CCNA6-9

386 Chapter 6  IP Routing
From the routing screen, I clicked the Edit button to the right of Dynamic Routing. I then
was able to configure RIP and the network number and then clicked on the interfaces I didn’t
want RIP to be broadcast out. The interfaces that RIP will broadcast out will be unchecked.
These are called passive interfaces and we’ll talk about it more in a minute. No reason to
broadcast RIP out an interface where no routers will be.
Routing Information Protocol (RIP) 387
From the SDM screen, we can see that we’re done with R3.
871W
Here is the last router’s RIP configuration:
871W#config t
871W(config)#no ip route 0.0.0.0 0.0.0.0 10.1.11.1
871W(config)#router rip
871W(config-router)#network 10.0.0.0
871W(config-router)#do sh ip route
10.0.0.0/24 is subnetted, 12 subnets
C 10.1.11.0 is directly connected, Vlan1
R 10.1.10.0 [120/1] via 10.1.11.1, 00:00:23, Vlan1
R 10.1.9.0 [120/3] via 10.1.11.1, 00:00:23, Vlan1
R 10.1.8.0 [120/3] via 10.1.11.1, 00:00:23, Vlan1
C 10.1.12.0 is directly connected, Dot11Radio0
R 10.1.3.0 [120/2] via 10.1.11.1, 00:00:23, Vlan1
R 10.1.2.0 [120/2] via 10.1.11.1, 00:00:23, Vlan1
R 10.1.1.0 [120/2] via 10.1.11.1, 00:00:23, Vlan1
R 10.1.7.0 [120/3] via 10.1.11.1, 00:00:24, Vlan1
R 10.1.6.0 [120/3] via 10.1.11.1, 00:00:24, Vlan1
R 10.1.5.0 [120/1] via 10.1.11.1, 00:00:24, Vlan1
R 10.1.4.0 [120/2] via 10.1.11.1, 00:00:24, Vlan1
871W#
Finally, all routes showing in the routing table are RIP injected routes.
It’s important to remember administrative distances and why we needed to either remove
the static routes before we added RIP routing or set them higher than 120 as we did.
By default, directly connected routes have an administrative distance of 0, static routes have
an administrative distance of 1, and RIP has an administrative distance of 120. I call RIP the
“gossip protocol” because it reminds me of junior high school, where if you hear a rumor
(advertised route), it just has to be true without exception. And that pretty much sums up how
RIP behaves on an internetwork—rumor mill as protocol!
Verifying the RIP Routing Tables
Each routing table should now have all directly connected routes as well as RIP-injected routes
received from neighboring routers.
This output shows us the contents of the Corp routing table:
10.0.0.0/24 is subnetted, 12 subnets
R 10.1.11.0 [120/1] via 10.1.5.2, 00:00:28, Serial0/2/0
R 10.1.10.0 [120/1] via 10.1.5.2, 00:00:28, Serial0/2/0
388 Chapter 6  IP Routing
R 10.1.9.0 [120/1] via 10.1.4.2, 00:00:26, Serial0/1/0
R 10.1.8.0 [120/1] via 10.1.4.2, 00:00:26, Serial0/1/0
R 10.1.12.0 [120/2] via 10.1.5.2, 00:00:28, Serial0/2/0
C 10.1.3.0 is directly connected, Serial0/0/1
C 10.1.2.0 is directly connected, Serial0/0/0
C 10.1.1.0 is directly connected, FastEthernet0/1
R 10.1.7.0 [120/1] via 10.1.3.2, 00:00:07, Serial0/0/1
[120/1] via 10.1.2.2, 00:00:10, Serial0/0/0
R 10.1.6.0 [120/1] via 10.1.3.2, 00:00:07, Serial0/0/1
[120/1] via 10.1.2.2, 00:00:10, Serial0/0/0
C 10.1.5.0 is directly connected, Serial0/2/0
C 10.1.4.0 is directly connected, Serial0/1/0
This output shows us that the routing table has the same entries that it had when we were
using static routes—except for that R. The R means that the networks were added dynamically
using the RIP routing protocol. The [120/1] is the administrative distance of the route (120)
along with the number of hops to that remote network (1). From the Corp router, all networks
are one hop away except network 10.1.12.0, which is two hops away.
