IP Routing in a Larger Network
Figure 5.4 shows the three 2500 routers connected via a WAN and the 2621 router connected via the Ethernet network off 2500A. Each router also has an Ethernet network connected.
The first step is to configure each router with the correct configuration. Table 5.1 shows the IP address scheme I used to configure the network. After we go over how the network is configured, I will discuss how to configure IP routing. Each network in the following table has a 24-bit subnet mask (255.255.255.0).
Router configuration is a fairly simple process, since you just need to add IP addresses to your interfaces and then perform a no shutdown on the interfaces. It will get a tad bit more complex in the next section, but first, let’s configure the IP addresses in the network.
2621A Configuration
To configure the 2621 router, you just need to add an IP address to interface
FastEthernet 0/0. Configuring the hostnames of each router will make identification
easier. Here is how I did that:
Router>en
Router#config t
Router (config)#
hostname 2621A
2621A(Config)#
interface fa0/0
2621A(Config-if)#
ip address 172.16.10.1 255.255.255.0
2621A(Config-if)#
no shut
To view the IP routing tables created on a Cisco router, use the privileged mode command show ip route. The command output is shown as follows.
Notice that only the configured network is shown in the routing table. This means the router only knows how to get to network 172.16.10.0.
2621A# sh ip route
Codes: C - connected, S - static, I - IGRP, 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, E – EGP i - IS-IS, L1 - IS-IS level-
1, L2 - IS-IS level-2, * - candidate default U - per-user
static route, o - ODR, P - periodic downloaded static
route T - traffic engineered route
Gateway of last resort is not set
172.16.0.0/24 is subnetted, 1 subnets
C 172.16.10.0 is directly connected, FastEthernet0/0
2621A#
The preceding routing table shows the directly connected network 172.16.10.0. Notice the “C”; this 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 with their abbreviations. In the interest of brevity, the codes will be abbreviated in the rest of this chapter.
2501A Configuration
It is now time to configure the next router. To configure 2501A, two interfaces
need to be configured: Ethernet 0 and serial 0.
The preceding configuration configured serial 0 into network 172.16.20.0 and Ethernet 0 into network 172.16.10.0. The show ip route command displays the following:
Notice that router 2501A knows how to get to networks 172.16.10.0 and 172.16.20.0. Router 2621 and Router A can communicate because they are on the same LAN.
2501B Configuration
The configuration of 2501B is more of the same, except that you also need to add the clock rate command to the DCE interfaces connected to both serial interfaces. For more information on the DCE interfaces and the clock rate command.
The above configuration configured the hostname and IP addresses as well as the clock rate on the serial interfaces. The output of the following show ip route command displays the directly connected networks of 172.16.20.0, 172.16.30.0, and 172.16.40.0.
Router A and Router B can communicate because they are on the same WAN network. However, Router B cannot communicate with the 2621 router because it does not know about network 172.16.10.0. Router A can ping both the 2621 router and 2501B, but 2501B and 2621 cannot see each other.
2501C Configuration
The configuration of 2501C is the same as 2501A except that it has different network IDs.
Interface Ethernet 0 is configured to participate in the 172.16.50.0 network, and serial 0 is configured into the 172.16.40.0 WAN network. The output of the show ip route command, displayed below, shows the directly connected networks on router 2501C.
Router 2501C can communicate with 2501B since they are on the same WAN network. However, by default, 2501C cannot see any other router or network.
The first step is to configure each router with the correct configuration. Table 5.1 shows the IP address scheme I used to configure the network. After we go over how the network is configured, I will discuss how to configure IP routing. Each network in the following table has a 24-bit subnet mask (255.255.255.0).
