EIGRP (Enhanced IGRP) is a hybrid routing protocol. It has some characteristics from both the distance vector routing and the link state routing.
• It is a Cisco proprietary. It only works on Cisco devices.
• It is a classless routing protocol; this means that it supports the CIDR, the VLSM and the discontiguous networks.
• The maximum hop count is ‘255’.
• It uses ‘DUAL’ (Diffusion Update Algorithm) to find the best path to the networks.
• It can load balance between up to six unequal cost paths.
• It uses the ‘successor route’ and the ‘feasible successor route’ for fast convergence.
• The ‘successor route’ is the route that is installed in the routing table in order to use it to reach a certain network.
• The feasible successor route can be considered as a backup for the successor route.
The default metrics used by the EIGRP protocol to calculate the path cost are the ‘delay’ and the ‘bandwidth’.
However, EIGRP protocol can use the delay, the bandwidth, the load, and the reliability as the path cost metrics.
• The routers that are running EIGRP discover neighbors using the ‘hello’ messages.
• The routers must have the following conditions to form a neighborship,
• Only neighbors can exchange routes with each other using the multicast IP address ‘126.96.36.199’.
Routers that are running EIGRP contain three tables,
The neighbor table: it contains the EIGRP neighbors.
The topology table: it contains the ‘EIGRP topology, including the successor and the feasible successor routes.
The routing table: it contains the routes that are currently used to route the data.
To configure the EIGRP protocol on a router, we use the following commands,
Router(config)# router eigrp AS number
Router(config-router)# network IP address wildcard mask
Router(config-router)# no auto-summary
The ‘Administrative Distance’ (AD) is a number assigned to every routing protocol.
Figure (14.1), lists the ‘AD’ for every routing protocol,
Figure 14.1: the administrative distance
In figure (14.2),
Suppose that ‘R3’ has the RIP protocol and the OSPF protocol running on it.
The RIP protocol is telling R3 that, to reach the network ‘188.8.131.52/30’, send the data to R1.
The OSPF protocol is telling R3 that, to reach network ‘184.108.40.206/30’, send the data to R2.
Which path will R3 use to reach ‘220.127.116.11/30’?
It will use the path that was installed in the routing table by the routing protocol that has the lowest AD.
Because the RIP protocol’s AD is ‘120’. Moreover, because the OSPF protocol’s AD is ‘110’, R3 will use the path installed by the OSPF protocol.
Therefore, to reach the network ‘18.104.22.168/30’, R3 will send the data to R2.
In figure (14.3), suppose that R1 needs to send some data to a certain destination that exists in the internet. In addition, R1 searched for this destination in its routing table to determine the next hop. However, R1 did not find any entry for this destination in its routing table. What should R1 do?
In this case, R1 should send the data to the ‘default route’.
The default route is an entry in the routing table that is used by the router in case that it did not find any entry for the destination network in its routing table.
The default route entry in the routing table is ‘0.0.0.0’ and the subnet mask is ‘0.0.0.0’.
To configure the default route we use one of the following commands in the global configuration mode,
Router(config)# ip ute 0.0.0.0 0.0.0.0 next hop IP address
Router(config)# ip route 0.0.0.0 0.0.0.0 exit interface
Another method is to configure a default network is using the following command,
Router(confide)# ip default-network network IP address
In this method, the network IP must be reachable using one of the routing protocols.