So while yes, it’s true that RIP has worked in our little internetwork, it’s not the solution
for every enterprise. That’s because this technique has a maximum hop count of only 15 (16
is deemed unreachable). Plus, it performs full routing-table updates every 30 seconds, which
would bring a larger internetwork to a painful crawl pretty quick!
There’s one more thing I want to show you about RIP routing tables and the parameters
used to advertise remote networks. Notice, as an example, that the following routing table
shows [120/15] in the 10.1.3.0 network metric. This means that the administrative distance
is 120, the default for RIP, but the hop count is 15. Remember that each time a router sends
out an update to a neighbor router, it increments the hop count by one for each route.
R3#sh ip route
10.0.0.0/24 is subnetted, 12 subnets
C 10.1.11.0 is directly connected, FastEthernet0/1
C 10.1.10.0 is directly connected, FastEthernet0/0
R 10.1.9.0 [120/2] via 10.1.5.1, 00:00:15, Serial0/0/1
R 10.1.8.0 [120/2] via 10.1.5.1, 00:00:15, Serial0/0/1
R 10.1.12.0 [120/1] via 10.1.11.2, 00:00:00, FastEthernet0/1
R 10.1.3.0 [120/15] via 10.1.5.1, 00:00:15, Serial0/0/1
R 10.1.2.0 [120/1] via 10.1.5.1, 00:00:15, Serial0/0/1
R 10.1.1.0 [120/1] via 10.1.5.1, 00:00:15, Serial0/0/1
R 10.1.7.0 [120/2] via 10.1.5.1, 00:00:15, Serial0/0/1
R 10.1.6.0 [120/2] via 10.1.5.1, 00:00:15, Serial0/0/1
C 10.1.5.0 is directly connected, Serial0/0/1
R 10.1.4.0 [120/1] via 10.1.5.1, 00:00:15, Serial0/0/1
R3#
Routing Information Protocol (RIP) 389
So this [120/15] is really bad because the next router that receives the table from router
R3 will just discard the route to network 10.1.3.0 since the hop count would then be 16, which
is invalid.
If a router receives a routing update that contains a higher-cost path to a network
that’s already in its routing table, the update will be ignored.
Configuring RIP Routing Example 2
Before we move onto learning more about RIP configurations, let’s take a look at Figure 6.16.
In this example, we first will find and implement our subnets and then add the RIP configuration
to the router.
FIGURE 6 . 1 6 RIP routing example 2
For this configuration, we are going to consider that the Lab_B and Lab_C routers are
already configured and we just need to configure the Lab_A router. We will use the network
ID of 192.168.164.0/28. The s0/0 interface of Lab_A will use the last available IP address in
the eighth subnet and the fa0/0 will use the last available IP address in the second subnet. Do
not consider the zero subnet valid.
Before we start, you do know that /28 is a 255.255.255.240 mask, right? And that we have
a block size of 16 in the fourth octet? It is very important that you know this, and if you need
another review of Chapters 2 and 3, that’s okay! Reviewing subnetting will never hurt you.
Since we have a block size of 16, our subnets are 16 (remember we are not starting at zero
for this example), 32, 48, 64, 80, 96, 112, 128, 144, etc. The eighth subnet (which we will use
for the s0/0 interface) is subnet 128. The valid host range for the 128 subnet is 129 through
142, and 143 is the broadcast address of the 128 subnet. The second subnet (which we will use
for the fa0/0 interface) is the 32 subnet. The valid hosts are 33 through 46, and 47 is the broadcast
address of the 32 subnet.
