EIGRP Routing Protocols – The full form of EIGRP is Enhance Interior Gateway Routing Protocol (EIGRP) is an advanced distance-vector routing protocol that is used on a computer network to help automate routing decisions and configuration. The protocol was designed by Cisco Systems as a proprietary protocol, available only on Cisco routers. EIGRP is used on a router to share routes with other routers within the same autonomous system. Unlike other well-known routing protocols, such as RIP, EIGRP only send incremental updates, reducing the workload on the router and the amount of data that needs to be transmitted. Routers running EIGRP must become neighbors before exchanging routing information. To dynamically discover neighbors, EIGRP routers use the multicast address of 224.0.0.10. Each EIGRP router stores routing and topology information in three tables.
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Neighbor table – stores information about EIGRP neighbors
Topology table – store routing information learned from neighboring routers
Routing table – stores the best routes
EIGRP neighbors: EIGRP must establish a neighbor relationship with other EIGRP neighboring routers before exchanging routing information. To establish neighbor relationships, routers send hello packets every couple of seconds. Hello, packets are sent to the multicast address of 224.0.0.10.
The following fields in hello packets must be identical in order for routers to become neighbors:
- ASN (autonomous system number)
- Subnet number
- K values (component of metric)
Feasible and reported distance: Feasible distance (FD) is the metric of the best route to reach a network. That route will be listed in the routing table. Reported Distance (RD) is the metric advertised by a neighboring router for a specific route. In other words, it is the metric of the route used by the neighboring router to reach the network.
Successor and feasible successor: A successor is a route with the best metric to reach a destination. The route is stored in the routing table. A feasible successor is a backup path to reach that same destination that can be used immediately if the successor route fails. These backup routes are stored in the topology table.
EIGRP topology table: EIGRO topology table contains all learned routes to a destination, the table holds all routes received from a neighbor, successors, and feasible successors for every route, and the interface on which updates were received. The table also holds all locally connected subnets included in an EIGRP process.
Table of Contents
EIGRP divided into two routes
Internal EIGRP route: These routes are developed within a specific autonomous system by EIGRP routes that are members of the same autonomous system. The AD of an internal EIGRP is 90.
External EIGRP route: These routes are developed within EIGRP routing tables by manual or automatic redistribution, and they represent networks that are originated outside of the EIGRP autonomous system. It has an AD of 170.
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EIGRP Routing Protocols composite and vector metrics
Bandwidth: Minimum bandwidth is in kilobits per second along the path from router to destination network.
Load: Number in range 1 to 255; 255 being saturated
Total delay: Delay, in 10s of microseconds, along the path from router to destination network.
Reliability: Number in range 1 to 255; 255 being the most reliable
MTU: Minimum path Maximum Transmission Unit is never used in the metric calculation
Hop count: Number of routers a packet passes through when routing to a remote network, used to limit the EIGRP AS. EIGRP maintains a hop count forever route; however, the hop count is not used in the metric calculation, by default it is set to 100 and can be changed to any value between 1 and 255.
Syntax / EIGRP Routing Protocol Packet Tracer:
Router(config)# router eigrp 1
Router(config-router)# network x.x.x.x
Router(config-router)# network x.x.x.x
Configuration of eigrp on router A is
Router(config)# router eigrp 1
Router(config-router)# network 10.0.0.0
Router(config-router)# network 172.16.0.0
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