Case Study Discontiguous Subnets

In Figure 5.13, another router has been added to the internetwork with a subnet 10.33.32.0/20 on its El interface. The problem is that the other subnet of network 10.0.0.0, 10.33.0.0/20, is connected to Barney, and the only route between the subnets is via 192.168.83.0 and 192.168.12.0—two entirely different networks. As a result, network 10.0.0.0 is discontiguous.

Figure 5.13. Classful protocols such as RIP and IGRP cannot route a topology in which the subnets of network 10.0.0.0 are separated by different networks.

Figure 5.13. Classful protocols such as RIP and IGRP cannot route a topology in which the subnets of network 10.0.0.0 are separated by different networks.

Barney will consider itself a border router between network 10.0.0.0 and network 192.168.83.0; likewise, Ernest_T will consider itself a border router between 10.0.0.0 and 192.168.12.0. Both will advertise a summary route of 10.0.0.0, and as a result Andy will be fooled into thinking that it has two equal-cost paths to the same network. Andy will load share on the links to Barney and Ernest_T, and there is now only a 50-50 chance that packets to network 10.0.0.0 will reach the correct subnet.

The solution is to configure subnets of network 10.0.0.0 on the same links on which 192.168.83.0/24 and 192.168.12.192/27 reside. This is accomplished with secondary IP addresses, as follows:

Barney(config)#interface e0

Barney(config-if)#ip address 10.33.55.1 255.255.240.0 secondary

Andy(config)#interface e1

Andy(config-if)#ip address 10.33.55.2 255.255.240.0 secondary

Andy(config-if)#interface e2

Andy(config-if)#ip address 10.33.75.1 255.255.240.0 secondary

Andy(config-if)#router rip Andy(config-router)#network 10.0.0.0

Ernest_T(config)#interface e0

Ernest_T(config-if)#ip address 10.33.75.2 255.255.240.0 secondary

Because Andy did not previously have an interface on network 10.0.0.0, a network statement is added to the RIP process. The result of the configuration can be seen in Figure 5.14. The existing logical network structure remains in place, and a contiguous network 10.0.0.0 is "overlaid" onto it.

Figure 5.14. Secondary addresses are used to connect the subnets of network 10.0.0.0 across the same links on which other network addresses exist.

Figure 5.14. Secondary addresses are used to connect the subnets of network 10.0.0.0 across the same links on which other network addresses exist.

Figure 5.15 shows Ernest_T's routing table. Of interest here are the dual, equal-cost routes associated with next-hop addresses 10.33.75.1 and 192.168.12.195.

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