached bit for expense metric. Set to 0 and not supported by the JUNOS software.
Bit 4 Attached bit for delay metric. Set to 0 and not supported by the JUNOS software.
Bit 3 Attached bit for default metric. Used by an L2 router to advertise connectivity to the
IS-IS backbone into an L1 area.
Bit 2 Overload bit. Used to alert other IS-IS routers to not use the information advertised
in this LSP.
Bits 0 and 1 Designates the capabilities of the router. An L1 router sets these to 0x01. An
L1/L2 router or L2 router sets these to 0x03.
TLVs (Variable) This field contains the summary database information from the local router.
In the “Partial Sequence Number PDU” section earlier in this chapter, we showed an IS-IS
exchange between Riesling and Cabernet. Cabernet requested an LSP from Riesling using a PSNP
and Riesling responded. The actual LSP information was removed from the earlier capture and is
included here:
May 9 15:22:26 Received L2 LSP Riesling.00-00, interface e3-0/2/0.101
May 9 15:22:26 from Riesling
May 9 15:22:26 sequence 0x336, checksum 0x9a76, lifetime 1188
May 9 15:22:26 max area 0, length 263
May 9 15:22:26 no partition repair, no database overload
May 9 15:22:26 IS type 3, metric type 0
May 9 15:22:26 area address 49.0001 (3)
May 9 15:22:26 speaks IP
Protocol Specifics 297
May 9 15:22:26 IP router id: 192.168.0.1
May 9 15:22:26 IP address 192.168.0.1
May 9 15:22:26 dyn hostname Riesling
May 9 15:22:26 IS neighbor Merlot.00, metric: 10
May 9 15:22:26 IP address: 192.168.2.1
May 9 15:22:26 Neighbor's IP address: 192.168.2.2
May 9 15:22:26 IS neighbor Cabernet.00, metric: 10
May 9 15:22:26 IP address: 192.168.1.1
May 9 15:22:26 Neighbor's IP address: 192.168.1.2
May 9 15:22:26 IP prefix: 192.168.0.1/32 metric 0 up
May 9 15:22:26 IP prefix: 192.168.1.0/30 metric 10 up
May 9 15:22:26 IP prefix: 192.168.2.0/30 metric 10 up
May 9 15:22:26 IP prefix: 192.168.0.0/24 metric 10 up
May 9 15:22:26 IP prefix: 192.168.1.0/24 metric 10 up
May 9 15:22:26 IP prefix: 200.0.3.0/24 metric 10 up
The highlighted portion of the output shows the sequence number, lifetime, checksum, and
overload setting. The IS type, currently set to 3, shows that Riesling is capable of communicating
at both Level 1 and Level 2. Also included are the IS-IS neighbors of Merlot and Cabernet
with appropriate IP addressing information. You can also observe the IP subnets and metrics
advertised by Riesling.
Common TLVs
Each of the IS-IS PDUs we have discussed contained some TLV triples. While the entire listing
of TLV values is outside the scope of this book, the list below points out some common TLVs.
You can observe many of these in the Riesling output in the “Link-State PDU” section earlier
in this chapter.
TLV 1—Area Addresses
TLV 2—IS Reachability
TLV 6—IS Neighbors
TLV 8—Padding
TLV 9—LSP Entry
TLV 10—Authentication
TLV 128—IP Internal Reachability
TLV 129—Protocols Supported
TLV 130—IP External Reachability
TLV 132—IP Interface Address
TLV 137—Dynamic Hostname Mapping
298 Chapter 7 Intermediate System to Intermediate System (IS-IS)
Command-Line Interface
Up to this point in the chapter, we’ve been talking about IS-IS from a theoretical point of view.
Let’s now discuss how to use the protocol on a Juniper Networks router. We first look at the
configuration of the protocol; then we examine some JUNOS software commands you can use
to troubleshoot the operation of IS-IS.
Configuration Commands
The configuration of IS-IS within the JUNOS software requires three main steps. You first assign
a NET ID to the router. Then you configure each router interface using the family iso command.
Finally, you configure the protocol itself within [edit protocols]. Let’s examine each
step in more detail.
Network Entity Title Assignment
Recall from the “Addressing” section earlier in this chapter that the ISO NSAP address encodes
the system ID of the router and its area address. This information is critical to allow an IS-IS
adjacency to form. You should configure the router’s NET ID on a reliable and stable router
interface; that way, an interface failure does not mean the loss of the NET address. It is currently
a best practice to assign the NET ID to the router’s loopback interface (lo0).
This command assigns a NET ID to Merlot’s lo0 interface:
[edit interfaces lo0 unit 0]
user@Merlot# set family iso address 47.0005.8083.0000.1921.6800.5001.00
This results in the following configuration:
[edit interfaces lo0]
user@Merlot# show
unit 0 {
family inet {
address 192.168.5.1/32;
}
family iso {
address 47.0005.8083.0000.1921.6800.5001.00;
}
}
Remember to set the N-Selector byte to a value of 0x00 to allow your IS-IS
adjacencies to form.
