Cisco WS-C3550-12G Software Configuration Manual page 539

Chapter 22 Configuring IP Unicast Routing
In Figure
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 22-2 IP Classless Routing
128.20.1.0
In Figure
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.
Figure 22-3 No IP Classless Routing
128.20.0.0
128.20.1.0
To prevent the switch from forwarding packets destined for unrecognized subnets to the best supernet
route possible, you can disable classless routing behavior.
78-11194-03
22-2, classless routing is enabled. When the host sends a packet to 120.20.4.1, instead of
128.0.0.0/8
128.20.4.1
IP classless
128.20.0.0
128.20.2.0
128.20.4.1
22-3, the router in network 128.20.0.0 is connected to subnets 128.20.1.0, 128.20.2.0, and
128.0.0.0/8
128.20.2.0
128.20.4.1
128.20.3.0
Host
128.20.4.1
Bit bucket
128.20.3.0
Host
Catalyst 3550 Multilayer Switch Software Configuration Guide
Configuring IP Addressing
22-9