Chapter 38 Configuring IP Unicast Routing

Configuring IP Addressing

Classless Routing

By default, classless routing behavior is enabled on the switch when it is configured to route. With classless routing, if a router receives packets for a subnet of a network with no default route, the router forwards the packet to the best supernet route. A supernet consists of contiguous blocks of Class C address spaces used to simulate a single, larger address space and is designed to relieve the pressure on the rapidly depleting Class B address space.

In Figure 38-2, classless routing is enabled. When the host sends a packet to 120.20.4.1, instead of discarding the packet, the router forwards it to the best supernet route. If you disable classless routing and a router receives packets destined for a subnet of a network with no network default route, the router discards the packet.

Figure 38-2 IP Classless Routing

128.0.0.0/8

128.20.4.1

128.20.0.0

IP classless

128.20.1.0

128.20.2.0

128.20.3.0

128.20.4.1

Host

45749

In Figure 38-3, the router in network 128.20.0.0 is connected to subnets 128.20.1.0, 128.20.2.0, and

128.20.3.0.If the host sends a packet to 120.20.4.1, because there is no network default route, the router discards the packet.

Catalyst 3750-E and 3560-E Switch Software Configuration Guide

38-8

OL-9775-02

 

 

Page 868
Image 868
Cisco Systems 3750E manual Classless Routing, 38-8