Command-Line Interface 299
Configuring Physical Interfaces
An interface on a Juniper Networks router accepts only IP packets by default. To allow other
protocol types to enter the router, you must configure the interface to recognize those packets.
This means that each interface must be aware that IS-IS packets with a Network-Layer Protocol
ID value of 0x83 are important. You use the family iso command to accomplish this, as
shown in the following:
[edit]
user@Cabernet# set interfaces fe-0/1/0 unit 0 family iso
user@Cabernet# set interfaces e3-0/2/0 unit 101 family iso
Cabernet now has two transit interfaces capable of running the IS-IS protocol. This is verified
when we issue the show interfaces terse command:
user@Cabernet> show interfaces terse
Interface Admin Link Proto Local Remote
fe-0/1/0 up up
fe-0/1/0.0 up up inet 10.0.8.3/24
iso
fe-0/1/1 up down
fe-0/1/2 up down
fe-0/1/3 up down
e3-0/2/0 up up
e3-0/2/0.101 up up inet 192.168.1.2/30
iso
e3-0/2/1 up down
e3-0/2/2 up down
e3-0/2/3 up down
fxp0 up up
fxp0.0 up up inet 172.25.41.111/25
fxp1 up up
fxp1.0 up up tnp 4
gre up up
ipip up up
lo0 up up
lo0.0 up up inet 192.168.16.1 --> 0/0
iso 47.0005.8083.0000.1921.6801.6001.00
lsi up up
The presence of the iso keyword within the logical interface portion of the fe-0/1/0 and
e3-0/2/0 interfaces verifies the success of the configuration.
300 Chapter 7 Intermediate System to Intermediate System (IS-IS)
Configuring the Protocol
The final step in operating IS-IS within the JUNOS software is enabling the route protocol daemon,
rpd, to process IS-IS messages. You enable rpd within the [edit protocols isis] portion
of the configuration hierarchy. Each configured IS-IS interface operates at both Level 1 and
Level 2 by default. To prevent IS-IS from forming an adjacency at a particular level, you must
use the disable command. Let’s examine some different methods for configuring the protocol.
Figure 7.14 shows our sample network and the IS-IS level each interface should use.
FIGURE 7 . 1 4 IS-IS network-level configuration
Riesling
We’ve configured Riesling to use only Level 2 IS-IS packets to communicate with its neighbors:
[edit protocols]
user@Riesling# show
isis {
level 1 disable;
interface e3-0/2/0.101;
interface e3-0/2/3.100;
interface lo0.0;
}
The configuration of level 1 disable at the global IS-IS level allows individual interfaces
to be listed without requiring you to explicitly disable the level for each. This is a common practice
Area: 49.0001
Area: 47.0005.8083.0000
fe-0/1/0.0
L2 L2
L1/L2 L2
e3-0/2/0.101 e3-0/2/3.100
e3-0/2/3.100
L1
e3-0/2/0.101
L1
fe-0/0/0.0 L1
fe-0/0/0.0
Cabernet Merlot
Riesling
Shiraz
Command-Line Interface 301
for routers that use only one of the two possible levels. Interface lo0.0 is configured to allow adjacencies
to form with the neighboring routers. Recall from the “Network Entity Title Assignment”
section earlier in this chapter that the area address in the NET ID was placed on the loopback
interface. Neighbor adjacencies form only when the NET ID is on an operational IS-IS interface.
Cabernet
IS-IS adjacencies for Cabernet operate at both Level 1 and Level 2 with its neighbors. Its configuration
is as follows:
[edit protocols]
user@Cabernet# show
isis {
interface fe-0/1/0.0 {
level 2 disable;
}
interface e3-0/2/0.101;
interface lo0.0;
}
Interface lo0.0 is included, as before, to advertise the NET ID to its neighbors. The
inclusion of level 2 disable within the configuration of interface fe-0/1/0.0 allows
only IS-IS Level 1 packets to be sent and limits this neighbor relationship to an L1 adjacency.
Cabernet sends both L1 and L2 IS-IS Hello PDUs to Riesling based on the default
interface parameters for interface e3-0/2/0.101. Because Riesling is configured to only use
Level 2, only an L2 adjacency will form between these routers.
Shiraz
The single interface on Shiraz is operating with its neighbors at Level 1 only. The configuration
for Shiraz is:
[edit protocols]
user@Shiraz# show
isis {
level 2 disable;
interface all;
interface fxp0.0 {
disable;
}
}
The configuration of level 2 disable at the global IS-IS level mirrors the configuration of
Riesling. It again allows individual interfaces to be listed without requiring the explicit disabling
of the IS-IS level for each. We’ve used the keyword all in Shiraz’s configuration to allow IS-IS
to operate on any interface configured with the family iso command. This is a common configuration
when an IS-IS router is using every router interface.
