EIGRP doesn’t send messages with UDP or TCP; instead, a Cisco’s protocol called Reliable Transport Protocol (RTP) is used for communication between EIGRP-speaking routers. As the name implies, reliability is a key feature of this protocol, and it is designed to enable quick delivery of updates and tracking of data reception.
Five different packets types are used by EIGRP:
- Update – contains route information. When routing updates are sent in response to the metric or topology changes, reliable multicasts are used. In the event that only one router needs an update, for example when a new neighbor is discovered, unicasts are used.
- Query – a request for specific routes that always uses the reliable multicast method. Routers send queries when they realize they’ve lost the path to a particular network and are looking for alternative paths.
- Reply – sent in response to a query via the unicast method. Replies can include a specific route to the queried destination or declare that there is no known route.
- Hello – used to discover EIGRP neighbors. It is sent via unreliable multicast and no acknowledgment is required.
- Acknowledgment (ACK) – sent in response to an update and is always unicast. ACKs are not sent reliably.
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