The router configuration is really a pretty straightforward process since you just need to add
IP addresses to your interfaces and then perform a no shutdown on those same interfaces. It gets
a tad more complex later on, but for right now, let’s configure the IP addresses in the network.
Corp Configuration
We need to configure five interfaces to configure the Corp router. And configuring the hostnames
of each router will make identification much easier. While we’re at it, why not set the
interface descriptions, banner, and router passwords too? It’s a really good idea to make a
habit of configuring these commands on every router.
To get started, I performed an erase startup-config on the router and reloaded, so we’ll
start in setup mode. I choose no to entering setup mode, which will get us straight to the username
prompt of the console. I’m going to configure all my routers this way except for R3,
which I’ll configure using the SDM just for fun. You can configure your routers either way.
Here’s how I did all that:
--- System Configuration Dialog ---
Would you like to enter the initial configuration dialog? [yes/no]: n
[output cut]
Press RETURN to get started!
Router>en
R3
R3 10.1.5.0 S0/0/0/ (DCE) 10.1.5.2
R3 10.1.10.0 F0/0 10.1.10.1
R3 10.1.11.0 F0/1 10.1.11.1
871W
871W 10.1.11.0 Vlan 1 10.1.11.2
871W 10.1.12.0 Dot11radio0 10.1.12.1
1242 AP
1242 AP 10.1.1.0 BVI 1 10.1.1.2
TABLE 6 . 1 Network Addressing for the IP Network (continued)
Router Network Address Interface Address
344 Chapter 6 IP Routing
Router#config t
Router(config)#hostname Corp
Corp(config)#enable secret todd
Corp(config)#interface fastEthernet 0/1
Corp(config-if)#ip address 10.1.1.1 255.255.255.0
Corp(config-if)#description Connection to 1242 AP
Corp(config-if)#no shutdown
Corp(config-if)#int s0/0/0
Corp(config-if)#ip address 10.1.2.1 255.255.255.0
Corp(config-if)#description 1st Connection to R1
Corp(config-if)#no shut
Corp(config-if)#int s0/0/1
Corp(config-if)#ip address 10.1.3.1 255.255.255.0
Corp(config-if)#description 2nd Connection to R1
Corp(config-if)#no shut
Corp(config-if)#int s0/1/0
Corp(config-if)#ip address 10.1.4.1 255.255.255.0
Corp(config-if)#description Connection to R2
Corp(config-if)#no shut
Corp(config-if)#int s0/2/0
Corp(config-if)#ip address 10.1.5.1 255.255.255.0
Corp(config-if)#description Connection to R3
Corp(config-if)#no shut
Corp(config-if)#line con 0
Corp(config-line)#password console
Corp(config-line)#login
Corp(config-line)#logging synchronous
Corp(config-line)#exec-timeout 0 0
Corp(config-line)#line aux 0
Corp(config-line)#password aux
Corp(config-line)#login
Corp(config-line)#exit
Corp(config)#line vty 0 ?
<1-1180> Last Line number
Corp(config)#line vty 0 1180
Corp(config-line)#password telnet
Corp(config-line)#login
Corp(config-line)#exit
Corp(config)#no ip domain-lookup
The IP Routing Process 345
Corp(config)#banner motd # This is my Corp 2811 ISR Router #
Corp(config-if)#^Z
Corp#copy running-config startup-config
Destination filename [startup-config]?[enter]
Building configuration...
[OK]
Corp#
If you have a hard time understanding this configuration process, refer back
to Chapter 4, “Cisco’s Internetworking Operating System (IOS) and Security
Device Manager (SDM).”
To view the IP routing tables created on a Cisco router, use the command show ip route.
The command output is shown as follows:
Corp#sh ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS
level-2, ia - IS-IS inter area, * - candidate default, U - per-user
static route, o - ODR, P - periodic downloaded static route
Gateway of last resort is not set
10.0.0.0/24 is subnetted, 1 subnets
C 10.1.1.0 is directly connected, FastEthernet0/1
Corp#
It’s important to remember that only configured, directly connected networks are going to
show up in the routing table. So why is it that I only see the FastEthernet0/1 interface in the
routing table? No worries—that’s just because you won’t see the serial interfaces come up
until the other side of the serial links is operational. As soon as we configure our R1, R2, and
R3 routers, all those interfaces should pop right up.
But did you notice the C on the left side of the output of the routing table? When you see
that there, it means that the network is directly connected. The codes for each type of connection
are listed at the top of the show ip route command, along with their abbreviations.
In the interest of brevity, the codes will be cut in the rest of this chapter.
346 Chapter 6 IP Routing
The Corp serial interface 0/0/1 is a DCE connection, which means that we need to add the
clock rate command to the interface. Remember that you don’t need to use the clock rate
command in production. Even though this is very true, it’s still imperative that you know how/
when you can use it and that you understand it really well when studying for your CCNA exam!
We can see our clocking with the show controllers command:
Corp#sh controllers s0/0/1
Interface Serial0/0/1
Hardware is GT96K
DCE V.35, clock rate 2000000
One last thing before we get into configuring the Remote routers: Did you notice the clock
rate is 2000000 under the s0/0/1 interface of the Corp router? That’s important because if you
think back to when we were configuring the Corp router, you’ll recall that I didn’t set the clock
rate. The reason I didn’t is because ISR routers will auto-detect a DCE-type cable and automatically
configure the clock rate—a really sweet feature!
