The Basics of EIGRP

About the Protocol

• The algorithm used for this advanced Distance Vector protocol is the Diffusing Update Algorithm.
• The metric is based upon Bandwidth and Delay values.
• For updates, EIGRP uses Update and Query packets that are sent to a multicast address.
• Split horizon and DUAL form the basis of loop prevention for EIGRP.
• EIGRP is a classless routing protocol that is capable of Variable Length Subnet Masking.
• Automatic summarization is on by default, but summarization and filtering can be accomplished anywhere inside the network.

Neighbor Adjacencies

EIGRP forms “neighbor relationships” as a key part of its operation. Hello packets are used to help maintain the relationship. A hold time dictates the assumption that a neighbor is no longer accessible and causes the removal of topology information learned from that neighbor. This hold timer value is reset when any packet is received from the neighbor, not just a Hello packet.

EIGRP uses the network type in order to dictate default Hello and Hold Time values:

• For all point-to-point types – the default Hello is 5 seconds and the default Hold is 15
• For all links with a bandwidth over 1 MB – the default is also 5 and 15 seconds respectively
• For all multi-point links with a bandwidth less than 1 MB – the default Hello is 60 seconds and the default Hold is 180 seconds

Interestingly, these values are carried in the Hello packets themselves and do not need to match in order for an adjacency to form (unlike OSPF).

Reliable Transport

By default, EIGRP sends updates and other information to multicast 224.0.0.10 and the associated multicast MAC address of 01-00-5E-00-00-0A.
For multicast packets that need to be reliably delivered, EIGRP waits until a RTO (retransmission timeout) before beginning a recovery action. This RTO value is based off of the SRTT (smooth round-trip time) for the neighbor. These values can be seen in the show ip eigrp neighbor command.
If the router sends out a reliable packet and does not receive an Acknowledgement from a neighbor, the router informs that neighbor to no longer listen to multicast until it is told to once again. The local router then begins unicasting the update information. Once the router begins unicasting, it will try for 16 times or the expiration of the Hold timer, whichever is greater. It will then reset the neighbor and declare a Retransmission Limit Exceeded error.
Note that not all EIGRP packets follow this reliable routine – just Updates and Queries. Hellos and acknowledgements are examples of packets that are not sent reliably.