302 Chapter 7 Intermediate System to Intermediate System (IS-IS)
Merlot
We’ve configured Merlot to use Level 2 with Riesling and Level 1 with all other routers:
[edit protocols]
user@Merlot# show
isis {
interface e3-0/2/3.100 {
level 1 disable;
}
interface all {
level 2 disable;
}
interface fxp0.0 {
disable;
}
}
We’ve also configured Merlot with the interface all command, which allows all operational
IS-IS capable interfaces to use the protocol. These interfaces use only Level 1 packets
to form adjacencies with Shiraz and Cabernet. We’ve configured interface e3-0/2/3.100
separately for Level 2 operations to Riesling. The listing of an individual interface within the
IS-IS configuration overrides the more generic use of interface all. This is very similar to
the JUNOS software default of a more specific parameter application taking precedence over
a less specific application. The exception here is that both of the applications occur within the
same configuration hierarchy.
There is one IS-IS configuration option that can’t be overridden with a more
specific application. When you set level 2 disable at the global IS-IS level, this
will cause all interfaces on the router to never use Level 2 PDUs. A specific interface
reference to level 1 disable (which normally activates Level 2) does not
take effect. In essence, no adjacencies ever form on the interface you specified.
Only use the global application when you really mean it!
Disabling the fxp0 Interface
You may recall from Chapter 1, “The Components of a Juniper Networks Router,” that the fxp0
interface on a Juniper Networks router has a special purpose. It should be used only for out-ofband
access to the Routing Engine. Packets can’t be forwarded from a transit interface across
the backbone of the router and out the management interface. However, fxp0 is still an operational
interface on the router and IS-IS adjacencies can be formed using this interface.
Command-Line Interface 303
Verification and Troubleshooting Commands
Once IS-IS is configured on your network, you probably want to know if it is working correctly.
The JUNOS software provides command-line interface (CLI) commands that verify and assist
in troubleshooting your configuration.
In Figure 7.15, we’ve added some IP addressing information to our sample network. We use
this common diagram to explore the various commands.
FIGURE 7 . 1 5 IS-IS network addressing
show isis adjacency
You can verify that your IS-IS adjacencies are working by using the show isis adjacency
command. This is often the first command you’ll use when troubleshooting IS-IS. When a neighbor
appears in the output, you can safely assume that packets are traversing the physical interface,
IS-IS PDUs have been exchanged, and the link-state databases are synchronized.
Using Figure 7.14 as a guide, imagine that the fxp0 interfaces of Shiraz and Riesling are configured
for IS-IS and an L2 adjacency forms between those routers. Riesling now believes that
it has a direct connection to Shiraz, when in fact it should not. Packets transiting Riesling and
destined for Shiraz will attempt to be forwarded out the management interface, but will in fact
be dropped from the network. Compounding this issue is the fact that both Shiraz and Riesling
advertise their relationship into the IS-IS network and other routers view this “virtual” connection
as a viable network link.
In short, nothing good can come from enabling the fxp0 interface within a routing protocol.
Therefore, it is a good practice to explicitly disable the management interface when using the
interface all syntax.
Area: 49.0001
.2 Area: 47.0005.8083.0000 .2
.3 .1
.2
10.0.8/24
.1 .1
192.168.1/30 192.168.2/30
Cabernet Merlot
Riesling
Shiraz
304 Chapter 7 Intermediate System to Intermediate System (IS-IS)
user@Cabernet> show isis adjacency
Interface System L State Hold (secs) SNPA
e3-0/2/0.101 Riesling 2 Up 23
fe-0/1/0.0 Shiraz 1 Up 7 0:90:69:99:9c:0
fe-0/1/0.0 Merlot 1 Up 24 0:90:69:97:c4:0
The column entries provide important information to you at a glance.
Interface This identifies the logical interface on which IS-IS has formed an adjacency. If
an expected entry is not listed here, first verify that the interface is configured within [edit
protocols isis]. A second possible cause of this problem results from omitting the family
iso command on that interface.
System The automatic system ID-to-router hostname mapping is shown here. Until this resolution
occurs, the system ID value itself is displayed.
L (Level) This indicates the IS-IS adjacency level with that neighbor. Possible values are 1, 2, or
3. A value of 3 indicates both a Level 1 and a Level 2 adjacency on a point-to-point interface. A
“!” symbol next to a level value denotes no IP information is present on the interfaces. Remember
that a Juniper Networks router uses only CLNP packets to form an IS-IS adjacency.
State Indicates the current state of the IS-IS adjacency. Possible values include:
Up
Down
New
One-Way
Initializing
Rejected
Hold Displays the time remaining before the local router removes the IS-IS adjacency.
SNPA The Sub-Network Point of Attachment (SNPA) is the data-link address used to reach the
neighbor on a broadcast media. Ethernet links use the MAC address of the neighbor as the SNPA.
show isis adjacency detail
Adding the detail option to the show isis adjacency command provides additional information
about each IS-IS adjacency:
user@Cabernet> show isis adjacency detail
Riesling
Interface: e3-0/2/0.101, Level: 2, State: Up, Expires in 25 secs
Priority: 0, Up/Down transitions: 1, Last transition: 08:18:11 ago
Circuit type: 3, Speaks: IP, IPv6
Restart capable: No
IP addresses: 192.168.1.1
Command-Line Interface 305
Shiraz
Interface: fe-0/1/0.0, Level: 1, State: Up, Expires in 8 secs
Priority: 64, Up/Down transitions: 1, Last transition: 00:06:21 ago
Circuit type: 1, Speaks: IP, IPv6, MAC address: 0:90:69:99:9c:0
Restart capable: No
LAN id: Shiraz.02, IP addresses: 10.0.8.2
The first line of the output for each neighbor closely resembles the normal adjacency output.
