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Sep 27

The EIGRP Explained

The EIGRP Explained

Enhanced Interior Gateway Routing Protocol (EIGRP) is an advanced distance vector interior routing protocol (IGP), and works on the calculation based on bandwidth and delay to find the best route to reach a subnet. It is a Cisco proprietary protocol, but in 2013 Cisco made it an open protocol so that it can be used on devices of other vendors, though its advanced features are still maintained and controlled by Cisco. It supports variable length subnet mask (VLSM), sends routing updates to the multicast IP address, and supports fast convergence. It supports both Ipv4 and Ipv6 networks. The default administrative distance of EIGRP is 5 for summary routes, 90 for internal routes, and 170 for external routes. EIGRP, when implemented, forms three tables – neighbor table, topology table, and routing table.

To save the resources of a network, EIGRP sends only hello packets over the stable network and only the changes in the routing table, instead of the entire routing table. It maintains a stable network topology based on the neighbour discovery and maintenance by sending hello packets at regular intervals. EIGRP also saves information about the feasible successor, in case the route from the successor path becomes unavailable. After a link fails, using the feasible successor, EIGRP can converge the network within milliseconds. It also supports route summarization and load balancing. Manual route summarization helps in improving the stability of the network and reduces the size of the routing table. It is the only routing protocol that supports unequal cost load balancing. In this, we can use variance to put traffic on a non-default path while load balancing. On EIGRP we can also set the pace for the packet transmission over the network to save the bandwidth. By default,  EIGRP uses 50 percent of the available bandwidth, which can however be changed by using the interface command ip bandwidth-percent eigrp 2 percentage.

For neighbour relations to form, the autonomous number (ASN number), the k value, subnet mask, and authentication, if applicable, must match. The k value is the metric of an EIGRP route and uses 5 variables – k1 (bandwidth), k2 (load), k3 (delay), k4 (reliability), and k5 (MTU). Out of these, only k1 and k3 are used by default. To check the EIGRP information exchanged between neighbouring routers use the command show ip eigrp traffic. It is very easy to deploy on a network. To enable EIGRP on a router, use the command router eigrp ASN number. After this specify the network available on the interfaces of the router using network subnet command. This command assumes a classful network by default. To enable EIGPR on a specific interface we can use network network-id wildcard-mask command. We can verify these steps by using show ip eigrp neibhors, show ip route, and show ip protocols command. The D character at the start of the route depicts the EIGRP route. To check all the entries in a topology table of a network use show ip eigrp topology network command. Auto-summary is enabled by default and must be disabled if using classless addresses. To disable auto-summary use the command no auto-summary. Despite all these advantages, EIGRP is still a distance vector routing protocol that depends on the routes provided by the directly connected neighbours and it does not support future application through opaque LSAs. With all its pros and cons, we can conclude that EIGRP is a very impactful routing protocol in real-time implementation.

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