The Routing Table: A Closer Look - Routing Protocols and Concepts - page 364

Is it possible to have a classful routing protocol configured yet a classless routing behavior
implemented? If yes, give an example. If no, explain why not.

Yes. The routing table could be populated with routes from a classful routing protocol like RIPv1, yet
implement classless routing behavior because the ip classless command is configured.

Explanation:

Building the routing table: The router learns about remote networks through RIPv1, a classful rout-
ing protocol. This means that the routing updates do not include a subnet mask and therefore cannot
support discontiguous networks or VLSM. The routing protocol is only used to populate the routing
table. The routing protocol has no effect on the routing table lookup process.

Lookup process: The router has been configured with classless routing behavior ( ip classless ). If
there is a match with the parent route, such as 172.16.0.0 (a 16-bit match), but not a match with any
of the child routes, the router will continue to search the routing table for a lesser match.

This could include the default route. If a lesser match is found, the router will forward the packet
using that router. Otherwise, it will drop the packet.

Determine the Route

Using Example 8-4, determine the route used to forward the packet based on the following scenarios.

Example 8-4 Routing Table for B3

B3> show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
<output omitted>
* - candidate default, U - per-user static route, o - ODR
P - periodic downloaded static route

Gateway of last resort is 10.10.10.9 to network 0.0.0.0

10.0.0.0/30 is subnetted, 3 subnets
R 10.10.10.0 [120/1] via 10.10.10.9, 00:00:22, Serial0/0/0
R 10.10.10.4 [120/1] via 10.10.10.9, 00:00:22, Serial0/0/0
C 10.10.10.8 is directly connected, Serial0/0/0
172.16.0.0/16 is variably subnetted, 8 subnets, 6 masks
R 172.16.0.0/18 [120/1] via 10.10.10.9, 00:00:22, Serial0/0/0
R 172.16.68.0/22 [120/2] via 10.10.10.9, 00:00:22, Serial0/0/0
R 172.16.72.0/23 [120/2] via 10.10.10.9, 00:00:22, Serial0/0/0
R 172.16.128.0/20 [120/1] via 10.10.10.9, 00:00:22, Serial0/0/0
R 172.16.160.0/21 [120/2] via 10.10.10.9, 00:00:22, Serial0/0/0
R 172.16.176.0/22 [120/2] via 10.10.10.9, 00:00:22, Serial0/0/0
C 172.16.188.0/23 is directly connected, FastEthernet0/0
C 172.16.190.0/24 is directly connected, FastEthernet0/1
R* 0.0.0.0/0 [120/1] via 10.10.10.9, 00:00:22, Serial0/0/0