So, here is what our configuration on the Lab_A router will look like:
Lab_A(config)#interface s0/0
Lab_A(config-if)#ip address 192.168.164.142 255.255.255.240
Lab_A(config-if)#no shutdown
Lab_A Lab_B Lab_C
fa0/0 fa0/0
s0/0
(DCE) s0/0
s0/1
(DCE)
s0/0
390 Chapter 6  IP Routing
Lab_A(config-if)#interface fa0/0
Lab_A(config-if)#ip address 192.168.164.46 255.255.255.240
Lab_A(config-if)#no shutdown
Lab_A(config-if)#router rip
Lab_A(config-router)#network 192.168.164.0
Lab_A(config-router)#^Z
Lab_A#
Finding the subnets and configuring the last valid host should be pretty straightforward. If
not, head back to Chapter 3. However, what I really want you to notice is that although we
added two subnets to the Lab_A router, we only had one network statement under RIP. Sometimes
it is hard to remember that you configure only the classful network statement, which
means you turn all host bits off.
This was the real purpose of this second RIP configuration example—to remind you of
classful network addressing. And it never hurts to practice subnetting, right?
Holding Down RIP Propagations
You probably don’t want your RIP network advertised everywhere on your LAN and WAN.
There’s not a whole lot to be gained by advertising your RIP network to the Internet, now, is there?
There’s a few different ways to stop unwanted RIP updates from propagating across your
LANs and WANs, and the easiest one is through the passive-interface command that I
showed you during the R3 configuration. This command prevents RIP update broadcasts
from being sent out a specified interface, yet that same interface can still receive RIP updates.
Here’s an example of how to configure a passive-interface on a router using the CLI:
Lab_A#config t
Lab_A(config)#router rip
Lab_A(config-router)#network 192.168.10.0
Lab_A(config-router)#passive-interface serial 0/0
This command will stop RIP updates from being propagated out serial interface 0/0, but
serial interface 0/0 can still receive RIP updates. This is easily done within the SDM configuration
as well, as I demonstrated with the R3 router.
RIP Version 2 (RIPv2)
Let’s spend a couple of minutes discussing RIPv2 before we move into the distance-vector,
Cisco-proprietary routing protocol IGRP.
RIP version 2 is mostly the same as RIP version 1. Both RIPv1 and RIPv2 are distance-vector
protocols, which means that each router running RIP sends its complete routing tables out all
active interfaces at periodic time intervals. Also, the timers and loop-avoidance schemes are the
same in both RIP versions (i.e., holddown timers and split horizon rule). Both RIPv1 and RIPv2
are configured as classful addressing (but RIPv2 is considered classless because subnet information
is sent with each route update), and both have the same administrative distance (120).
Routing Information Protocol (RIP) 391
But there are some important differences that make RIPv2 more scalable than RIPv1. And
I’ve got to add a word of advice here before we move on; I’m definitely not advocating using
RIP of either version in your network. But since RIP is an open standard, you can use RIP with
any brand of router. You can also use OSPF (discussed in Chapter 7) since OSPF is an open
standard as well. RIP just requires too much bandwidth, making it pretty intensive to use in
your network. Why go there when you have other, more elegant options?
Table 6.3 discusses the differences between RIPv1 and RIPv2.
Should We Really Use RIP in an Internetwork?
You have been hired as a consultant to install a couple of Cisco routers into a growing network.
They have a couple of old Unix routers that they want to keep in the network. These
routers do not support any routing protocol except RIP. I guess this means you just have to
run RIP on the entire network.
Well, yes and no. You can run RIP on a router connecting that old network, but you certainly
don’t need to run RIP throughout the whole internetwork!
You can do what is called redistribution, which is basically translating from one type of routing
protocol to another. This means that you can support those old routers using RIP but use
Enhanced IGRP, for example, on the rest of your network.
This will stop RIP routes from being sent all over the internetwork and eating up all that
precious bandwidth.
TABLE 6 . 3 RIPv1 vs. RIPv2
RIPv1 RIPv2
Distance vector Distance vector
Maximum hop count of 15 Maximum hop count of 15
Classful Classless
Broadcast based Uses multicast 224.0.0.9
No support for VLSM Supports VLSM networks
No authentication Allows for MD5 authentication
No support for discontiguous networks Supports discontiguous networks