EIGRP: Neighbor Discovery Process

The EIGRP process starts by discovering new neighbors by receiving a Hello packet.  As soon as a new neighbor is discovered it is put into a ‘pending’ state, this is done so no routing information is exchanged until the neighborship has been confirmed to be good.

The originating device sends an empty Update packet with the Init flag set, this Update contains a nonzero sequence but no routing information.  The Init flag is an indication to the neighbor that this new adjacency is starting from the beginning and requests to send the full routing database once bi-directional visibility is confirmed.

The Receiving device replies with the sequence number from the original Init Update back to the originating device.

While the neighbor is set into a pending state, no reliable packets are sent.  A neighbor is moved from pending to Up state if if acknowledges the null update received from the router and sends its own null update with the Init flag set.

After this three way handshake the routers will exchange complete routing information using Update packets.  Once synchronization is complete, EIGRP neighbors will only send updates to advise neighbors of routing changes and will not send the full routing information database unless manually invoked.

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