Friday, November 5, 2010


EIGRP is considered one of the more user-friendly routing protocols in the IGP world. I'm going to walk through the basics and advanced configuration of EIGRP in the next few articles.
Unlike other protocols which only keeps the best route and throws the rest away (resulting in having to recalculate if the primary route goes down), EIGRP is able to quickly switch to one backup route if the primary link is down. EIGRP is the only protocol that supports backup routes.

OSPF is one of the most advanced routing protocols in the world, but it is more difficult to configure. EIGRP supports all of OSPF's features with the ease of configuration of traditional Distance Vector protocols.

Unlike OSPF's requirement to implement summarization on ABR or ASBR's, EIGRP can summarize wherever you want it. This makes EIGRP very flexible in summarization.

It is also difficult to effectively implement load-balancing across unequal-cost paths in other routing protocols. Typically, such an implementation results in having a reduction of throughput for the higher speed link. EIGRP, however, does this very quickly and easily.

Expandability is also one of the key factors designers kept in mind while creating the protocol. EIGRP is able to route multiple routed protocols without having to majorly rewrite the protocol like others.

Because EIGRP doesn't know the entire network topology, but only the networks their neighbors tell them about, the protocol is classified under Advanced Distance Vector protocol.

There are two terms to describe distance in EIGRP:
-Feasible Distance (FD)
-Advertised Distance (AD)

Feasible Distance is the distance for the router to reach the destination.

Advertised Distance is the distance for the router who told you about the destination (which is, the router's neighbor) to reach it.

Remember that I mentioned above that EIGRP keeps a backup route? These are the two terms you'll need to get familiar with:
-Feasible Successor

The Successor refers to the primary route, while the Feasible Successor refers to the backup link. In order for a route to become a Feasible Successor, its Advertised Distance must be less than the Feasible Distance of your successor. In other words, your neighbor must have a better route to the destination than yourself for the route to be a backup.

This is a loop prevention mechanism. From the router's perspective, if your neighbor's advertised distance is higher than your feasible distance, there is a chance that your neighbor is routing through you.

Here's an example:

In this topology, R1 has two ways to get to the destination network: Through R2, and through R3.

It first looks for the successor by looking at the lowest cost path. Both R2 and R3 has an AD of 10. The path through R2 has a FD of 610, while the path through R3 has a FD of 1010.

The best path to the destination network (Successor) is through R2 as it has the lowest FD.

The path through R3 also becomes the backup (Feasible Successor) as it has the second lowest FD and the AD is lower than the FD of the successor.

However, this method sometimes block valid routes. If R3's route to the destination is 1500 instead of 10, then it wouldn't be chosen as the backup. Instead of instantaneously switching over to the route, R1 would take a longer time to switch over having to query R3 to make sure that it still has a real route.

However, let's look at what would happen if R3 didn't have the link to the destination to begin with:

R2 now has the Advertised Distance of 610 and R3 has the Advertised Distance of 1610.

The path through R2 becomes the primary because it has the best FD and AD is lower than FD.

However, the path through R3 will not be selected as the backup despite having the next best FD because the AD is higher than the FD of the successor.

There are two states a route can sit in:
-Active Route
-Passive Route

Contrary to first impressions, an active route is actually a bad thing. Active means that it is actively searching for a path to the route. Passive simply means that the path has been found.

You can see the state of the routes by looking at the topology table:
show ip eigrp topology

EIGRP has 5 different messages which they use to communicate and form neighborship:

Hello forms relationships.

Update sends updates.

Query asks about routes.

Reply is a response to a query.

Ack is an acknowledgment to any of the messages above (except Hello).

Unlike RIP which sends the whole routing table constantly, EIGRP uses triggered updates. After the initial exchange, EIGRP only queries for, and receives routes it actually needs.


The practical (default) metric is 256*(BANDWIDTH+DELAY)

BANDWIDTH = 10^7/Link Bandwidth
DELAY is expressed in Microseconds/10

Both the bandwidth and delay are statically entered values on a link. Each link has a predefined set of these. We can look at these by typing:
show int s0/0

To set the bandwidth, we can use:
bandwidth 1544

To set the delay, we can use:
delay 20000

Reliability and Load are the actual dynamic values. Cisco recommends never to use the full metric. Neighbors will only form if both routers have the same K Values.

The EIGRP routing process maintains three tables:
-Neighbor Table
-Topology Table
-Routing Table

We'll talk about these in upcoming articles.

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