Question & Answer: How are EIGRP messages encapsulated? How are neighbor adjacencies formed using EIGRP?…..

EIGRP stands for Enhanced Interior Gateway Routing Protocol. EIGRP which is an advanced distance vector routing protocol has evolved from IGRP( Interior Gateway Routing Protocol) and it allows routers to share routing information with other routers and if the routing table entry in one router changes, it is automaticaly conveyed to other routers reducing the overhead on the network administrator who does not have make the changes in the routing table manually.

If a router is using EIGRP, it keeps a copy of the neighbour`s routing table,however if a routeto the destination is not found then the router queries the neighbour for the route, who in turn queries his neighbour for the route, if any changes are made in the routing table, they are updated automatically.

A router keep sending “hello” packets periodically to check the status of the neighbouring routers, if a router does not receive the “hello” packet,it is declared as inoperative.

Encapsulation Of EIGRP messages:

In an EIGRP message the data portion of the message is encapsulated in a packet.The data field is called as TLV(type,length,value), the information in the TLV relevant to EIGRP is EIGRP parameters,IP internal routes, IP external routes). In every EIGRP packet an EIGRP packet header is included, the TLV and EIGRP packet header are then attached in the IPv4 packet. In the IPv4 packet the field named protocol is set to 88 to indicate EIGRP and the IPv4 destination address is multicaste 224.0.0.10. given below is the diagram which shows how encapsulation is done.

Neighbour Adjacencies using EIGRP:

Given below is the step by step procedure of how two routers communicate using EIFRP

• Suppose there are two routers R1 and R2. as soon the two routers start using EIGRP, the R1 router sends a hello packet to R2 router.
• The R2 router on receiving the “hello” packet sends an “update” packet. The update packet has the initialisation bit attached signifying that it is the initialisation process. The neighbour adjacency will not be developed till the time R2 doesnot send a “hello” packet.
• The R2 sends a “hello”packet to R1 and the process for neighbour adjacency has started.
• After the “hello”packet is received by R1, it sends an ACK (acknowledge) packet to R2 to confirm that it has recieved the update packets, the R1 router updates the information in its Topology table.
• Now R1 sends the update packet to R2, R2 on receiving the packet will save the information in the toplogy table.