Additional information gathered from this command includes the configured (DIS) priority
value. Riesling is advertising a value of 0 on the point-to-point link (because no DIS is elected
on this interface), while Shiraz is advertising a value of 64 on the Ethernet link. The Circuit Type
entry details the local level configuration of the router. Cabernet’s interface is configured for a
circuit type of 3 (both L1 and L2), but the actual adjacency is reporting only Level 2. Riesling
is either configured for only L2, or an IS-IS area mismatch occurred at Level 1 between the two
routers. The Ethernet link to Shiraz shows the MAC address (SNPA) of the neighbor as well as
the address of the LAN pseudonode—Shiraz.02.
clear isis adjacency
The clear isis adjacency command enables you to remove an IS-IS adjacency from the local
router. New IS-IS Hello PDUs and sequence number PDUs are exchanged and the adjacency is
reestablished. This is helpful to guarantee that “fresh” information is entered in the link-state
database correctly. Using the clear isis adjacency command itself clears all adjacencies;
adding a neighbor’s hostname clears just that single adjacency.
user@Shiraz> show isis adjacency
Interface System L State Hold (secs) SNPA
fe-0/0/0.0 Cabernet 1 Up 26 0:90:69:64:90:1f
fe-0/0/0.0 Merlot 1 Up 23 0:90:69:97:c4:0
user@Shiraz> clear isis adjacency Cabernet
user@Shiraz> show isis adjacency
Interface System L State Hold (secs) SNPA
fe-0/0/0.0 Cabernet 1 Initializing 26 0:90:69:64:90:1f
fe-0/0/0.0 Merlot 1 Up 22 0:90:69:97:c4:0
user@Shiraz> show isis adjacency
Interface System L State Hold (secs) SNPA
fe-0/0/0.0 Cabernet 1 Up 26 0:90:69:64:90:1f
fe-0/0/0.0 Merlot 1 Up 21 0:90:69:97:c4:0
After verifying the current adjacencies, Shiraz clears its connection with Cabernet. As the
adjacency starts to reform, the Initializing state quickly appears since the neighboring
router lists the local router in its IS-IS Hello PDU.
306 Chapter 7 Intermediate System to Intermediate System (IS-IS)
show isis interface
After you use the show isis adjacency command and see no neighbors, the show isis
interface command is the next best troubleshooting command available. It displays the interfaces
that are currently operational from the local router’s perspective.
user@Cabernet> show isis interface
IS-IS interface database:
Interface L CirID Level 1 DR Level 2 DR L1/L2 Metric
e3-0/2/0.101 3 0x1 Point to Point Point to Point 10/10
fe-0/1/0.0 1 0x2 Shiraz.02 Disabled 10/10
lo0.0 0 0x1 Passive Passive 0/0
Each of the columns in the output indicate the IS-IS configuration and operation of the
interfaces.
Interface This identifies the logical interface on which IS-IS is operating. Entries not listed
here are often caused by a misconfiguration within [edit protocols isis] or a missing
family iso command on that interface.
L (Level) This indicates the IS-IS levels each interface is configured to support. Possible values
are 0,1, 2, or 3. A value of 0 indicates that all operational IS-IS levels are currently in passive
mode. A value of 3 indicates that both Level 1 and Level 2 are operating on the interface.
CirID Each IS-IS interface is assigned a circuit ID value to identify the interface within the linkstate
database. The loopback interface and all point-to-point links share the locally significant
value of 0x01. Each broadcast link receives a unique value starting at 0x02 and incrementing by
1 for each new interface.
Level 1 DR / Level 2 DR Each interface lists the known DISs (if any) for that link. The
loopback interface is always listed as passive since no IS-IS adjacency can ever form on this virtual
interface. No DIS is ever elected on a point-to-point link, so the listing there is always Point
to Point. Each broadcast interface displays the known DIS for that interface. All interfaces
that are not configured for a particular IS-IS level show Disabled in this column.
L1/L2 Metric The advertised metrics of each interface are displayed here. IS-IS uses a default
interface metric of 10 for both Levels 1 and 2. The maximum metric value is a 6-bit value of 63.
