Note

Check both the destination and return path when a route fails.

Case Study: A Protocol Conflict

Figure 3.19 shows two routers connected by two Ethernet networks, one of which includes a bridge. This bridge handles traffic for several other links not shown and occasionally becomes congested. The host Milne is a mission-critical server; the network administrator is worried about traffic to Milne being delayed by the bridge, so a static host route has been added in Roo that will direct packets destined for Milne across the top Ethernet, avoiding the bridge.

Figure 3.19. A host route directs packets from Roo to Milne across the top Ethernet, avoiding the occasionally congested bridge.

Figure 3.19. A host route directs packets from Roo to Milne across the top Ethernet, avoiding the occasionally congested bridge.

This solution seems to be logical, but it isn't working. After the static route was added, packets routed through Roo no longer reach the server. Not only that, but packets routed through Kanga no longer reach the server, although no changes were made to that router.

The first step, as always, is to check the route table. Roo's route table (Figure 3.20) indicates that packets with a destination address of 172.16.20.75 will in fact be forwarded to Kanga's E1 interface, as desired. Kanga is directly connected to the destination network, so no more routing should be occurring; a quick check verifies that the Ethernet interfaces are functioning at both Kanga and at Milne.

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