The Routing Table: A Closer Look - Routing Protocols and Concepts - page 363

Routing Behavior

What happens when there is a match between the packet's destination IP address and a level 1 parent
route, but there is not a match with any of the level 2 child routes? You might assume that the routing
table lookup process continues looking for a less specific match in the routing table. However, you
will see that this might or might not be the case, depending on the configuration of the router.

Classful and Classless Routing Behavior

Routing behavior influences the process of searching for the preferred route using the no ip classless
or ip classless commands. Classless and classful routing behaviors are not the same as classless and
classful routing protocols. Classful and classless routing protocols affect how the routing table is pop-
ulated . Classful and classless routing behaviors determine how the routing table is searched after it is
populated .

In Table 8-3, the routing protocols are listed. Indicate which are classful and which are classless.

Table 8-3 Classful and Classless Routing Protocols

Routing Protocol Classful or Classless?

RIPv1 Classful

RIPv2 Classless

IGRP Classful

EIGRP Classless

OSPF Classless

IS-IS Classless

In contrast to classful and classless routing protocols, what determines whether the router operates
with classful or classless routing behavior?

The commands no ip classless and ip classless

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

Yes. The routing table could be populated with routes from a classless routing protocol like RIPv2, yet
implement classful routing behavior because the no ip classless command is configured.

Explanation:

Building the routing table: The router learns about remote networks through RIPv2, a classless rout-
ing protocol. This means that the routing updates included a subnet mask and therefore support dis-
contiguous networks and 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 classful routing behavior ( no 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 drop the packet. Even if a lesser match exists in the routing table
(less than the 16 bits of the parent route) such as a default route, that route will not be used.