Each IS-IS router is capable of calculating a total path cost of 1023, or 10 bits.
show isis hostname
You can use the show isis hostname command to verify the dynamic hostname resolution of
system ID values. This command is helpful when you suspect that multiple IS-IS routers have an
identical system ID configuration.
user@Cabernet> show isis hostname
IS-IS hostname database:
System ID Hostname Type
1921.6800.0001 Riesling Dynamic
Command-Line Interface 307
1921.6800.5001 Merlot Dynamic
1921.6800.8001 Shiraz Dynamic
1921.6801.6001 Cabernet Static
show isis spf log
The show isis spf log command shows the history of SPF calculations (Dijkstra Algorithm),
why it was performed, and the duration of the calculation. A constant and rapid SPF calculation
is sometimes caused by a flapping interface in your network. The show isis spf log command
can pinpoint the router that is connected to the interface because each flap causes a new
link-state PDU to be generated.
user@Cabernet> show isis spf log
IS-IS level 1 SPF log:
Start time Elapsed (secs) Count Reason
Thu May 2 21:07:12 0.000205 1 Periodic SPF
Thu May 2 22:32:32 0.000225 1 Updated LSP Shiraz.00-00
Thu May 2 22:33:09 0.000171 1 Updated LSP Shiraz.02-00
Thu May 2 22:33:16 0.000177 3 Updated LSP Shiraz.02-00
IS-IS level 2 SPF log:
Start time Elapsed (secs) Count Reason
Thu May 2 22:24:46 0.000166 1 Periodic SPF
Thu May 2 22:33:10 0.000125 1 Updated LSP Cabernet.00-00
Thu May 2 22:33:11 0.000134 1 Updated LSP Merlot.00-00
Thu May 2 22:33:23 0.000127 1 Updated LSP Cabernet.00-00
Remember that the SPF algorithm operates as the local router receives new LSPs.
Topology changes result in an Updated LSP. The regular refreshing of LSPs in the
network results in the Periodic LSP.
show isis statistics
The show isis statistics command is helpful to verify that IS-IS packets are being transmitted,
received, and processed by the local router:
user@Cabernet> show isis statistics
IS-IS statistics for Cabernet:
PDU type Received Processed Drops Sent Rexmit
LSP 301 301 0 101 0
IIH 1676 96 1580 25 0
CSNP 6695 6446 0 5989 0
308 Chapter 7 Intermediate System to Intermediate System (IS-IS)
PSNP 57 57 0 94 0
Unknown 0 0 0 0 0
Totals 8729 6900 1580 6209 0
Total packets received: 8729 Sent: 6184
SNP queue length: 0 Drops: 0
LSP queue length: 0 Drops: 0
SPF runs: 165
Fragments rebuilt: 103
LSP regenerations: 75
Purges initiated: 5
show isis route
The handy show isis route command displays the results of the SPF calculation before the
routes are placed into the JUNOS software routing table. Although the same information can be
gathered from the output of show route protocol isis, this command places an IS-IS slant on
the data to aid in troubleshooting. For example, the next-hop router is displayed by IS-IS hostname
and not IP address. The type of the metric (internal versus external) can also be seen. Finally,
each route shows the exact SPF calculation used (the version) to select the route from the database.
user@Cabernet> show isis route
IS-IS routing table Current version: L1: 84 L2: 85
Prefix L Version Metric Type Interface Via
192.168.0.0/24 2 85 20 int e3-0/2/0.101 Riesling
192.168.2.0/30 2 85 20 int e3-0/2/0.101 Riesling
192.168.5.0/24 1 84 20 int fe-0/1/0.0 Merlot
192.168.10.0/24 1 84 20 int fe-0/1/0.0 Shiraz
192.168.11.0/24 1 84 20 int fe-0/1/0.0 Shiraz
200.0.3.0/24 2 85 20 int e3-0/2/0.101 Riesling
200.0.6.0/24 1 84 20 int fe-0/1/0.0 Merlot
200.0.7.0/24 1 84 20 int fe-0/1/0.0 Shiraz
show isis database
The show isis database command, along with its detail and extensive variations, is the
final stop in troubleshooting IS-IS. Simply put, if information is not in the database, then it will
never appear in the routing table. This version of the command displays summary information
on a per-level basis. Each link-state PDU shows its name, remaining lifetime, and attributes:
user@Cabernet> show isis database
IS-IS level 1 link-state database:
LSP ID Sequence Checksum Lifetime Attributes
Command-Line Interface 309
Merlot.00-00 0x31 0x781a 1049 L1 L2 Attached
Shiraz.00-00 0x39 0xf8b 835 L1
Shiraz.02-00 0x37 0x7611 941 L1
Cabernet.00-00 0x2d 0xc362 1015 L1 L2 Attached
4 LSPs
IS-IS level 2 link-state database:
LSP ID Sequence Checksum Lifetime Attributes
Riesling.00-00 0x3c 0x6ca1 1120 L1 L2
Merlot.00-00 0x37 0xc288 1047 L1 L2
Cabernet.00-00 0x37 0x66d9 1015 L1 L2
3 LSPs
show isis database detail
The detail option for the show isis database command provides more information about
each LSP in the link-state database. The advertised prefixes from each router, the metric for each
route, and the origin (internal versus external) of each route is visible when you issue this
command.
user@Cabernet> show isis database detail
IS-IS level 1 link-state database:
Merlot.00-00 Sequence: 0x31, Checksum: 0x781a, Lifetime: 919 secs
IS neighbor: Shiraz.02 Metric: 10
IP prefix: 200.0.6.0/24 Metric: 10 External
IP prefix: 192.168.7.0/24 Metric: 10 External
IP prefix: 192.168.6.0/24 Metric: 10 External
IP prefix: 192.168.5.0/24 Metric: 10 External
IP prefix: 192.168.5.1/32 Metric: 0 Internal
IP prefix: 10.0.8.0/24 Metric: 10 Internal
show isis database extensive
You use the show isis database extensive command to view each piece of data advertised
from each router into the IS-IS network. In addition to the information shown using show isis
database detail, the extensive option provides the LSP header information as well as each
TLV triple advertised. We’ll examine only a single Level 1 LSP, Merlot.00-00. The entire linkstate
database from our small sample network takes over six pages to display.