Network Addressing for the IP Network
| Router | Network | Address | Interface |
| 2621A | 172.16.10.0 | f0/0 | 172.16.10.1 |
| 2501A | 172.16.10.0 | e0 | 172.16.10.2 |
| 2501A | 172.16.20.0 | s0 | 172.16.20.1 |
| 2501B | 172.16.20.0 | s0 | 172.16.20.2 |
| 2501B | 172.16.40.0 | s1 | 172.16.40.1 |
| 2501B | 172.16.30.0 | e0 | 172.16.30.1 |
| 2501C | 172.16.40.0 | s0 | 172.16.40.2 |
| 2501C | 172.16.50.0 | e0 | 172.16.50.1 |
Router configuration is a fairly simple process, since you just need to add IP addresses to your interfaces and then perform a no shutdown on the interfaces. It will get a tad bit more complex in the next section, but first, let’s configure the IP addresses in the network.
2621A Configuration
To configure the 2621 router, you just need to add an IP address to interface
FastEthernet 0/0. Configuring the hostnames of each router will make identification
easier. Here is how I did that:
Router>en
Router#config t
Router (config)#
hostname 2621A
2621A(Config)#
interface fa0/0
2621A(Config-if)#
ip address 172.16.10.1 255.255.255.0
2621A(Config-if)#
no shut
To view the IP routing tables created on a Cisco router, use the privileged mode command show ip route. The command output is shown as follows.
Notice that only the configured network is shown in the routing table. This means the router only knows how to get to network 172.16.10.0.
2621A# sh ip route
Codes: C - connected, S - static, I - IGRP, 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, E – EGP i - IS-IS, L1 - IS-IS level-
1, L2 - IS-IS level-2, * - candidate default U - per-user
static route, o - ODR, P - periodic downloaded static
route T - traffic engineered route
Gateway of last resort is not set
172.16.0.0/24 is subnetted, 1 subnets
C 172.16.10.0 is directly connected, FastEthernet0/0
2621A#
The preceding routing table shows the directly connected network 172.16.10.0. Notice the “C”; this 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 with their abbreviations. In the interest of brevity, the codes will be abbreviated in the rest of this chapter.
2501A Configuration
It is now time to configure the next router. To configure 2501A, two interfaces
need to be configured: Ethernet 0 and serial 0.
Router> en
Router# config t
Router(config)# hostname 2501A
2501A(config)#int e0
2501A(config-if)#ip address 172.16.10.2 255.255.255.0
2501A(config-if)#no shut
2501A(config-if)#int s0
2501A(config-if)#ip address 172.16.20.1 255.255.255.0
2501A(config-if)#no shut
Router# config t
Router(config)# hostname 2501A
2501A(config)#int e0
2501A(config-if)#ip address 172.16.10.2 255.255.255.0
2501A(config-if)#no shut
2501A(config-if)#int s0
2501A(config-if)#ip address 172.16.20.1 255.255.255.0
2501A(config-if)#no shut
The preceding configuration configured serial 0 into network 172.16.20.0 and Ethernet 0 into network 172.16.10.0. The show ip route command displays the following:
2501A#sh ip route
Codes: C - connected, S - static, I - IGRP, R - RIP,
M – [output cut]
Gateway of last resort is not set
172.16.0.0/24 is subnetted, 2 subnets
C 172.16.20.0 is directly connected, Serial0
C 172.16.10.0 is directly connected, Ethernet0
2501A#
Codes: C - connected, S - static, I - IGRP, R - RIP,
M – [output cut]
Gateway of last resort is not set
172.16.0.0/24 is subnetted, 2 subnets
C 172.16.20.0 is directly connected, Serial0
C 172.16.10.0 is directly connected, Ethernet0
2501A#
Notice that router 2501A knows how to get to networks 172.16.10.0 and 172.16.20.0. Router 2621 and Router A can communicate because they are on the same LAN.
2501B Configuration
The configuration of 2501B is more of the same, except that you also need to add the clock rate command to the DCE interfaces connected to both serial interfaces. For more information on the DCE interfaces and the clock rate command.
