What is EIGRP (Enhanced Interior Gateway Routing Protocol)

Enhanced Interior Gateway Routing Protocol (EIGRP) is an advanced distance vector routing protocol based on the principles of the Interior Gateway Routing Protocol (IGRP).

EIGRP is a successor to the Interior Gateway Routing Protocol (IGRP). Both are owned by Cisco and operate only on their devices. Cisco introduced EIGRP because it needed a protocol with faster converging abilities, route selection, and calculation, and the ability to record information from neighboring devices.

EIGRP has the following characteristics:

  1. Advanced operational efficiency
  2. Capabilities of both link state and distance vector
  3. A classless routing protocol
  4. Unique features include the use of Reliable Transport Protocol (RTP), a diffusing update algorithm (DUAL), updates and updated information about neighbors
  5. Faster converging because it precalculates routes and does not broadcast hold-down timer packets before converging

Advantages of EIGRP 

  • EIGRP uses AS (Autonomous System) numbers ranging from  1-65535 to identify a collection of routers that share the same information.
  • EIGRP has less convergent time and is more efficient
  • EIGRP supports both auto and manual route summarization
  • Supports multiple routed protocols like IP, IPX, and Apple talks
  • EIGRP converges rapidly in the event of link failure
  • EIGRP can load balance equal and unequal cost path. By default, EIGRP supports 4 load balancing paths. It can be extended to 6 paths 

EIGRP Packet types 

There are 5 packet types

  1. Hello Packet: In normal scenarios, Hello packets are sent in 5 seconds. Hello, packets are sent between directly connected neighbors. Hello packets are sent as Multicast
  2. Update: Update packets are used to send triggered updates. EIGRP won’t send periodic updates like distance vector protocols. They send triggered updates only when a change occurs. Update packets are sent as Multicast or unicast
  3. Query: Query packets are sent when the successor path is failed and there is no feasible successor (backup path). Query packets are sent as multicast
  4. Reply: Reply for query packets. Reply packets are sent as unicast
  5. ACK: Ack packets are sent as Acknowledgement for update and Reply packets. Send as Unicast

Configuration

  • 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.
  • Let’s look at an example of enabling EIGRP for autonomous system 20 on a router connected to two networks, with the network numbers being 10.3.1.0/24 and 172.16.10.0/24:
Router(config)#router eigrp 20 
Router(config-router)#network 172.16.0.0
Router(config-router)#network 10.0.0.0
  • Remember—as with RIP, you use the classful network address, which is all subnet and host bits turned off.
  • Remember – the AS number is irrelevant – as long as all routers use the same number.
  • The AS number can be any number from 1 to 65,535.
  • Let’s say you didn’t want an interface to use EIGRP. You would do this by using the passive-interface command.
Router(config)#router eigrp 20
Router(config-router)#passive-interface serial 0/0
  • Use the no auto-summary command for EIGRP to work with discontiguous networks.

Commands to Verify EIGRP

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 entires 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 in your network

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