Identify Cisco Express Forwarding concepts

Network Principles

    Identify Cisco Express Forwarding concepts

  • Cisco Express Forwarding concepts¬†

    The CEF is referred as a Cisco express forwarding. It is an advanced one in layer 3 switching technology, which is mainly used in the internet or else core networks to enrich the overall performance of the network. However, CEF is the Cisco proprietary protocol, remarkable vendors of the high capacity or multi layer switches provides a similar functionality at where the layer 3 routing or switching is taking place in the hardware rather than the CPU or software.

    Identify Cisco express forwarding concepts

    The CEF is most often used to improve the speed of the packet switching by deducting the overhead as well as delays which is introduced by means of other routing techniques. This CEF also optimizes the scalability for the networks with a dynamic and large traffic pattern. It offers the capability to switch the packets via device in an efficient way when keeping the load on a router processor as low.

    The CEF is mainly made up of 2 different main components such as FIB and Adjacency table. Those 2 components will updates same time automatically as a routing table.


    The routing itself will not truly optimize for the rapid look ups. It mostly comprises of lots of information which is crucial to its construction, however, it is not so essential for the routing lookups like administrative an origin distances of a route, their ages, metrics and much more.

    The FIB is referred as a forwarding information base. It contains the destination reachability details and the next hop details. This detail is mostly used by a router to decide the forwarding decision. Then the FIB is mainly organized as the multiway trie that allows for easy lookups and very efficient. Within the FIB and the routing table there will be a one on one relationship will exist between the routes.

    The CEF uses this FIB to make the IP destination based switching decisions. It is conceptually very similar to the information base, or routing table. It also maintains the mirror of a forwarding information in an IP routing table. When a topology or routing changes may occur in a network, a routing tables are updated as well as those changes will reflect on the FIB.

    The entries in a routing table will need some recursive lookups such as: after matching the entry of the destination network, next hop information might comprise next hop's IP address but not an egress interface, so that the IP address of the next hop has to be searched up in a next iteration of the routing table and this recursion depth is literally theoretical. After matching the immediate entry in a routing table which identifies an egress interface finally. Then the final IP address of the next hop at the time of lookup process must be matched further in ARP or the similar mapping table for an egress interface to determine how to design the header of the layer 2 frame. All those shortcomings can also be improved by: destination prefixes from a routing table can also be stored in the separate data structure is called as forwarding information base. Rather than transferring the plain IP address of the next hop from a routing table through FIB, every entry in a forwarding information base which represents the prefix of the destination can rather comprise the pointer into the specific entry in an adjacency table which stores an appropriate information of the egress interface and layer 2 header frame indication.

    The routing table recursion is resolved when making the entries of FIB as well as setting up a pointer into appropriate entries of the adjacency table. This FIB will only store the destination prefixes and the forwarding information is stored as the rewrites information of the layer 2 in an adjacency table and also FIB points entries towards appropriate adjacency table entries.

    All the FIB entries will describe that the networks are reachable through specific next hop point to a similar entries of the adjacency table that may contain intended egress information and layer 2 header towards the next hop.

    Adjacency Table

    Once the forwarding information base is checked, then the next hop in the troubleshooting a CEF lies to rewrite information in an adjacency table. An adjacency table comprises the rewrite information which CEF uses to switch the packets. Every adjacency entry stores the pre computed frame headers are used while forwarding the packet using the FIB entry referencing the associated entry. This adjacency table is highly populated as an adjacencies are discovered. Every time when an adjacency entry is produced, such as via the ARP protocol, the link layer headers for that the adjacent node is pre computed as well as stored in an adjacency table.

    The routes may have mostly more than the 1 path per entry, allowing it possible to use the CEF to switch the packets when loading over the multiple paths. Additional to the next hop of the interface adjacencies, few exception adjacencies will exist to expedite the switching for a non standard condition.

    Typically a router has only the handful neighbors, a next hop into all of the known destination. All destinations are reachable via a specific next hop are using the similar layer-two rewrites information and to reach any networks behind the specific adjacent next hop, then the packets may be encapsulated towards the frame which has the similar layer two headers addressed and also sent out the same egress interfaces. Once the perfect entry is chose, encapsulating a packet, and forwarding that out the egress interfaces will be done in an effective rapid way, as the required data are available readily.

    Once the FIB as well as adjacency table are produced, then the routing tables are not used anyway for route packet for which all the forwarding information is found in an adjacency table or FIB. The routing table will become a more source of the routing data to build an adjacency table and FIB contents, but it is not required to use route packets anymore.

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