EIGRP stands for Enhanced Interior Gateway Routing Protocol. It is considered a hybrid routing protocol because it uses features from both link-state and distance vector routing protocols.
EIGRP is a Cisco proprietary routing protocol. Meaning that it can only run on Cisco routers.
EIGRP has an administrative distance of 90. EIGRP can also learn routes from other routing protocols. These are known as external routes and have an administrative distance of 170.
EIGRP Tables
EIGRP keeps track of three different types of tables. The neighbor table, topology table, and routing table.
The neighbor table gets populated by hello packets. Rather than sending out hello packets one at a time to every router, EIGRP sends them out as a multicast. Only when they are acknowledged are they unicast.
The topology table keeps track of many of the routers in the EIGRP network. While it is not a complete map of the topology like the name implies it does contain different routes to each network. These different routes are known as the successor and feasible successor.
Shortest-Path Calculation
EIGRP uses DUAL to determine loop free paths. Once DUAL calulates the successor route (or the best route) it gets placed in the routing table. A feasible successor (or backup route) is also calculated, but it does not get placed into the routing table. A backup route is determined so that fast convergence can take place and DUAL won’t have to run again incase the best route goes down for some reason.
Two of the main criteria that EIGRP uses to calculate the best route is the Bandwidth and Delay of the link. There are other criteria, but those two are the default.
How to Configure EIGRP
To configure EIGRP follow these steps:
Router EIGRP 100
Network 192.168.1.0 255.255.255.0
EIGRP uses Autonomous System (AS) Numbers. For each EIGRP router in the network you will need to make sure they all the the same AS number, in this case the AS number is 100. If you have configured EIGRP and it isn’t working, double check the AS numbers on each router.
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