Network Warrior, 2nd Edition

Configuring and Managing Switching Paths

Configuring switching paths is done both globally and at the interface level, allowing you the flexibility of configuring different switching paths on each interface. For example, you may want to disable CEF on an interface to see whether it’s causing problems.

Process Switching

To force a router to use process switching, turn off all other switching methods.

Here, I’m showing the performance of a Cisco 2621XM router with about 600k of traffic running over serial interface s0/1:

R1#sho int s0/1 | include minute
  5 minute input rate 630000 bits/sec, 391 packets/sec
  5 minute output rate 627000 bits/sec, 391 packets/sec

The normal switching method for this interface on this router is CEF. To see which switching path is running on interface s0/1, use the show ip interface s0/1 | include switching command:

R1#sho ip interface s0/1 | include switching
  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

Notice that fast switching and CEF are both enabled. CEF will try to switch the packet first. If CEF cannot switch the packet, it will punt the packet to the next best available switching path—fast switching. If fast switching cannot process the packet, the router will process-switch the packet. If all the other switching paths are turned off, ...