EIGRP Messages

EIGRP uses various message types to initiate and maintain neighbor relationships, and to maintain an accurate routing table. It is designed to conserve bandwidth and router resources by sending messages only when needed and only to those neighbors that need to receive them.

Packet Types:

EIGRP uses five Packet types:

• Hello: Identifies neighbors and serves as a keepalive mechanism
• Update: Reliably sends route information
• Query: Reliably requests specific route information
• Reply: Reliably responds to a query
• Ack: Acknowledgment

EIGRP is reliable, but hellos and ACKs are not acknowledged. The acknowledgment to a query is a reply.

If a reliable packet is not acknowledged, EIGRP periodically retransmits the packet to the nonresponding neighbor as a unicast. EIGRP has a window size of one, so no other traffic is sent to this neighbor until it responds. After 16 unacknowledged retransmissions, the neighbor is removed from the neighbor table.

Neighbor Discovery and Route Exchange:

When EIGRP first starts, it uses hellos to build a neighbor table. Neighbors are directly attached routers that have a matching AS number and k values. (The timers don't have to agree.) The process of neighbor discover and route exchange betwork two EIGRP routes is as follows:

• Step 1: Router A sends out a hello.
• Step 2: Router B sends back a hello and an update. The update contains routing information.
• Step 3: Router A acknowledges the update.
• Step 4: Router A sends its update.
• Step 5: Router B acknowledges.

When two routers are EIGRP neighbors, they use hellos between them as keepalives. Additional route information is sent only if a route is lost or a new route is discovered. A neighbor is considered lost if no hello is received within three hello periods (called hold-time). The default hello/hold timers are as follows:

• 5 Seconds/15 Seconds for multipoint circuits with bandwidth greater than T1 and for Point-to-point media.
• 60 Seconds/180 seconds for multipoint circuits with bandwidth less than or equal to T1.

 The Exchange process can be viewed using debug ip eigrp packets, and the update process can be seen using debug ip eigrp. the neighbor table can be seen with the command show ip eigrp neighbors.