which it knows the major network?

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

C.
auto-summary

D.
no auto-summary

Explanation:
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 25.1.6.0/24 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, 25.1.5.0/30 and 25.1.1.0/24), it will not use a statically configured default route to forward
traffic to an unknown subnet of that network (in this case 25.1.6.0/24). 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.
Objective:
Routing Fundamentals
Sub-Objective:
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