EIGRP Neighbour Discovery

• We have 2 routers called R1 and R2 and they are configured for EIGRP. As soon as we enable it for the interface they will start sending hello packets. In this example R1 is the first router to send a hello packet.
• As soon as R2 receives the hello packet from R1 it will respond by sending update packets that contain all the routing information that it has in its routing table. (The only routes that are not sent on this interface are the one that R2 learned on this interface because of split-horizon.).
• The update packet that R2 will send has the initialization bit set so we know this is the “initialization process”.  At this moment there is still no neighbour adjacency until R2 has sent a hello packet to R1.
• R1 is of course not the only one sending hello packets. As soon as R2 sends a hello packet to R1 we can continue to setup a neighbour adjacency.
• After both routers have exchanged hello packets we will establish the neighbour adjacency. R1 will send an ACK to let R2 know he received the update packets. The routing information in the update packets will be saved in the EIGRP topology table.
• R2 is anxious to receive routing information as well so R1 will send update packets to R2 who will save this information in its EIGRP topology table.
• After receiving the update packets R2 will send an ACK back to R1 to let him know everything is ok.