A router is running a classful routing protocol. Which command will enable this router to select a default route when routing to an unknown subnet of a network for which it knows the major network?
A. ip classless
B. no ip classless
D. no auto-summary
Correct Answer: A
The ip classless command causes a routing protocol to change its default behavior of discarding any traffic that is bound for unknown subnets of a known classful network. If the command is enabled, the router tries to match the most number of bits possible against the route in its routing table. Alternatively, the router will use the default route rather than dropping the packet.
For an example of this behavior, examine the diagram below. The ip route 0.0.0.0 0.0.0.0 serial 0/0 command has been issued on Router B. If the 188.8.131.52/24 network is unknown to Router B, then under normal circumstances, Router B would NOT use its configured default route. Instead, it would drop any packets addressed to that unknown network, because when a router knows a route to a major classful network or its subnets (in this case, 184.108.40.206/30 and 220.127.116.11/24), it will not use a statically configured default route to forward traffic to an unknown subnet of that network (in this case 18.104.22.168/24). In the scenario described in the diagram, Router B will drop the packet. However, if the ip classless command has been executed, it will use the default route and send the traffic to Router A.
The ip classless command is a global configuration mode command enabled by default in Cisco IOS version 12.0 and later. If the default route is learned from IS-IS or OSPF, as opposed to being statically configured as in the above example, the ip classless command is not necessary for the router to use the default route.
The no ip classless command on routers will disable the forwarding of packets destined to an unknown subnet of a known classful network. Therefore, it is an incorrect option.
The auto-summary command is used to allow automatic summarization of subnet routes into network-level routes. This is a command executed in router configuration mode.
Classless routing protocols such as Routing Information Protocol version 2 (RIPv2) and Enhanced Interior Gateway Routing Protocol (EIGRP) perform automatic route summarization at classful boundaries. The no auto-summary command is used to turn off this route summarization.
Configure, verify, and troubleshoot IPv4 and IPv6 static routing
Cisco > Articles > Cisco Certification > CCNP > CCNP Self-Study: Advanced IP Addressing
Cisco > Cisco IOS IP Addressing Services Command Reference > IP Addressing Commands > ip classless