How does EIGRP Neighbor Adjacency begin?

The process of EIGRP (Enhanced Interior Gateway Routing Protocol) neighbor adjacency begins with the exchange of hello packets. When EIGRP is enabled on an interface, it starts by sending out hello packets to discover and establish relationships with neighboring routers. These hello packets are crucial for routers to identify which neighbors are present on the same network segment and to monitor their status.

Once a router receives a hello packet from another EIGRP-enabled router, it can respond with its own hello packets, leading to the establishment of the neighbor adjacency. This step is fundamental for the EIGRP operation since it lays the groundwork for further protocol exchanges, such as the sharing of routes and updates. The hello protocol includes parameters like the hold time, which determines how long a router will consider a neighbor to be valid without receiving a hello packet.

In contrast, sending query packets or update packets involves subsequent actions that occur after the adjacency has been established. Query packets are used for route discovery when a router needs additional information, while update packets share routing information with neighbors. Establishing ACK packets helps assure reliable delivery of messages, which is part of the EIGRP's enhanced reliability but occurs after the neighbor relationship has been confirmed. Therefore, the process initiates with the hello