File:EIGRP Neighbor Discovery sequence diagram - en.png

English: Sequence diagram fro Neighbor discovery in EIGRP.

(1) Router A sends a multicast HELLO and Router B discovers it. (2) Router B sends an expedited HELLO and starts the process of sending its topology table to Router A. In addition, Router sends the NULL UPDATE packet with the INIT-Flag. The second packet is queued, but it cannot be sent until the first is acknowledged. (3) Router A receives the first UPDATE packet and processes it as a DUAL event. If the UPDATE contains topology information, the packet will be processed and stored in a topology table. Router B sends its first and only UPDATE packet with an accompanied ACK. (4) Router B receives UPDATE packet 100 from Router A. Router B can dequeue packet 10 from A's transmission list since the UPDATE acknowledged 10. It can now send UPDATE packet 11 and with an acknowledgment of Router A's UPDATE. (5) Router A receives the last UPDATE packet from Router B and acknowledges it. The acknowledgment gets lost. (6) Router B later retransmits the UPDATE packet to Router A.

(7) Router A detects the duplicate and simply acknowledges the packet. Router B dequeues packet 11 from A's transmission list, and both routers are up and synchronized.