user@Cabernet> show isis database extensive
IS-IS level 1 link-state database:
Merlot.00-00 Sequence: 0x31, Checksum: 0x781a, Lifetime: 969 secs
310 Chapter 7 Intermediate System to Intermediate System (IS-IS)
IS neighbor: Shiraz.02 Metric: 10
IP prefix: 200.0.6.0/24 Metric: 10 External
IP prefix: 192.168.7.0/24 Metric: 10 External
IP prefix: 192.168.6.0/24 Metric: 10 External
IP prefix: 192.168.5.0/24 Metric: 10 External
IP prefix: 192.168.5.1/32 Metric: 0 Internal
IP prefix: 10.0.8.0/24 Metric: 10 Internal
Header: LSP ID: Merlot.00-00, Length: 222 bytes
Allocated length: 222 bytes, Router ID: 192.168.5.1
Remaining lifetime: 969 secs, Level: 1,Interface: 4
Estimated free bytes: 0, Actual free bytes: 0
Aging timer expires in: 969 secs
Protocols: IP
Packet: LSP ID: Merlot.00-00, Length: 222 bytes, Lifetime : 1198 secs
Checksum: 0x781a, Sequence: 0x31, Attributes: 0xb
NLPID: 0x83, Fixed length: 27 bytes, Version: 1, Sysid length: 0 bytes
Packet type: 18, Packet version: 1, Max area: 0
TLVs:
Area address: 47.0005.8083.00 (6)
Speaks: IP
Speaks: IPv6
IP router id: 192.168.5.1
IP address: 192.168.5.1
Hostname: Merlot
IS neighbor: Shiraz.02, Internal, Metric: default 10
IS neighbor: Shiraz.02, Metric: default 10
IP address: 10.0.8.1
IP prefix: 10.0.8.0/24, Internal, Metric: default 10
IP prefix: 192.168.5.1/32, Internal, Metric: default 0
IP prefix: 10.0.8.0/24 metric 10 up
IP prefix: 192.168.5.1/32 metric 0 up
IP external prefix: 192.168.5.0/24, Internal, Metric: default 10
IP external prefix: 192.168.6.0/24, Internal, Metric: default 10
IP external prefix: 192.168.7.0/24, Internal, Metric: default 10
IP external prefix: 200.0.6.0/24, Internal, Metric: default 10
IP prefix: 192.168.5.0/24 metric 10 up
IP prefix: 192.168.6.0/24 metric 10 up
Comparison to OSPF 311
IP prefix: 192.168.7.0/24 metric 10 up
IP prefix: 200.0.6.0/24 metric 10 up
No queued transmissions
Comparison to OSPF
IS-IS and Open Shortest Path First (OSPF) are the main protocols ISPs use within their routing
domains. These two protocols share many similarities but have distinct differences as well. Let’s
finish our discussion of IS-IS by examining these points.
The similarities between IS-IS and OSPF include:
Link-state protocols Both protocols are based on the concept of a link-state database. Network
information is flooded throughout the network, and each router maintains a complete
copy of this data.
Hierarchical network designs The flooding of information is bounded by the design of the
network. Both protocols support a hierarchical design concept that bounds the update flooding.
The IS-IS level is comparable to an OSPF area.
Hello protocol for adjacencies Network link information is advertised after two routers form
an adjacency relationship. The concept of a hello packet is common to both IS-IS and OSPF.
This hello packet forms and maintains the adjacency.
Pseudonode election on broadcast media To reduce the amount of information in the linkstate
database, both protocols utilize the concept of a pseudonode on broadcast links. A router
is elected to represent the link to the remainder of the network.
IS-IS and OSPF approach these basic operational concepts in different ways. These differences
include:
Election of a new pseudonode Within IS-IS, the election of the pseudonode is deterministic—
the router with the best criteria will always become the DIS. In addition, there is no provision
or requirement for a backup DIS.
OSPF approaches this issue from a different perspective. The Designated Router (DR) may not
be the router with the best criteria—which makes it a nondeterministic system. New elections
are conducted only upon the failure of the current DR, resulting in a new backup DR. The previous
BDR automatically assumes the DR responsibility.
Routing propagation An entire link-state PDU is readvertised upon a network change in an
IS-IS network. A similar change in an OSPF network, however, means that only a specific linkstate
advertisement (LSA) need be flooded.
Formatting Updates IS-IS updates contain multiple (Type, Length, Value) triples to advertise
information. The addition of a new TLV makes the protocol very easy to alter since an IS-IS
router uses only the TLVs it understands. OSPF routers process only known Link-State Advertisements
(LSA) and a protocol alteration requires a new standardized LSA definition that all
vendors can agree on.
