EIGRP Updates Breakdown

So, in order for the global routing table to have EIGRP routes in it, EIGRP has to first be enabled, then go through a process through with it determines which routes are Feasible successors (Best routes) and only then install them to the global routing table.

EIGRP has five basic messages it uses for this EIGRP collaboration between its neighbors.

  • Hello
  • Update
  • Query
  • Reply
  • ACK

EIGRP uses the Update and the ACK messages to exchange EIGRP route information. Updates contain the route info and ACK are used by neighbors to acknowledged they got the update.

Update messages have the following info in them:

  • Prefix information
  • Prefix Length
  • Metric Components (bandwidth, delay, reliability, and load
  • Nonmetric items: MTU and hop count

To put a visual on this, we will start with a basic configuration:

When all routers are configured with the basic EIGRP config, a neighborship process starts to occur. Every router will start sending out hello packets, find neighbors, and then exchange updates with their eigrp routing tables:

Router 1

router eigrp 1
network 192.168.60.0
network 172.16.1.0

Router 2

router eigrp 1
network 192.168.60.0
network 172.17.1.0

Router 3

router eigrp 1
network 192.168.60.0
network 172.18.1.0

EIGRP first sends out updates to find neighbors:

Neighbors are found and established. After this neighbors send update messages with full EIGRP route information (Prefix, Prefix Length, Metric Components & non metric items mentioned above).

During this update the full EIGRP table/topology routes are sent. So in our case Router 1 is sending an update with he following prefix (172.16.1.0/24 and 192.168.60.0/24) to routers 2 and 3. In a similar way routers 2 and 3 are sending their FULL EIGRP topology tables to one another. During this process each router calculates the metric for each route that they are learning from their neighbors.

As mentioned before each EIGRP enabled router/neighbor calculates the best route to each neighbors routes and adds 1 to its hop count:

From router 1’s perspective we can see the following metrics for network 172.16.1.0/24:

As you can see minimum bandwidth is 10000000, Delay is 5000msec, and hop count is 0.

Now lets look from router 3’s perspective for network 172.16.1.0 which lives off Router 1:

You can see that the minimum bandwidth changed to 100000, (Router 3 looks at the interface on which it learned this route and chooses the lower value between the advertised interface and the one it learned the route from. In this case the interface value is lower then the one advertised by the neighbor. The total delay is increased to 5100 from 5000 so that means the original delay of 5000 and the added delay of 100 from Router 3’s interface comes up with a total delay of 5100. Lastly, the hop count was increased to 1.

 

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