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RTP, Reliable Transport Protocol of EIGRP

RTP is a Cisco proprietry protocol, used in EIGRP to manage the communication of messages between EIGRP-Speaking Routers.Reliable Transport or Delivery of EIGRP packets meansAcknowledgement is required from the receiving Router and the packet should be delivered in order. Ordered delivery is ensured by including two sequence numbers in the packet. One sequence Number is assigned by the sending router for the packet order Number and that is incremented by one each time the sending Router sends a packet. Another Number is the Last Acknowledgement number received from destination or Neighbor Router.Why ordered delivery of EIGRP Packets is important?

Let me give you a simplest example to explain question. Suppose an EIGRP Router’s Link went down and up after some time. But the neighbor Router receives a message of Link-up first and then Link-Down after! What will happen? Out of order execution of packets, can result in corruption of the Routing Database.
Guaranteed delivery is accomplished by a Cisco Proprietary process called Reliable Multicast which uses the reserved class D address 224.0.0.10. Each neighbor receiving a reliable multicast pakcet must unicast an acknowledgment. If EIGP does not get a reply from a neighbor it Unicasts to that Number for sixteen (16) times, if still the Router does not get Reply then that Neighbor router is decalared as dead.

Some types of EIGRP packets are also sent unreliably, means NO Acknowledgement and Ordered-delivery is required for those packets.

Let’s see types of EIGRP Packets and whether they are delivered Reliably or Un-Reliably.
Types of EIGRP Pakcets that are delivered Reliably
  • Updates convey route information. Unlike RIP and IGRP updates, these packets are transmitted only when necessary, contain only necessary information, and are sent only to routers that require the information. When updates are required by a specific router, they are unicast. When updates are required by multiple routers, such as upon a metric or topology change, they are multicast. Updates always use reliable delivery.
  • Queries and Replies are used by the DUAL finite state machine to manage its diffusing computations. Queries can be multicast or unicast, and replies are always unicast. Both queries and replies use reliable delivery.
Types of EIGRP Packets that are delivered Un-reliably
  • Hellos are used by the neighbor discovery and recovery process. Hello packets are multicast and use unreliable delivery.
  • Acknowledgments (ACKs) are Hello packets with no data in them. ACKs are always unicast and use unreliable delivery.
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