EIGRP Reliable Transport Protocol

Five types of EIGRP packets exist, further categorized as reliable packets and unreliable packets. The reliable EIGRP packets are as follows :

• Update ‚ Update packets contain EIGRP routing updates sent to an EIGRP neighbor.

• Query ‚ Queries are sent to neighbors when a route is not available and the router needs to ask the status of the route for fast convergence.

• Reply ‚ Reply packets to the queries contain the status of the route being queried for.

The unreliable EIGRP packets are as follows:

• Hello ‚ Hello packets are used to establish EIGRP neighbor relationships across a link.

• Acknowledgment ‚ Acknowledgment packets ensure reliable delivery of EIGRP packets.

All the EIGRP packets are sent through EIGRP multicast address 224.0.0.10. Every EIGRP-enabled device automatically listens to the 224.0.0.10 address. Because this is a multicast address and multiple devices receive the EIGRP packets at once, EIGRP needs its own transport protocol to ensure reliable delivery of EIGRP packets. This protocol is the EIGRP Reliable Transport Protocol (RTP). The router keeps a transmission list for every neighbor. When a reliable EIGRP packet is sent to the neighbor, the sending router expects an acknowledgment to be sent back from the neighbor indicating that the reliable EIGRP packet has been received. EIGRP RTP maintains the transport window size of only one unacknowledged packet. Therefore, every single reliable packet must be acknowledged before the next reliable EIGRP packet can be sent out. The router retransmits the unacknowledged packet until an acknowledgment is received. If no acknowledgment is received, EIGRP RTP retransmits the same packet up to 16 times. If no acknowledgment is received after 16 retransmissions, EIGRP resets the neighbor relationship.

In a multiaccess LAN network, sending a multicast update could pose a problem if the transport window size is 1. As discussed previously, with reliable multicast traffic, the next reliable multicast packet is not transmitted until all peers have acknowledged the previous multicast packet. If one or more EIGRP neighbors in a multiaccess LAN network are slow or fail to acknowledge the EIGRP packet, all the other neighbors will suffer from this.

For example, if there are three routers on an Ethernet segment and Router 1 sends a multicast EIGRP update, it won't send another multicast EIGRP packet on the Ethernet until it receives an acknowledgment from the other two routers. Now assume that Router 2 successfully sends an acknowledgment packet to Router 1, but Router 3 has a problem sending the acknowledgment packet. Router 1 could potentially stop sending any more EIGRP packets, and Router 2 would be affected even though the problem lies on Router 3. EIGRP RTP avoids this problem by retransmitting the unacknowledged EIGRP packet as a unicast packet to the neighbor that has not acknowledged the previous EIGRP packet, and it continues to send EIGRP multicast packets to the neigh-bor that has already acknowledged the EIGRP packet. The router retransmits the unacknowledged EIGRP packet as a unicast 16 times to a neighbor. If the neighbor still has not acknowledged the EIGRP packet after 16 retries, EIGRP resets the neighbor relationship and the whole process starts over. The 16- retry timeout period usually runs from 50 to 80 seconds.