EIGRP Router Initialization and Neighbor Adjacency Process

In the context of the initial router communication for the purpose of establishing an EIGRP neighbor adjacency, it is not necessary for routers to have a fully established adjacency to exchange update packets.

When router R1 sends a hello packet to R2, R2 can immediately respond with an update packet, which includes information about routes, but also an initialization bit set to 1. This indicates that the router is in the initialization phase, implying that a full neighbor relationship has not yet been established.

During this phase, routers begin synchronizing routing information. The process is symmetric:

  • From R1's perspective, it sends a hello to R2, receives an update in response, and then sends an acknowledgment (ACK).
  • From R2's perspective, it sends a hello to R1, receives an update in response, and then sends an ACK.

The initialization bit is crucial because it informs the receiving router that the update packet contains the entire routing table from the sender, which signifies the start of a fresh synchronization. This ensures the receiving router can process the packet appropriately with the understanding that it represents a complete initial exchange.

https://networklessons.com/eigrp/detailed-look-of-eigrp-neighbor-adjacency