Introduction to EIGRP
EIGRP otherwise known as the Enhanced Interior Gateway Routing Protocol is considered to be the CISCO’s second proprietary routing protocol that was released first in 1992. It was considered as a classless advancement to CISCO’s first proprietary routing protocol IGRP. Since this is a CISCO proprietary protocol, would be only running on the CISCO routers.

We are going to discuss some of the most notable features of EIGRP, but if you wish to have the better results, you are needed to go through the training courses which would be gained by joining the SPOTO

EIGRP Includes the Following:
RTP – Reliable Transport Protocol.
Bounded updates – The updates are only sent if and when there would be the change in topology and to affected routers.
It utilizes the DUAL algorithm (Diffusion Update Algorithm) so as to find the best paths.
It would be established adjacencies with neighboring routers.
It would be maintaining the neighbor table, the topology table, and the routing table.

The operation of EIGRP would be having some similarities with link-state routing protocols like the OSPF, however, it would be still a distance vector routing protocol.

EIGRP Packet Types
The packet types in EIGRP are quite crucial to understand about how it would be exchanging routing information. It is considered quite important to note that there are 5 packet types that EIGRP would be utilized to maintain adjacencies. Some of them are mentioned below, we could see that some of them come in pair.

Hello packets
The Hello packet is considered as the first packet that would be sent when EIGRP is configured, this packet is considered to be sent to discover neighbors and form adjacencies with those neighbors. The hello packet would be usually sent in the intervals of 60 seconds on slow links and at intervals of 5 seconds on links with a bandwidth that would be exceeding 1.544mbps like the T1 links.

Update
In EIGRP, the update packets are utilized to send routing information to its neighbors. The routing updates that are sent by EIGRP would be usually partial and bounded. This means that unless there would be is a change in topology, the updates wouldn’t usually be sent. When a topology change would be detected, the EIGRP update packets are might be either sent as a unicast to a single affected router or it would be sent as a multicast to several affected routers in the routing domain.

Acknowledgment (ACK)
The ACK packets would be utilized in order to verify that updates or other types of messages were received or not. RTP would be ensuring that ACK messages are delivered by utilizing Reliable delivery.

Query and reply packets
When a router is considered to be missing a route, it would be the work of the query and reply packets so as to probe neighbors for the missing routes. The queries wouldn’t be usually sent as multicast messages, while the replies would be usually unicast messages.

EIGRP and routes propagation
When routers are being configured with EIGRP boot up, the hello messages would be usually sent to all the routers in the domain so as to form adjacencies, when the neighbors reply, they would be forming neighbor relationships. The routers then would be sending updates which would be containing their information to the neighbors. When this is done, the routers would be drawing up a topology table with all the best routes and alternative or backup paths. From this, the best path is determined and utilized for packet forwarding.

So, these were the few things that you needed to know about the Layer 3 EIGRP. If you wish to learn more, you could gain it through the training courses provided by the SPOTO.