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EIGRP for CCNA students


Enhanced Interior Gateway Routing is a dynamic routing Protocol which is used to find the best path between any two layer-3 devices to deliver the packet. EIGRP works on network layer Protocol of OSI model and uses the protocol number 88. EIGRP was developed by Cisco System as a proprietary protocol, which can be used only on Cisco routers but in 2013 partial functionality of EIGRP was converted to an open standard. EIGRP AD value for iEIGRP is 90 and 170 for eEIGRP.
    1. It is also known as Hybrid Routing protocol.
    2. It uses DUAL algorithm.
    3. It supports IP, IPX and AppleTalk.
    4. It has 2 route states (Active and Passive)
    Fig 1.1- EIGRP Basics

    EIGRP Terms
    Successor – A successor route is the best route to a remote network. A successor route is used by EIGRP to forward traffic to a destination and is stored in the routing table.
    Feasible successor – A feasible successor is a path whose reported distance is less the feasible distance and it is considered a backup route. Only the one with the best metric (the successor) is copied and placed in the routing table.
    Feasible distance – This is the best metric along all paths to a remote network.
    Reported/advertised distance – This is the metric of a remote network, as reported by a neighbor.
    Active Route – It means that the router is actively trying to find out a route for the destination, if the successor fails.
    Passive Route – It means that the router has a backup route.

    EIGRP Packet Types
    Hello: These messages are keep alive messages which are exchanged between two devices operating EIGRP. For example if Router A is directly connected with Router B and Router A is not receiving Hello packets from Router B for a specified period of time, Router A will know that Router B is down.
    Update: Updates are used to send route information to the neighbor routers. When a new neighbor is discovered, update packets are sent so the neighbor router can build up its topology table.
    Query: These messages are multicast when the device is declared dead and it has no routes to it in its topology table.
    Reply: These messages are the acknowledgment of the query message sent to the originator of the query message stating the route to the network which has been asked in the query message.
    ACK: It is used to acknowledge EIGRP update, queries, and replies. Acks are hello packets that contain no data.
    Note: However we need to remember that Hello and acknowledgment packets do not require any acknowledgment. Reply, query, update messages are reliable messages requires acknowledgement.
    EIGRP Tables
    Neighbor Table: As its name suggests this table consists of information of neighbor routers whose information is collected using Hellos. It consists of all directed neighbors. The “show ip eigrp neighbor” command lists the information about neighbors. 
    Topology Table: This table contain a lot of information i.e. All the paths to destination learnt by the EIGRP neighbors. The command “show ip eigrp topology” shows the topology table.
    Routing Table: This is the table that has the best possible route to a destination. The command “show ip route” shows all routes. To specifically see the EIGRP route in routing table “show ip route eigrp” command is used.
    Advantages of EIGRP
      1. EIGRP supports both classful and classless routing.
      2. Support for VLSM & CIDR.
      3. Supports MD5 authentication on routers running EIGRP.
      4. Provides routing support for multiple Network layer protocols: IP, IPX, AppleTalk, and even IPv6.
      5. In EIGRP the routers can communicate to other routers which can be maximum 255 hops away (by default it is 100 hops).
      6. EIGRP supports load balancing over equal cost (maximum 32 equal cost path, by default it’s 4) & un-equal cost path.
      7. Whenever any new network is added, modified or deleted EIGRP only sends incremental updates, reducing the workload on the router and the amount of data that needs to be transmitted.
      As we have go through the basic of EIGRP in this article, in further we will be continuing with EIGRP configuration, Authentication, Summarization and Load balancing in EIGRP. I hope you all guys liked this article. Stay connected with Networks baseline.

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