Specifications

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Cisco MWR 2941 Mobile Wireless Edge Router Release 3.5 Software Configuration Guide, Cisco IOS Release 15.1(3)MR
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Chapter 13 Configuring Cisco Express Forwarding
Information About Cisco Express Forwarding
• Frame Relay
• High-Level Data Link Control (HDLC)
• PPP
• Spatial Reuse Protocol (SRP)
• TokenRing
• Tunnels
Main Components of Cisco Express Forwarding Operation
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 on page 13-3.
Adjacency tables maintain Layer 2 next-hop addresses for all FIB entries. For more information, see the
“Cisco Express Forwarding Adjacency Tables Overview” section on page 13-4.
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:
• 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 invalidation of cache entries.
FIB Overview
Cisco Express Forwarding uses a 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.
Cisco Express Forwarding 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.