Main Components Of Cef; Fib Overview; Cef Fib And Load Balancing; Cef Adjacency Tables Overview - Cisco Catalyst 3850 Configuration Manual

CEF FIB and Load Balancing

Main Components of CEF

Information conventionally stored in a route cache is stored in several data structures for Cisco Express
Forwarding switching. The data structures provide optimized lookup for efficient packet forwarding. The
two main components of Cisco Express Forwarding operation are the forwarding information base (FIB)
and the adjacency tables.
The FIB is conceptually similar to a routing table or information base. A router uses this lookup table to
make destination-based switching decisions during Cisco Express Forwarding operation. The FIB is
updated when changes occur in the network and contains all routes known at the time. For more
information, see the FIB Overview section.
Adjacency tables maintain Layer 2 next-hop addresses for all FIB entries. For more information, see the
CEF Adjacency Tables Overview section.
This separation of the reachability information (in the Cisco Express Forwarding table) and the forwarding
information (in the adjacency table), provides a number of benefits:
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FIB Overview

Cisco Express Forwarding uses a forwarding information base (FIB) to make IP destination prefix-based
switching decisions.
The FIB contains the prefixes from the IP routing table structured in a way that is optimized for forwarding.
When routing or topology changes occur in the network, the IP routing table is updated, and those changes
are reflected in the FIB. The FIB maintains next-hop address information based on the information in the IP
routing table.
Because there is a one-to-one correlation between FIB entries and routing table entries, the FIB contains all
known routes and eliminates the need for the route cache maintenance that is associated with switching
paths such as those used in fast switching and optimum switching.
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CEF FIB and Load Balancing
Several paths can lead to a destination prefix. This occurs, for example, when a router is configured for
simultaneous load balancing and redundancy. For each resolved path, the FIB contains a pointer for the
adjacency corresponding to the next hop interface for that path.

CEF Adjacency Tables Overview

A node is said to be adjacent to another node if the node can be reached with a single hop across a link
layer (Layer 2). Cisco Express Forwarding stores forwarding information (outbound interface and MAC
header rewrite) for adjacent nodes in a data structure called the adjacency table. Cisco Express Forwarding
uses adjacency tables to prepend Layer 2 addressing information to packets. The adjacency tables maintain
Layer 2 next-hop addresses for all FIB entries.
The following sections provide additional information about adjacencies:
IP Switching Cisco Express Forwarding Configuration Guide, Cisco IOS XE Release 3SE (Catalyst 3850
Switches)
4
The adjacency table can be built separately from the Cisco Express Forwarding table, allowing both to
be built without any packets being process-switched.
The MAC header rewrite used to forward a packet is not stored in cache entries, so changes in a MAC
header rewrite string do not require validation of cache entries.
CEF FIB and Load Balancing, page 4
Main Components of CEF