Look at this example:
Ethernet 6 is up, line protocol is up Internet address is 192.192.15.1, subnet mask is 255.255.255.0 Broadcast address is 192.192.15.255 Address determined by non-volatile memory MTU is 1500 bytes Helper address is 192.192.12.5 Outgoing access list is not set Proxy ARP is enabled Security level is default Split horizon is enabled ICMP redirects are always sent ICMP unreachables are always sent ICMP mask replies are never sent IP autonomous switching is enabled IP autonomous switching on the same interface is disabled ^^^^^^^^^^^^^^^^^^^^^^^^^^^ Gateway Discovery is disabled IP accounting is disabled TCP/IP header compression is disabled Probe proxy name replies are disabled
If you enable fast or autonomous switching on an interface, packets coming from any other interface on the router are fast-switched (or autonomous-switched) to that interface. If you enable same-interface fast or autonomous switching, packets whose source and destination address are the same are fast or autonomous switched.
You can use same-interface fast or autonomous switching in cases where you have Frame Relay or Asynchronous Transfer Mode (ATM) WAN links configured as subinterfaces on the same main interface. Another situation is when you are using secondary networks on LAN interfaces, as during IP address migration. In order to enable same-interface fast switching, use the ip route-cache same-interface configuration command.