1.3.a Adjacencies

EIGRP’s operations can be conceptually simplified into three basic steps:

Step 1. Neighbor Discovery: Through the exchange of Hello messages, EIGRP-speaking routers discover one another, compare parameters (for example, autonomous system numbers, K-values, and network addresses), and determine if they should form a neighborship.

Step 2. Topology Exchange: If neighboring EIGRP routers decide to form a neighborship, they exchange their full topology tables with each other. However, after the neighborship is established, only changes to the existing topology are communicated between the routers. This approach makes EIGRP much more efficient than a routing protocol such as RIP, which advertises its entire list of known networks at regular intervals.

Step 3. Choosing Routes: Once an EIGRP router’s topology table is populated, the EIGRP process examines all learned network routes and selects the best route to each network. EIGRP considers a network route with the lowest metric to be the best route to that network.

Step 1 in detail

Step 1. Router R1 wants to see if there are any EIGRP-speaking routers off of its Gig 0/1 interface with which it might possibly form a neighborship (also known as an adjacency). So, it multicasts an EIGRP Hello message to the well-known EIGRP multicast address of 224.0.0.10 asking for any EIGRP-speaking routers to identify themselves.

Step 2. Upon receipt of router R1’s Hello message, router R2 sends a unicast Update message back to router R1’s IP address of 10.1.1.1. This Update message contains router R2’s full EIGRP topology table (R2 has only one Loopback configured).

Step 3. Router R1 receives router R2’s Update and responds with an Acknowledgement (ACK) unicast message sent to router R2’s IP address of 10.1.1.2.

Step 4. The process then repeats itself with the roles reversed. Specifically, router R2 sends a Hello message to the EIGRP multicast address of 224.0.0.10.

Step 5. Router R1 responds to router R2’s Hello message with a unicast Update containing router R1’s full EIGRP topology table. This Update message is destined for router R2’s IP address of 10.1.1.2 with the sequence number 10. As we can see this update contains no route (yet).

Step 6. Router R2 receives router R1’s routing information and responds with an ACK (Acknowledge 10) unicast message sent to router R1’s IP address of 10.1.1.1.

At this point, an EIGRP neighborship has been established between routers R1 and R2. The routers will periodically exchange Hello messages, to confirm each router’s neighbor is still there. However, this is the last time the routers exchange their full routing information.

Subsequent topology changes are advertised via partial updates, rather than the full updates used during neighbor establishment. Also, notice that the Update messages during neighbor establishment were sent as unicast messages. However, future Update messages are sent as multicast messages, destined for 224.0.0.10. This helps ensure all EIGRP-speaking routers on a segment receive the Update messages.