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ip route 10.1.30.0 255.255.255.0 10.1.10.2 ip route 10.1.10.0 255.255.255.0 192.168.1.194 ip route 192.168.1.192 255.255.255.224 192.168.1.

If Pooh needs to send a packet to host 10.1.30.25, it will look into its route table and find that the subnet is reachable via 10.1.10.2. Because that address is not on a directly connected network, Pooh must again consult the table to find that network 10.1.10.0 is reachable via 192.168.1.194. That subnet is also not directly connected, so a third table lookup is called for. Pooh will find that 192.168.1.192 is reachable via 192.168.1.66, which is on a directly connected subnet. The packet can now be forwarded.

Because each table lookup costs processor time, under normal circumstances forcing a router to perform multiple lookups is a poor design decision. Fast switching significantly reduces these adverse effects by limiting the recursive lookups to the first packet to each destination, but a justification should still be identified before using such a design.

Figure 3.12 shows an example of an instance in which recursive lookups may be useful. Here, Sanderz reaches all networks via Heffalump. However, the network administrator plans to eliminate Heffalump and repoint all of Sanderz's routes through Woozle. The first 12 entries point not to Heffalump, but to the appropriate router attached to the 10.87.14.0 subnet. The last entry specifies that the 10.87.14.0 subnet is reached via Heffalump.

Figure 3.12. Configuring Sanderz for recursive lookups enables the network administrator to redirect all of that router's exit traffic from Heffalump to Woozle by changing one route entry.

Figure 3.12. Configuring Sanderz for recursive lookups enables the network administrator to redirect all of that router's exit traffic from Heffalump to Woozle by changing one route entry.

With this configuration, all of Sanderz's entries can be repointed through Woozle simply by changing the last static entry:

Sanderz(config)# ip route 10.87.14.0 255.255.255.0 10.23.5.95 Sanderz(config)# no ip route 10.87.14.0 255.255.255.0 10.23.5.20

Had all the static routes referenced 10.23.5.20 as the next-hop address, it would have been necessary to delete all 13 lines and type 13 new lines. Nevertheless, the effort saved in retyping static routes must be weighed carefully against the extra processing burden that recursive lookups put on the router.

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