IP fast switching on the same interface

[CARLO CIANFARANI]

Hi all,

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: 224.0.0.5

  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 ?

Thanks, Carlo.

[Richard Burts]

Carlo

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.

HTH

Rick

[Umesh Shetty]

Hi Richard,

Two years too late !!! but I have a question on this. Does CEF by default support same interface switching in Hardware ?

Thanks in Advance

Umesh

[Joseph W. Doherty]

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Posting

Generally the small routers do everything in software, including CEF.  L3 switches, and some high end routers, support forwarding with special hardware.

[Richard Burts]

Umesh

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.

HTH

Rick