Deploying Cisco Express Forwarding – CEF-Based Multilayer Switching
Common CEF problems and solutions

CEF is the fastest means of switching Layer 3 packets in hardware. The CEF tables stored in hardware are populated from information gathered by the route processor. Troubleshooting CEF operations therefore has two primary steps:

  • Ensure that the normal Layer 3 operations on the route processor are functioning properly so that the switch tables will be populated with accurate and complete information.
  • Verify that information from the route processor has properly populated the FIB and adjacency table, and is being used by CEF to switch Layer 3 packets in hardware.

Troubleshooting CEF is, in essence, verifying that packets are indeed receiving the full benefit of CEF switching and not being "punted" to a slower packet switching or processing method. The Cisco term "punt" describes an action of sending a packet "down" to the next fastest switching level. The following list defines the order of preferred Cisco IOS switching methods, from fastest to slowest.

  • Distributed CEF
  • CEF
  • Fast switching
  • Process switching

A punt occurs when the preferred switching method did not produce a valid path or in CEF, a valid adjacency. If the CEF lookup process fails to find a valid entry in the FIB, CEF will install a punt adjacency to the less preferred system. CEF will punt all packets with that adjacency to the next best switching mode, in order to forward all the packets by some means even if that means is less efficient.

Some basic CEF problems and associated solutions are described in the CEF Problems and Solutions table.