EIGRP (Enhanced Interior Gateway Routing Protocol) by Siby T. R

The Enhanced Interior Gateway Routing Protocol (EIGRP) is a Cisco-proprietary routing protocol for IP. It's actually based on IGRP, with many enhancements built into it. These characteristics include:

Fast convergence
Loop-free topology
VLSM and route summarization
Multicast and incremental updates
Routes for multiple routed protocols (Supports IP, IPX, APPLE TALK)

What is Convergence?

A router that stores a database, called a routing table, of the network addresses of network devices and the most efficient routes to them. Each router independently runs a routing algorithm to calculate metrics and build a routing table based on this information. In dynamic routing, routing tables are created dynamically by obtaining the network information from other routers. When a new addresses is added, updates are sent to routers across the network that describe changes in the network topology. When all of the routers agree this is called convergence.

What is VLSM?

Variable Length Subnet Mask (VLSM) is a network design strategy where subnet masks can have varying sizes. This enables network designers to use multiple masks for different subnets of a single class. With VLSM a network address space can be divided into a hierarchy of subnets with different sizes.

Characteristics of EIGRP

Both offer load balancing across six paths (equal or unequal).

They have similar metric structures.
EIGRP has faster convergence (triggered updates and saving a neighbor's routing table locally).
EIGRP has less network overhead, since it uses incremental updates.

EIGRP and IGRP use the same metric structure. Both can use bandwidth, delay, reliability, and MTU when computing a best metric path to a destination. By default, only bandwidth and delay are used in the metric computation. One interesting point about these protocols is that if you have some routers in your network running IGRP and others running EIGRP, and both sets have the same autonomous system number, routing information will automatically be shared between the two.

EIGRP uses the Diffusing Update Algorithm (DUAL) to update the routing table. This algorithm can enable very fast convergence by storing a neighbor's routing information in a local topology table. If a primary route in the routing table fails, DUAL can take a backup route from the topology table and place this into the routing table without necessarily having to talk to other EIGRP neighboring routers to find an alternative path to the destination.

Interaction with Other EIGRP Routers

EIGRP uses hello packets to discover and maintain neighbor relationships, much as OSPF does. EIGRP generates hello packets every 5 seconds on LAN, point-to-point, and multipoint connections with speeds of at least T1/E1 speeds. Otherwise, hellos are generated every 60 seconds. The dead interval period is three times the hello interval. EIGRP uses the multicast address of for the destination in the hello packets.

For EIGRP routers to become neighbors, the following information must match:

The AS number
The K-values (these enable/disable the different metric components)

When two routers determine whether they will become neighbors, they go through the following process:

1. The first router generates a Hello with configuration information.
2. If the configuration information matches, the second router responds with an Update message with topology information.
3. The first router responds with an ACK message, acknowledging the receipt of the second's ACK.
4. The first router sends its topology to the second router via an Update message.
5. The second router responds back with an ACK.

At this point, the two routers have converged.

Here are the message types for which an EIGRP router expects an ACK back:

Update Contains a routing update
Query Asks a neighboring router to validate routing information
Reply Responds to a query message

If an EIGRP router doesn't receive an ACK from these three packet types, the router will try a total of 16 times to resend the information. After this, the router declares the neighbor dead. When a router sends a hello packet, no corresponding ACK is expected.

Configuring EIGRP

Setting up EIGRP is almost as simple as configuring IGRP:

Router(config)# router eigrp autonomous_system_#
Router(config-router)# network IP_network_#

You must specify the AS number when configure EIGRP. Even though EIGRP is classless, you must configure it as a classful protocol when specifying your network numbers with the network command.

Troubleshooting EIGRP

show ip protocols
show ip route
show ip eigrp neighbors
show ip eigrp topology
show ip eigrp traffic
debug ip eigrp

Siby.T.R I am a free lance technical consultant I am interested to learn and publish my thoughts in ccna and system administration side. I hope you will be enjoying my articles if you are a technical person or a technical student or willing to accept technical things. Thanks and Regards Siby

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