Cisco Catalyst 9500 series Configuration Manual page 64

Configuring IP Unicast Routing
Classless Routing
In the figure, 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 3: IP Classless Routing
In the figure , 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.
Figure 4: No IP Classless Routing
To prevent the Device from forwarding packets destined for unrecognized subnets to the best supernet route
possible, you can disable classless routing behavior.
Routing Configuration Guide, Cisco IOS XE Everest 16.6.x (Catalyst 9500 Switches)
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