EIGRP Messages
EIGRP uses various message types to
- Initiate and maintain neighbor relationships
- To maintain an accurate routing table.
- In order to conserve bandwidth and router resources it sends messages only when needed, and those neighbors that need to receive them.
Packet Types
EIGRP uses five packet types:
- Hello—Multicast for neighbor discovery/recovery.Identifies neighbors and serves as a keepalive mechanism.Require no acknowledgment. A hello with no data is also used as an acknowledgment (ack). Acks are always sent using a unicast address and contain a non-zero acknowledgment number.
- Update—Reliably sends route information.Updates are used to convey reachability of destinations. When a new neighbor is discovered, update packets are sent so the neighbor can build up its topology table. In this case, update packets are unicast. In other cases, such as a link cost change, updates are multicast. Updates are always transmitted reliably.
- Query—Reliably requests specific route information.Queries are sent when destinations go into Active state. Queries are always multicast unless they are sent in response to a received query. In this case, it is unicast back to the successor that originated the query. Queries are transmitted reliably.
- Reply—Reliably responds to a a query.Replies are sent when destinations go into Active state.Replies are always sent in response to queries to indicate to the originator that it does not need to go into Active state because it has feasible successors. Replies are unicast to the originator of the query. Both queries and replies are transmitted reliably.
- ACK—Acknowledgment. Request packets are used in route server applications. They can be multicast or unicast. Requests are transmitted unreliably.
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