CCNP Route 300-101 – Cisco Express Forwarding (CEF)

by Samuel Mitchell, posted in Cisco, Routing & Switching

The Cisco Express Forwarding (CEF) is a Topology-Based switching technology. It is enabled by default on most cisco router and layer 3 switches.

device(config)# ip cef

The CEF is made up of two tables:

  • Forwarding Information Base (FIB)
  • Adjacency table

Forwarding Information Base (FIB) table – maintains next-hop Layer 3 address information based on the information in the IP routing table. CEF use the FIB to make IP destination prefix-based switching decisions. The command to show FIB below:

show ip cef

Adjacency Table – maintains Layer 2 next-hop addresses for all FIB entries. If the information is not available, ARP is used to discover this information. The command to show adjacency table below:

show adjacency

Type of Adjacency That requires Special Handling

  • Null – Packets destined for a Null0 interface
  • Glean – used for directly connected routes. Tells router to check ARP table
  • Punt – used for packets that cant be forwarded by CEF. it is sent to next level switching method
  • Discard – packets discarded by access-list or other policy
  • Drop – packets cant forward because of encapsulation error or unsupported protocol.

CEF can be enabled in one of two modes:

  • Central CEF mode – When CEF mode is enabled, the CEF FIB and adjacency tables reside on the route processor, and the route processor performs the express forwarding. You can use CEF mode when line cards are not available for CEF switching, or when you need to use features not compatible with distributed CEF switching.
  • Distributed CEF (dCEF) mode – When dCEF is enabled, line cards maintain identical copies of the FIB and adjacency tables. The line cards can perform the express forwarding by themselves, relieving the main processor – Gigabit Route Processor (GRP) – of involvement in the switching operation. This is the only switching method available on the Cisco 12000 Series Router.

Packets that CEF cannot handle:

  • IP Header Option
  • Expiring TTL
  • Tunnel interface
  • Exceed MTU
  • IGMP Redirect

 

Reference:

https://www.cisco.com/c/en/us/support/docs/routers/12000-series-routers/47321-ciscoef.html#cef-ops

https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipswitch_cef/configuration/15-mt/isw-cef-15-mt-book/isw-cef-overview.html?referring_site=RE&pos=1&page=https://www.cisco.com/c/en/us/support/docs/routers/12000-series-routers/47321-ciscoef.html#GUID-993D4B0C-C032-420D-8304-F56AAB1CECC6

 

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