Enhanced IGRP (EIGRP)

Enhanced IGRP (EIGRP):-is a classless, enhanced distance-vector protocol that gives us a real edge over another Cisco proprietary protocol, EIGRP uses the concept of an autonomous system to describe the set of contiguous routers that run the same routing protocol and share routing information. EIGRP is sometimes referred to as a hybrid routing protocol because it has characteristics of both distance-vector and link-state protocols. EIGRP has a maximum hop count of 255 (the default is set to 100).

There are a number of powerful features that make EIGRP a real standout from IGRP and other protocols. The main ones are listed here:

  1. Support for IP and IPv6 (and some other useless routed protocols) via protocol-dependent modules
  2. Considered classless (same as RIPv2 and OSPF)
  3. Support for VLSM / CIDR
  4. Support for summaries and dis-contiguous networks
  5. Efficient neighbor discovery
  6. Communication via Reliable Transport Protocol (RTP)
  7. Best path selection via Diffusing Update Algorithm (DUAL)

Protocol-Dependent Modules

One of the most interesting features of EIGRP is that it provides routing support for multiple Network layer protocols: IP, IPX, AppleTalk, and now IPv6. (Obviously we won’t use IPX and AppleTalk, but EIGRP does support them.) The only other routing protocol that comes close and supports multiple network layer protocols is Intermediate System-to-Intermediate System (IS-IS). EIGRP supports different Network layer protocols through the use of protocol-dependent modules (PDMs). Each EIGRP PDM will maintain a separate series of tables containing the routing information that applies to a specific protocol. 

Neighbor Discovery

Before EIGRP routers are willing to exchange routes with each other, they must become neighbors.

There are three conditions that must be met for neighborship establishment:

  1. Hello or ACK received
  2. AS numbers match
  3. Identical metrics (K values)

Link-state protocols tend to use Hello messages to establish neighborship (also called adjacencies) because they normally do not send out periodic route updates and there has to be some mechanism to help neighbors realize when a new peer has moved in or an old one has left or gone down. To maintain the neighborship relationship, EIGRP routers must also continue receiving Hellos from their neighbors.EIGRP routers that belong to different autonomous systems (ASes) don’t automatically share routing information and they don’t become neighbors. This behavior can be a real benefit when used in larger networks to reduce the amount of route information propagated through a specific AS. The only catch is that you might have to take care of redistribution between the different ASes manually.


  • Feasible distance

    This is the best metric along all paths to a remote network, including the metric to the neighbor that is advertising that remote network. This is the route that you will find in the routing table because it is considered the best path.
  • Reported/advertised distance

    This is the metric of a remote network, as reported by a neighbor.It is also the routing table metric of the neighbor and is the same as the second number in parentheses as displayed in the topology table, the first number being the feasible distance.
  • Neighbor table

    Each router keeps state information about adjacent neighbors. When a newly discovered neighbor is learned, the address and interface of the neighbor are recorded,and this information is held in the neighbor table, stored in RAM. There is one neighbor table for each protocol-dependent module. Sequence numbers are used to match acknowledgments with update packets. The last sequence number received from the neighbor is recorded.
  • Topology table

    It contains all destinations advertised by neighboring routers, holding each destination address and a list of neighbors that have advertised the destination. For each neighbor, the advertised metric, which comes only from the neighbor’s routing table, is recorded. If the neighbor is advertising this destination, it must be using the route to forward packets.The neighbor and topology tables are stored in RAM and maintained through the use of Hello and update packets
  • Feasible successor

    A feasible successor is a path whose reported distance is less than the feasible distance, and it is considered a backup route. EIGRP will keep up to six feasible successors in the topology table. Only the one with the best metric (the successor) is copied and placed in the routing table. The show ip eigrp topology command will display all the EIGRP feasible successor routes known to a router.
  • Successor

    A successor to neighbor route having least cost path towards the destination.


