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.
