Introduction to EIGRP

In this lesson, we’ll take a look at EIGRP (Enhanced Interior Gateway Routing Protocol) which is Cisco’s routing protocol. If you are unfamiliar with distance vector and RIP I highly recommend to read my Introduction to RIP first before continuing.

EIGRP stands for Enhanced Interior Gateway Routing Protocol and is a routing protocol created by Cisco. Originally, it was only available on Cisco hardware but since a few years, it’s now an open standard. EIGRP is called a hybrid or advanced distance vector protocol and most of the rules that apply to RIP also apply here:

  • Split Horizon
  • Route Poisoning
  • Poison Reverse

EIGRP Tables

EIGRP routers will start sending hello packets to other routers just like OSPF does, if you send hello packets and you receive them you will become neighbors. EIGRP neighbors will exchange routing information which will be saved in the topology table. The best path from the topology table will be copied in the routing table.

In this lesson, we'll take a look at EIGRP (Enhanced Interior Gateway Routing Protocol) which is Cisco's routing protocol. If you are unfamiliar with distance vector and RIP I highly recommend to read my Introduction to RIP first before continuing. EIGRP stands for Enhanced Interior Gateway Routing

Selecting the best path with EIGRP works a bit different than other routing protocols so let’s see it in action:

eigrp demo 3 routers

We have three routers named R1, R2 and R3. We are going to calculate the best path to the destination which is behind R3.

EIGRP uses a rich set of metrics namely bandwidth, delay, load and reliability which we will cover later. These values will be put into a formula and each link will be assigned a metric. The lower these metrics the better.

In the picture above I have assigned some values on the interfaces, if you would look on a real EIGRP router you’ll see the numbers are very high and a bit annoying to work with.

eigrp advertised distance

R3 will advertise to R2 its metric towards the destination. Basically R3 is saying to R2: “It costs me 5 to get there”. This is called the advertised distance. R2 has a topology table and in this topology table it will save this metric, the advertised distance to reach this destination is 5.

The advertised distance is also called the reported distance.

eigrp feasible distance

We are not done yet since there is something else that R2 will save in its topology table. We know the advertised distance is 5 since this is what R3 told us. We also know the metric of the link between R2 and R3 since this is directly connected. R2 now knows the metric for the total path to the destination, this total path is called the feasible distance and it will be saved in the topology table.

You have now learned two important concepts of EIGRP. The advertised distance, your neighbor tells you how far it is for him to reach the destination and the feasible distance which is your total distance to get to the destination.

Let’s continue!

eigrp advertised feasible distance

We are not done yet since R1 is also running EIGRP. R2 is sending its feasible distance towards R1 which is 15. R1 will save this information in the topology table as the advertised distance. R2 is “telling” R1 the distance is 15.

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Forum Replies

  1. Rene,
    I’m using the Boson Netsim 9.0 simulator and I configured a lab to match what you laid out in this lesson, what has me baffled is how and where do you see the hop count message of 16, when I shutdown the interface on my R3 router (LAN) I turned on debug IP Rip on my R2 router to view the updates being sent from R3. I did see the metric count reach 4 and after that the route was deleted for network, I never saw the hop count reach 16…just curious about this.
    Otherwise a very good explanation and write up RIP protocol
    It could be the

    ... Continue reading in our forum

  2. Hello Sameer

    This is a very good question. What is happening here is that R2 has obtained its original hop count of 1 to network from R3. So since R3 has told R2 that the distance to is 1, it puts it in its routing table.

    After the procedure that is described, R3 sends a new hop count to R2 of 2. Now you say since this hop count is higher than that which is already in the routing table of R2, why does R2 replace it? Well, it will replace it because it is an updated piece of information.

    So, the previous entry of metric 1 that was learned fr

    ... Continue reading in our forum

  3. Hello Sameer

    RIP requests are only sent under special circumstances, when a router requires that it be provided with immediate routing information. The most common example of this is when a router is first powered on. After initializing, the router will typically send an RIP Request on its attached networks to ask for the latest information about routes from any neighboring routers. The only other situation in which RIP requests are sent is when they are to be used for diagnostic purposes.

    So yes, it is rare to find requests.

    Responses on the other hand are sen

    ... Continue reading in our forum

  4. Super, Nice explanation Liz, now i understand the behaviour of RIPV1.

  5. Thanks again Lazaros,
    Now it is clear for me.

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