EIGRP fundamentals

• Last Updated : 05 Aug, 2021

Dynamic routing Protocol performs the same function as static routing Protocol does. In dynamic routing Protocol, if the destination is unreachable then another entry, in the routing table, to the same destination can be used. One of the routing protocols is EIGRP. 
EIGRP: 
Enhanced Interior Gateway Routing Protocol (EIGRP) is a dynamic routing protocol that is used to find the best path between any two-layer 3 devices to deliver the packet. EIGRP works on network layer Protocol of OSI model and uses protocol number 88. It uses metrics to find out the best path between two layer 3 devices (router or layer 3 switches) operating EIGRP. Administrative Distance for EIGRP are:- 
 

EIGRP routesAD valuesSummary Routes5Internal Routes90external routes170

It uses some messages to communicate with the neighbour devices that operate EIGRP. These are:- 

1. Hello message-These messages are kept alive messages which are exchanged between two devices operating EIGRP. These messages are used for neighbour discovery/recovery, if there is any device operating EIGRP or if any device(operating EIGRP) coming up again. 
   These messages are used for neighbor discovery if multicast at 224.0.0.10. It contains values like AS number, k values, etc. 
   These messages are used as acknowledgment when unicast. A hello with no data is used as the acknowledgment.
2. NULL update-It is used to calculate SRTT(Smooth Round Trip Timer) and RTO(Retransmission Time Out). 
   SRTT: The time is taken by a packet to reach the neighboring router and the acknowledgment of the packet to reach the local router. 

   RTO: If a multicast fails then unicast is being sent to that router. RTO is the time for which the local router waits for an acknowledgment of the packet.

3. Full Update – After exchanging hello messages or after the neighbourship is formed, these messages are exchanged. This message contains all the best routes.
4. Partial update-These messages are exchanged when there is a topology change and new links are added. It contains only the new routes, not all the routes. These messages are multicast.
5. Query message-These messages are multicast when the device is declared dead and it has no routes to it in its topology table.
6. Reply message – These messages are the acknowledgment of the query message sent to the originator of the query message stating the route to the network which has been asked in the query message.
7. Acknowledgement message 
   It is used to acknowledge EIGRP updates, queries, and replies. Acks are hello packets that contain no data. 

   Note:-Hello and acknowledgment packets do not require any acknowledgment. 
   Reply, query, update messages are reliable messages i.e require acknowledgement.

Composite matrix-The EIGRP composite metric calculation can use up to 5 variables, but only 2 are used by default (K1 and K3). The composite metric values are : 

K1 (bandwidth) 
K2 (load) 
K3 (delay) 
K4 (reliability) 
K5 (MTU) 

The lowest bandwidth, load, delay, reliability, MTU along the path between the source and the destination is considered in the composite matrix in order to calculate the cost. 
Note:- Generally, only k1 and k3 values are used for metric calculation by EIGRP. The values are 10100 for k1, k2, k3, k4, k5 respectively. 
criteria To form EIGRP neighbourship, these criteria should be fulfilled:- 
 

1. k values should match.
2. Autonomous system number should match. (AS is a group of networks running under a single administrative control) .
3. authentication should match (if applied). EIGRP supports MD5 authentication only.
4. subnet mask should be the same.

Timers:- 
Hello timer- The interval in which EIGRP sends a hello message on an interface. It is 5 seconds by default. 
Dead timer- The interval in which the neighbor will be declared dead if it is not able to send the hello packet. It is 15 seconds by default.