Figure 71 A simple internetwork with static routes

In this simple example, the networks' administrators have collaborated on a route redistribution scheme that they believe will minimize their workload as well as network traffic loads. The internetwork is relatively small, consisting of three different networks, one of which supports a stub network. Each network uses its own address space and a different dynamic routing protocol. Given the innate incompatibility of the three different routing protocols, the administrators chose not to redistribute routing information among their networks. Rather, they aggregated the routes into network numbers, and statically defined paths for them. Table 7-1 summarizes the routing tables of the three gateway routers. Router D connects a small, stub network to the other networks. As such, this router uses its serial port as a default gateway for all packets addressed to any IP address that does not belong to 192.168.126.

Note For more information on redistributing routing information among dissimilar routing protocols, refer to "Internetworking with Dissimilar Protocols."

Table 7-1: Statically Defined Routes

Router

Destination

Next Hop

A

172.16.0.0

B

A

192.168.0.0

C

B

10.0.0.0

A

B

192.168.0.0

C

C

10.0.0.0

A

C

172.16.0.0

B

C

192.168.126.0

D

In this scenario, Router A forwards all packets addressed to any hosts within the 172.16 network address space to Router B. Router A also forwards all packets addressed to hosts within network 192.168 to Router C. Router B forwards all packets addressed to any hosts within the 192.168 address space to Router C. Router B forwards packets addressed to hosts within Network 10 to Router

The Mechanics of Routing Protocols

A. Router C forwards all packets destined for Network 10 to Router A, those packets destined for 172.16 to Router B. Additionally, Router C forwards packets addressed to 192.168.126 to Router D, its stub network. This network is a stub because it is literally a dead end in the network. There is only one way in---and one way out. This small network depends completely on its link to Router C, and Router C itself, for connectivity to all the internetworked hosts.

In this example, a failure will result in unreachable destinations despite the fact that an alternative path is available for use. In Figure 7-2, the transmission facility between Gateway Routers A and C has failed.

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