  • Reliable Transport Protocol (RTP)

    EIGRP uses a proprietary protocol calledReliable Transport Protocol (RTP) to manage thecommunication of messages between EIGRP-speaking routers. And as the name suggests, reliability is a key concern of this protocol. Cisco has designed a mechanism that leverages multicasts and unicasts to deliver updates quickly and to track the receipt of the data.When EIGRP sends multicast traffic, it uses the Class D address router is aware of who its neighbors are, and for each multicast it sends out, it maintainsa list of the neighbors who have replied. If EIGRP doesn’t get a reply from a neighbor, it willswitch to using unicasts to resend the same data. If it still doesn’t get a reply after 16 unicastattempts, the neighbor is declared dead. People often refer to this process as reliable multicast
  • Diffusing Update Algorithm (DUAL)

    EIGRP usesDiffusing Update Algorithm (DUAL)for selecting and maintaining the best path to each remote network. This algorithm allows for the following:
    1. Backup route determination if one is available
    2. Support of VLSMs
    3. Dynamic route recoveries
    4. Queries for an alternate route if no route can be found
    DUAL provides EIGRP with possibly the fastest route convergence time among all protocols.

    The key to EIGRP’s speedy convergence is twofold: First, EIGRP routers maintain a copy of all of their neighbors’ routes, which they use to calculate their own cost to each remote network. If the best path goes down, it may be as simple as examining the contents of the topology table to select the best replacement route. Second, if there isn’t a good alternative in the local topology table, EIGRP routers very quickly ask their neighbors for help finding one they aren’t afraid to ask directions! Relying on other routers and leveraging the information they provide accounts for the “diffusing” character of DUAL.
  • Multiple ASes

    EIGRP uses autonomous system numbers to identify the collection of routers that share route information. Only routers that have the same autonomous system numbers share routes. In large networks, you can easily end up with really complicated topology and route tables, and that can markedly slow convergence during diffusing computation operations.
  • VLSM Support and Summarization

    As one of the more sophisticated classless routing protocols, EIGRP supports the use of Variable Length Subnet Masks. This is really important because it allows for the conservation of address space through the use of subnet masks that more closely fit the host requirements, such as using 30-bit subnet masks for point-to-point networks. And because thesubnet mask is propagated with every route update, EIGRP also supports the use of dis-contiguous subnets, something that gives us a lot more flexibility when designing the network’sIP address plan.
  • Route Discovery and Maintenance

    The hybrid nature of EIGRP is fully revealed in its approach to route discovery and maintenance. many link-state protocols, EIGRP supports the concept of neighbors that are discovered viaa Hello process and whose states are monitored, many distance-vector protocols, EIGRPuses the routing-by-rumor mechanism that implies many routers neverhear about a route update firsthand. Instead, they hear about it from another router that mayalso have heard about it from another one, and so on.
  • Neighborship table

    The neighborship table(usually referred to as the neighbor table) records information about routers with whom neighborship relationships have been formed.
  • Topology table

    The topology table stores the route advertisements about every route in the internetwork received from each neighbor.
  • Route table

    The route table stores the routes that are currently used to make routing decisions. There would be separate copies of each of these tables for each protocol that is actively being supported by EIGRP, whether it’s IP or IPv6. EIGRP Metrics
    1. Bandwidth
    2. Delay
    3. Load
    4. Reliability
    5. maximum transmission unit (MTU)

Configuring EIGRP

Although EIGRP can be configured for IP, IPv6, IPX, and AppleTalk, as a future Cisco.

There are two modes from which EIGRP commands are entered: router configuration mode and interface configuration mode. Router configuration mode enables the protocol, determines which networks will run EIGRP, and sets global characteristics. Interface configuration mode allows customization of summaries, metrics, timers, and bandwidth.

To start an EIGRP session on a router, use the router eigrp command followed by the autonomous system number of your network. You then enter the network numbers connected to the router using the network command followed by the network number.

An example of enabling EIGRP for autonomous system 20 on a router connected to two networks, with the network numbers being and

Router#config t
Router(config)#router eigrp 20

AS number is irrelevant—that is, as long as all routers use the same number! You can use any number from 1 to 65,535.

To stop EIGRP from working on a specific interface, such as a BRI interface ora serial connection to the Internet. To do that, you would flag the interface as passive using the passive-interfacecommand,

Router(config)#router eigrp 20
Router(config-router)#passive-interface serial 0/1

EIGRP Troubleshooting Commands

  • Command Description/Function

    show ip route :- Shows the entire routing table

    show ip route eigrp :- Shows only EIGRP entries in the routing table

    show ip eigrp neighbors:- Shows all EIGRP neighbors

    show ip eigrp topology:- Shows entries in the EIGRP topology table

    debug eigrp packet :- Shows Hello packets sent/received between adjacent routers

    Debug ip eigrp notification :-Shows EIGRP changes and updates as they occur onyour network


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