312 Chapter 7 Intermediate System to Intermediate System (IS-IS)
Reliance on IP Two IS-IS routers can form an adjacency without the presence of IP addressing
since NSAP addresses and CLNP processing are all that are required. OSPF, on the other hand,
uses its own IP protocol number, so a valid IP addressing structure is required.
Summary
In this chapter, we reviewed the basic concepts behind the operation of link-state protocols. For
IS-IS, this means that routers form adjacencies, flood network information into the network, and
use the Dijkstra (shortest path first) Algorithm to calculate the total cost to each node in the network.
You can configure an IS-IS network to support multiple levels that provide an informationflooding
boundary.
We then discussed the data packets used by IS-IS routers. Once adjacent using IS-IS Hello
PDUs, the routers synchronize their databases using Complete Sequence Number PDUs and
Partial Sequence Number PDUs. This synchronization process advertises only the header information
of the database contents. The actual network data is advertised within a Link-State PDU
and is flooded throughout the network.
Finally, we covered the configuration and operation of IS-IS on a Juniper Networks router.
We found that there are three major steps to configuring the protocol: NET ID assignment,
interface configuration, and protocol setup. The JUNOS software provides several commands
that you can use to verify adjacencies, protocol configuration, and database contents.
We wrapped up our discussion with a comparison of IS-IS and OSPF (covered in Chapter 6)
by considering the similarities and differences between these two important protocols.
Exam Essentials
Be able to identify the portions of an IS-IS NSAP address. The NET ID assignment to an IS-IS
router is critical to the correct operation of the protocol. The NET ID contains the router’s area
address, system ID, and N-selector information. The N-selector must always be set to 0x00.
Know the various Protocol Data Units used by an IS-IS router. Four main PDUs are advertised
in an IS-IS network: the Hello, Link-State, Complete Sequence Number, and Partial
Sequence Number PDUs.
Understand how an IS-IS adjacency is formed. The two IS-IS levels have different criteria for
forming an adjacency. Both require a unique system ID, while a Level 1 adjacency also dictates
a common area address.
Be able to describe the election criteria for the Designated Intermediate System. On a broadcast
network, a single router is elected to represent the link information to the network. This
router is chosen based on the highest configured priority, with the highest SNPA being the only
tiebreaker.
Key Terms 313
Understand the steps required to configure IS-IS. Configuring the protocol on a Juniper Networks
router requires three main steps. First, you assign the NET ID; then, you configure each
interface to support IS-IS. Finally, you tell the routing process which interfaces to operate across.
Identify the JUNOS software commands that validate the operation of IS-IS. Various commands
allow you to check the status of adjacencies, interfaces, and the link-state database.
Key Terms
Before you take the exam, be certain you are familiar with the following terms:
adjacency IS-IS level
Authority and Format Indicator (AFI) link-state database
Complete Sequence Number PDU (CSNP) link-state PDU (LSP)
Designated Intermediate System (DIS) N-selector (SEL)
Dijkstra Algorithm Network Entity Title (NET)
DIS priority Partial Sequence Number PDU (PSNP)
Domain-Specific Part (DSP) Protocol Data Unit (PDU)
Hello messages system ID
intermediate system triple
IS-IS Hello (IIH) PDUs
314 Chapter 7 Intermediate System to Intermediate System (IS-IS)
Review Questions
1. Which of the following would be considered a private NSAP address?
A. 37.1010.1921.6806.4001.00
B. 39.0001.1921.6806.4001.00
C. 47.1010.1921.6806.4001.00
D. 49.0001.1921.6806.4001.00
2. What is the size of the system ID on a Juniper Networks router?
A. 3 bytes
B. 6 bytes
C. 13 bytes
D. 20 bytes
3. An IS-IS router uses which circuit ID to represent the node itself within the network?
A. 0x00
B. 0x01
C. 0x02
D. 0x03
4. An IS-IS router uses which PDU to request information missing in its database?
A. Hello
B. Link-State
C. Complete Sequence Number
D. Partial Sequence Number
5. The IS-IS Hello PDU is used for what network function?
A. To advertise information about connected networks
B. To form an adjacency with a neighbor
C. To inform the network about connected IS-IS routers
D. To prevent database information from flooding beyond the defined IS-IS levels
6. Information about the local link-state database is exchanged with a neighbor during an adjacency
formation. Which PDU accomplishes this?
A. Hello
B. Link-State
C. Complete Sequence Number
D. Partial Sequence Number
Review Questions 315
7. A link-state PDU advertises information using what format?
A. Type, Length, Value (TLV) encoding
B. Link-state advertisement (LSA) types
C. Connectionless Network Protocol structures
D. Link-layer encapsulation
8. An IS-IS router with a NET ID of 49.1234.4321.1921.6801.6001.00 can form a Level 1 adjacency
with which other system?
A. 49.4321.1921.6806.4001.00
B. 49.4321.1921.6801.6001.00
C. 49.1234.4321.1921.6806.4001.00
D. 49.1234.4321.1921.6801.6001.00
9. Which IS-IS adjacency state shows that bidirectional communication has occurred but that the
link-state databases are still converging?