R1 Configuration
Now we’re ready to configure the next router—R1. To make that happen correctly, keep in
mind that we have four interfaces to deal with: serial 0/0/0, serial 0/0/1, FastEthernet 0/0, and
FastEthernet 0/1. So let’s make sure we don’t forget to add the hostname, passwords, interface
descriptions, and banner to the router configuration. As I did with the Corp router, I erased
the configuration and reloaded.
Here’s the configuration I used:
R1#erase start
% Incomplete command.
R1#erase startup-config
Erasing the nvram filesystem will remove all configuration files!
Continue? [confirm][enter]
[OK]
Erase of nvram: complete
R1#reload
Proceed with reload? [confirm][enter]
[output cut]
%Error opening tftp://255.255.255.255/network-confg (Timed out)
%Error opening tftp://255.255.255.255/cisconet.cfg (Timed out)
--- System Configuration Dialog ---
Would you like to enter the initial configuration dialog? [yes/no]: n
The IP Routing Process 347
Before we move on, I really want to discuss the above output with you. First, notice that the
new 12.4 ISR routers will no longer take the command erase start. The router has only one
command after erase that starts with s, as shown here:
Router#erase s?
startup-config
I know, you’d think that the IOS would continue to accept the command, but nope—sorry!
The second thing I want to point out is that the output tells us the router is looking for a TFTP
host to see if it can download a configuration. When that fails, it goes straight into setup mode.
This gives you a great picture of the Cisco router default boot sequence we talked about in
Chapter 5.
Okay, let’s get back to configuring our router:
Press RETURN to get started!
Router>en
Router#config t
Router(config)#hostname R1
R1(config)#enable secret todd
R1(config)#int s0/0/0
R1(config-if)#ip address 10.1.2.2 255.255.255.0
R1(config-if)#Description 1st Connection to Corp Router
R1(config-if)#no shut
R1(config-if)#int s0/0/1
R1(config-if)#ip address 10.1.3.2 255.255.255.0
R1(config-if)#no shut
R1(config-if)#description 2nd connection to Corp Router
R1(config-if)#int f0/0
R1(config-if)#ip address 10.1.6.1 255.255.255.0
R1(config-if)#description Connection to HostA
R1(config-if)#no shut
R1(config-if)#int f0/1
R1(config-if)#ip address 10.1.7.1 255.255.255.0
R1(config-if)#description Connection to HostB
R1(config-if)#no shut
R1(config-if)#line con 0
R1(config-line)#password console
R1(config-line)#login
R1(config-line)#logging synchronous
R1(config-line)#exec-timeout 0 0
R1(config-line)#line aux 0
R1(config-line)#password aux
348 Chapter 6 IP Routing
R1(config-line)#login
R1(config-line)#exit
R1(config)#line vty 0 ?
<1-807> Last Line number
R1(config)#line vty 0 807
R1(config-line)#password telnet
R1(config-line)#login
R1(config-line)#banner motd # This is my R1 ISR Router #
R1(config)#no ip domain-lookup
R1(config)#exit
R1#copy run start
Destination filename [startup-config]?[enter]
Building configuration...
[OK]
R1#
Let’s take a look at our configuration of the interfaces.
R1#sh run | begin interface
interface FastEthernet0/0
description Connection to HostA
ip address 10.1.6.1 255.255.255.0
duplex auto
speed auto
!
interface FastEthernet0/1
description Connection to HostB
ip address 10.1.7.1 255.255.255.0
duplex auto
speed auto
!
interface Serial0/0/0
description 1st Connection to Corp Router
ip address 10.1.2.2 255.255.255.0
!
interface Serial0/0/1
description 2nd connection to Corp Router
ip address 10.1.3.2 255.255.255.0
!
The IP Routing Process 349
The show ip route command displays the following:
R1#show ip route
10.0.0.0/24 is subnetted, 4 subnets
C 10.1.3.0 is directly connected, Serial0/0/1
C 10.1.2.0 is directly connected, Serial0/0/0
C 10.1.7.0 is directly connected, FastEthernet0/1
C 10.1.6.0 is directly connected, FastEthernet0/0
R1#
Notice that router R1 knows how to get to networks 10.1.3.0, 10.1.2.0, 10.1.7.0, and
10.1.6.0. We can now ping to the Corp router from R1:
R1#10.1.2.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.1.2.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/4 ms
R1#
Now let’s go back to the Corp router and look at the routing table:
Corp#sh ip route
[output cut]
10.0.0.0/24 is subnetted, 4 subnets
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
Corp#
Since the serial links are up—remember, DCE is now detected automatically with ISR routers
and the clock rate is automatically added to the interface configuration—we can now see all
three. And once we configure R2 and R3, we’ll see two more networks in the routing table of the
Corp router. The Corp router can’t see either the 10.1.6.0 or 10.1.7.0 networks because we
don’t have any routing configured yet.
R2 Configuration
To configure R2, we’re going to do pretty much the same thing we did with the other two routers.
There are three interfaces: serial 0/2/0, FastEthernet 0/0/0, and Dot11radio 0/3/0 to deal with, and
again, we’ll be sure to add the hostname, passwords, interface descriptions, and a banner to the
router configuration:
Router>en
Router#config t
Router(config)#hostname R2
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