a question about the entries highlighted in show ip interface <>
R1#sh ip int s0/0
Serial0/0 is up, line protocol is up
Internet address is 10.0.0.1/30
Broadcast address is 255.255.255.255
Address determined by non-volatile memory
MTU is 1500 bytes
Helper address is not set
Directed broadcast forwarding is disabled
Multicast reserved groups joined: 188.8.131.52
Outgoing access list is not set
Inbound access list is not set
Proxy ARP is enabled
Local Proxy ARP is disabled
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 fast switching is enabled <----------------------------------------------------------------------------
IP fast switching on the same interface is enabled <-------------------------------------------
IP Flow switching is disabled
IP CEF switching is enabled
IP CEF Fast switching turbo vector
IP multicast fast switching is enabled
IP multicast distributed fast switching is disabled
IP route-cache flags are Fast, CEF
Router Discovery is disabled
IP output packet accounting is disabled
IP access violation accounting is disabled
TCP/IP header compression is disabled
RTP/IP header compression is disabled
Policy routing is disabled
Network address translation is disabled
BGP Policy Mapping is disabled
WCCP Redirect outbound is disabled
WCCP Redirect inbound is disabled
WCCP Redirect exclude is disabled
What is the difference between them ?
These two things are related to the way that Cisco did forwarding of IP traffic before they developed CEF. Before CEF most of the forwarding on IOS routers used process switching and fast switching. Process switching was the default method and fast switching was an enhancement to make packet forwarding more efficient. Fast switching built a cache based on traffic that it had processed. The fast switching cache was demand driven, which means that the router built the cache entries as it saw traffic coming through the router to be forwarded. By default the router would build the cache only for traffic that entered on one interface and exited on another interface. So by default traffic that entered on one interface and was forwarded out the same interface did not build a cache entry. There was a special config option to tell the router to also build cache entries when traffic was forwarded out the same interface. And this is what the second output is about - will the router build fast switching cache for packets switched back out the same interface. In this case that option is on.
Now that CEF is used on IOS routers Fast Switching and Fast Switching Same Interface are really not used. But they still show up in the show command output for backward compatibility with older IOS.
Two years too late !!! but I have a question on this. Does CEF by default support same interface switching in Hardware ?
Thanks in Advance
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Generally the small routers do everything in software, including CEF. L3 switches, and some high end routers, support forwarding with special hardware.
I would agree with Joseph that if CEF is implemented in hardware then a packet forwarded out the same interface that it arrived on will still be forwarded using CEF in hardware.
I would then go a step further with this observation. For Fast Switching, in which the cache is demand driven, Cisco implemented an efficiency in which by default it did not build a cache entry if the packet is forwarded out the interface on which it arrived. They then created an option that if you wanted that functionality that you could enable it. The implementation of CEF is different. The cache is not demand driven but is calculated before packet forwarding begins. The cache includes a forwarding table and an adjacency table (where the layer 2 rewrite information is stored). The adjacency table includes every adjacency, no matter which interface. So there is no concept or no dependency in CEF on same interface.