Cisco Express Forwarding

Cisco Express Forwarding (CEF), like MLS, aims to speed the data routing and forwarding process in a network. However, the two methods use different approaches.

CEF uses two components to optimize the lookup of the information required to route packets: the Forwarding Information Base (FIB) for the Layer 3 information and the adjacency table for the Layer 2 information.171

CEF creates an FIB by maintaining a copy of the forwarding information contained in the IP routing table. The information is indexed so that it can be quickly searched for matching entries as packets are processed. Whenever the routing table changes, the FIB is also changed so that it always contains up-to-date paths. A separate routing cache is not required.

The adjacency table contains Layer 2 frame header information, including next-hop addresses, for all FIB entries. Each FIB entry can point to multiple adjacency table entries, for example, if two paths exist between devices for load balancing.

After a packet is processed and the route is determined from the FIB, the Layer 2 next-hop and header information is retrieved from the adjacency table and a new frame is created to encapsulate the packet.

Cisco Express Forwarding can be enabled on a router (for example, on a Cisco 7500 Series router) or on a switch with Layer 3 functionality (such as the Catalyst

8540 switch).

Was this article helpful?

0 0

Post a comment