EIGRP (Enhanced Interior Gateway Routing Protocol) is a feature-rich routing protocol with a unique vocabulary and set of concepts you must master in order to pass the Cisco Certified Network Professional (CCNP ®) ROUTE exam.
The first component and process in EIGRP routing is setting up neighbor relationships. A router sets up relationships with other routers within its reach and with the same Autonomous System (defined by an Autonomous System Number). The ASN defines a domain of routers under a central authority’s control and identifies an instance of EIGRP.
These neighbor relationships are formed by a series of messages that the routers exchange with each other. RTP (Reliable Transport Protocol) provides sequencing and acknowledgement between EIGRP packets. Below are six types of messages that EIGRP routers exchange with the goal of setting up neighbor relationships:
The Hello packet is used to discover neighbors and keeps neighbor relationships alive. Hellos are sent to multicast address 126.96.36.199, and are therefore received by all EIGRP-enabled routers on a subnet with a single transmission.
Routers reply with Acknowledge packets. An Acknowledgement is simply a Hello with no data. Hello and Acknowledge packets do not use RTP. Rather, they are IP transmissions with the protocol type field set to 88, because EIGRP doesn’t need to know if the Hellos were received. A Hello packet is sent to 188.8.131.52 with the request “Are there any neighbors for my AS?” to any router willing to accept it. The packet then listens for replies in the form of an Acknowledgement with an Update containing the answering router’s topology table.
Update packets are sent to neighbors when changes occur. Most of the time, the Update message contains only route changes. The exception to this rule is when a new neighbor is brought up.
First, think of the Update as still taking place, except that the Update contains all the EIGRP routing information instead of just a single change.
Second, an initial Update is a unicast response to a neighbor attempting to form a relationship instead of a multicast. This is because, initially, Updates are destined as a reply to form specific neighbor relationships. If R2 is the router that is responding to R1’s initial Hello, R2 will send its entire table as an Update. R1 will then reply with an Acknowledgement and a unicast Update of its own table destined for R2. R2 will finalize this process with a final Acknowledgement.
After Acknowledgement, the relationship is established. When a route is added, Update packets are multicast to advertise the update to neighbors.
After neighbors are established, the multicast Query packet is sent when a route goes down and no backup route is available.
The Query packet is answered by a unicast Reply packet, which is returned by a neighboring router to provide a new route to replace the failed one.
The final message type is the Goodbye message. The Goodbye message allows a router to notify its neighbors when it is going down.
Understanding these six types of messages will certainly help you answer questions on Cisco’s CCNP ® ROUTE exam that relate to EIGRP and the neighbor-creation process.