Router>en
Router#config t
Router(config)#hostname 2501B
2501B(config)#int e0
2501B(config-if)#ip address 172.16.30.1 255.255.255.0
2501B(config-if)#no shut
2501B(config-if)#int s0
2501B(config-if)#ip address 172.16.20.2 255.255.255.0
2501B(config-if)#clock rate 64000
2501B(config-if)#no shut
2501B(config-if)#int s1
2501B(config-if)#ip address 172.16.40.1 255.255.255.0
2501B(config-if)#clock rate 64000
2501B(config-if)#no shut
Router#config t
Router(config)#hostname 2501B
2501B(config)#int e0
2501B(config-if)#ip address 172.16.30.1 255.255.255.0
2501B(config-if)#no shut
2501B(config-if)#int s0
2501B(config-if)#ip address 172.16.20.2 255.255.255.0
2501B(config-if)#clock rate 64000
2501B(config-if)#no shut
2501B(config-if)#int s1
2501B(config-if)#ip address 172.16.40.1 255.255.255.0
2501B(config-if)#clock rate 64000
2501B(config-if)#no shut
The above configuration configured the hostname and IP addresses as well as the clock rate on the serial interfaces. The output of the following show ip route command displays the directly connected networks of 172.16.20.0, 172.16.30.0, and 172.16.40.0.
2501B#sh ip route
Codes: C - connected, S - static, I - IGRP, R - RIP,
M – [output cut]
Gateway of last resort is not set
172.16.0.0/24 is subnetted, 3 subnets
C 172.16.40.0 is directly connected, Serial1
C 172.16.30.0 is directly connected, Ethernet0
C 172.16.20.0 is directly connected, Serial0
2501B#
Codes: C - connected, S - static, I - IGRP, R - RIP,
M – [output cut]
Gateway of last resort is not set
172.16.0.0/24 is subnetted, 3 subnets
C 172.16.40.0 is directly connected, Serial1
C 172.16.30.0 is directly connected, Ethernet0
C 172.16.20.0 is directly connected, Serial0
2501B#
Router A and Router B can communicate because they are on the same WAN network. However, Router B cannot communicate with the 2621 router because it does not know about network 172.16.10.0. Router A can ping both the 2621 router and 2501B, but 2501B and 2621 cannot see each other.
2501C Configuration
The configuration of 2501C is the same as 2501A except that it has different network IDs.
Router>en
Router#config t
Router(config)#hostname 2501C
2501C(config)#int e0
2501C(config-if)#ip address 172.16.50.1 255.255.255.0
2501C(config-if)#no shut
2501C(config-if)#int s0
2501C(config-if)#ip address 172.16.40.2 255.255.255.0
2501C(config-if)#no shut.
Router#config t
Router(config)#hostname 2501C
2501C(config)#int e0
2501C(config-if)#ip address 172.16.50.1 255.255.255.0
2501C(config-if)#no shut
2501C(config-if)#int s0
2501C(config-if)#ip address 172.16.40.2 255.255.255.0
2501C(config-if)#no shut.
Interface Ethernet 0 is configured to participate in the 172.16.50.0 network, and serial 0 is configured into the 172.16.40.0 WAN network. The output of the show ip route command, displayed below, shows the directly connected networks on router 2501C.
2501C#sh ip route
Codes: C - connected, S - static, I - IGRP, R - RIP,
M – [output cut]
Gateway of last resort is not set
172.16.0.0/24 is subnetted, 2 subnets
C 172.16.50.0 is directly connected, Ethernet0
C 172.16.40.0 is directly connected, Serial0
2501C#
Codes: C - connected, S - static, I - IGRP, R - RIP,
M – [output cut]
Gateway of last resort is not set
172.16.0.0/24 is subnetted, 2 subnets
C 172.16.50.0 is directly connected, Ethernet0
C 172.16.40.0 is directly connected, Serial0
2501C#
Router 2501C can communicate with 2501B since they are on the same WAN network. However, by default, 2501C cannot see any other router or network.
