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CEF uses a FIB to make IP destination prefix-based switching decisions. The FIB is conceptually similar to a routing table or information base. It maintains a mirror image of the forwarding information contained in the IP routing table. 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 route cache maintenance that is associated with switching paths such as fast switching and optimum switching.
3 Main types of packet switchng:
1. Packet Switching: This involves the router checking the packet destination header and matching it to the ROUTING TABLE.
2. Fast Switching: This was developed to improve speed of packet routing. So it kept another list in addition to the routing table which cached the specific IP address for recently forwarded packets. It also keeps a copy of the destination header so it would copy that instead of creating a new one for each new packet.
3. CEF (Cisco Express Forwarding): This did the same as fast switching, but it organized its table for all routing AHEAD of time and uses more sophisticated search algorithms & binary trees for faster more efficient searches.
So it short, it's a table with a cache of the best routes which would be calculated from the routing table, so they are ready to access instead of the router having to look to the routing table everytime and calculate the best route for a packet.
CEF: Is mainly used to increase packet switching speed by reducing the overhead and delays introduced by other routing techniques. 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 table maintains layer2 or switching information linked to a particular FIB entry, avoiding the need for an ARP request for each table lookup. There are several types of adjacencies
Routing IP Table:A routing table is a set of rules, often viewed in table format, that is used to determine where data packets traveling over an Internet Protocol (IP) network will be directed. All IP-enabled devices, including routers and switches, use routing tables.
A routing table contains the information necessary to forward a packet along the best path toward its destination. Each packet contains information about its origin and destination. When a packet is received, a network device examines the packet and matches it to the routing table entry providing the best match for its destination. The table then provides the device with instructions for sending the packet to the next hop on its route across the network.
CEF Table is one component of CEF protocol which is Cisco proprietary protocol used mainly in large core networks, to provide high speed packet switchin.
