IGP: Enhanced Interior Gateway Routing Protocol (EIGRP) Overview

Enhanced Interior Gateway Routing Protocol (EIGRP) is a Cisco proprietary protocol. It uses It is a hybrid protocol because it has features of the Link State and Distance Vector routing protocols. Confused? If you’re new with IGP, it is natural. You are not alone. There are millions to billions of people around the world who doesn’t know what the heck is EIGRP.

EIGRP is Cisco’s baby. It works on all Cisco devices. It is an advanced distance vector routing protocol that has some link state features. Like RIP, it has the hop count feature. Routes that reached the maximum hop cunt will be tagged as unreachable. Although it is not used as its metric, it limits the EIGRP AS when routing to a remote network. The default hop count is 100 and the value can vary between 1 – 255. And, like OSPF it does not send the whole routing table when there is a routing change.

In addition, EIGRP supports Classless Inter-Domain Routing (CIDR) and variable length subnet masking. Like OSPF and RIP v2, routes are not summarized on classful boundaries. However, be careful with not putting the “no auto-summary” command under the router process. The “no auto-summary” command disables or prevents automatic summarization of subnet routes into network-level routes.

Remember that EIGRP uses Diffusing Update ALgorithm (DUAL) as its algorithm in every routing decisions, prevents routing loops, and it is used to respond to changes in the routing topology. It is developed by J.J. Garcia-Luna-Aceves and uses three tables for the route calculation: Neighbor table, Topology table, and Routing table.

Lastly, internal EIGRP has an administrative distance (AD) value of 90, the external has a value of 170, and the summary has a value of 5.

EIGRP RELATED TOPICS:

1. EIGRP Metrics
2. The EIGRP Neighbor Table
3. The EIGRP Topology Table
4. The EIGRP Routing Table
5. EIGRP Packet Types
6. EIGRP Neighbor Adjacency
7. EIGRP Successor, Feasible Successor and Feasibility Condition
8. EIGRP Packet Header
9. EIGRP Show Commands

EIGRP CONFIGURATION:

1. EIGRP Configuration
2. EIGRP Redistribution
3. Configuring “no auto-summary” eigrp summarization
4. EIGRP Authentication
5. EIGRP Network Advertisement
6. EIGRP Passive Interface
7. EIGRP Unequal Path Cost Load Balancing
8. Changing Administrative Distance