A. New
B. One-Way
C. Initializing
D. Up
10. An authentication failure prompts which IS-IS adjacency state to appear?
A. New
B. Down
C. Initializing
D. Reject
11. What is the primary criterion for the election of the Designated Intermediate System (DIS) on a
broadcast link?
A. Highest system priority
B. Lowest system priority
C. Highest MAC address
D. Lowest MAC address
12. The following four routers are adjacent on a broadcast link. Which router is elected the Designated
Intermediate System?
A. Priority of 25 and MAC address of 00:90:69:90:50:11
B. Priority of 64 and MAC address of 00:90:69:96:87:46
C. Priority of 64 and MAC address of 00:90:69:56:70:79
D. Priority of 127 and MAC address of 00:90:69:31:55:91
316 Chapter 7 Intermediate System to Intermediate System (IS-IS)
13. What is the default priority value assigned to all IS-IS interfaces?
A. 0
B. 63
C. 64
D. 127
14. Which interface is primarily used for the assignment of the NET ID?
A. lo0
B. fxp0
C. fe-0/0/0.0
D. so-0/0/0.0
15. Given the following configuration:
protocols {
isis {
level 1 disable;
interface all;
interface fxp0.0 {
disable;
}
}
}
which statement is correct?
A. Only adjacencies on interface fxp0.0 will be established.
B. All operational interfaces will form only Level 1 adjacencies (except fxp0.0).
C. All operational interfaces will form only Level 2 adjacencies (except fxp0.0).
D. All operational interfaces will form both Level 1 and Level 2 adjacencies.
16. Which command allows a logical interface to accept and process IS-IS packets?
A. family inet
B. family iso
C. family isis
D. family clnp
Review Questions 317
17. Which command allows you to see the result of the SPF calculation before routes are sent to the
routing table?
A. show isis route
B. show isis adjacency
C. show isis database
D. show route protocol isis
18. You suspect that your neighbor may not be properly advertising its connected networks. Which
command best troubleshoots this problem?
A. show isis route
B. show isis database detail
C. show isis interface
D. show isis statistics
19. Which command displays the circuit IDs assigned by the local router as well as information
about elected DIS routers?
A. show isis route
B. show isis interface
C. show isis adjacency
D. show isis statistics
20. Which IS-IS command provides information about connected routers?
A. show isis adjacency
B. show isis spf log
C. show isis interface
D. show isis statistics
318 Chapter 7 Intermediate System to Intermediate System (IS-IS)
Answers to Review Questions
1. D. The presence of 49 in the Authority Format Indicator (AFI) position marks this NSAP
address as a private address.
2. B. All IS-IS routers, by definition, support a variable-length field between 1 and 8 bytes. The
JUNOS software implementation uses only a default value of 6.
3. A. The router node is always assigned a circuit ID of 0x00. This value is placed within the selector
byte of a NET ID. Point-to-point links share a value of 0x01, while broadcast links begin
their unique numbering at 0x02.
4. D. The Partial Sequence Number PDU (PSNP) is used during the adjacency formation process
when one of the routers determines its database is not synchronized.
5. B. The Hello PDU forms adjacencies with network neighbors at either Level 1 or Level 2. The
remaining functions are accomplished using a link-state PDU.
6. C. The Complete Sequence Number PDU (CSNP) is used to inform other IS-IS routers of the
contents of the local router’s database. This header information allows neighbors to determine
if they have a complete and updated set of data.
7. A. The TLV structure is the basis for all IS-IS LSP information. This encoding allows for easy
protocol scalability.
8. C. Only option C provides the same area address as that of the local router and a unique system
ID. These are the two requirements of forming a Level 1 adjacency.
9. C. When the local router sees itself in a neighbor’s link-state PDU, it understands that bidirectional
communication is achieved. This is a critical step before a fully functional adjacency is established.
10. D. The rejected state is seen when two routers have an authentication failure or an area mismatch.
11. A. The two possible criteria for DIS election are priority and MAC address. The first tiebreaker
is the highest system priority, followed by the highest MAC address.
12. D. The IS-IS router with the highest configured priority is always elected the DIS on the broadcast
segment.
13. C. The possible priority range is from 0 to 127; the JUNOS software default value is 64.
14. A. To ensure that the NET ID is always reachable, you should assign it to the loopback interface.
Options C and D are transit interfaces and susceptible to physical failure. The fxp0 interface is not
used because only network management traffic should use this interface.
15. C. The presence of the level 1 disable command at the global IS-IS level allows only Level 2
adjacencies to form.
16. B. The family iso command is the protocol family assigned to interfaces within the JUNOS
software.
Answers to Review Questions 319
17. A. To see routes after a SPF run, use the command show isis route. Option D shows the
routes after they are placed into the routing table.
18. B. A detailed examination of the database will always display the networks advertised by all
IS-IS routers.
19. B. Only show isis interface provides you with information about elected DIS routers and
circuit IDs on a per-interface basis.
20. A. Only show isis adjacency details information about other routers in the network. The
remaining commands display data about the local router only.
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