Cisco Express Forwarding
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Cisco Express Forwarding (CEF) is an advanced layer 3 switching technology used mainly in large core networks or the Internet.
[edit] Function
CEF is mainly used to increase packet switching speed, reducing the overhead and delays introduced by other routing techniques, increasing overall performance. CEF consists of two key components: The Forwarding Information Base (FIB) and adjacencies.
The FIB is similar to the routing table generated by multiple routing protocols, maintaining only the next-hop address for a particular IP-route.
The adjacency maintains layer 2 or switching information linked to a particular FIB entry, avoiding the need for an ARP request for each table lookup. There are five types of adjacencies:
- Null adjacency: Handles packets destined to a NULL interface. Packets with FIB entries pointing to NULL adjacencies will normally be dropped.
- Punt adjacency: Deals with packets that require special handling or can not be switched by CEF. Such packets are forwarded to the next switching layer (generally fast switching) where they can be forwarded correctly.
- Glean adjacency: Handles packets destined for currently attached hosts, but without layer 2 information.
- Discard adjacency: FIB entries pointing to this type of adjacency will be discarded.
- Drop adjacency: Packets pointing to this entry are dropped, but the prefix will be checked.
In order to take full advantage of CEF, it is recommended to use distributed CEF (dCEF), where there is a FIB table on each of the line cards. This avoids the need for querying the main processor or routing table in order to get the next-hop information, performing the fast switching on the line card itself.
CEF currently supports Ethernet, Frame Relay, ATM, PPP, FDDI, Tunnels and HDLC.
