|
Sometimes a router receives packets
destined for an unknown subnet of a network that has directly
connected subnets.
In order for the Cisco IOS software to forward these packets to the
best supernet route possible, use the
ip classless
global configuration command. A supernet route is a route that covers
a greater range of subnets with a single entry. For example, an
enterprise uses the entire subnet 10.10.0.0 /16, then a supernet route
for 10.10.10.0 /24 would be 10.10.0.0 /16. The
ip classless
command is enabled by default in Cisco IOS Software Release 11.3 and
later. To disable this feature, use the
no
form of this command. When this feature is disabled any
packets received that are destined for a subnet that numerically falls
within the router’s subnetwork addressing scheme will be discarded.
IP classless only affects the operation
of the forwarding processes in IOS. IP classless does not affect the
way the routing table is built. This is the essence of classful
routing. If one part of a major network is known, but the subnet
toward which the packet is destined within that major network is
unknown, the packet is dropped.
The most confusing aspect of this rule
is that the router only uses the default route if the major network
destination does not exist in the routing table. A router by default
assumes that all subnets of a directly connected network should be
present in the routing table. If a packet is received with an unknown
destination address within an unknown subnet of a directly attached
network, the router assumes that the subnet does not exist. So the
router will drop the packet even if there is a default route.
Configuring ip classless
on the router resolves this problem by allowing the router to ignore
the classful boundaries of the networks in its routing table and
simply route to the default route.
| |
Lab Activity
e-Lab Activity: Using Classless IP Routing
In this lab, the student will configure the router to use
classless routing when forwarding packets. 
|
|
|
Web Links