Enhanced Interior Gateway Routing Protocol is an advanced distance vector interior routing protocol 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 that supports variable length subnet mask (VLSM), sends routing updates to the multicast IP address, and supports fast convergence. The default administrative distance of EIGRP is 5 for summary routes, 90 for internal routes, and 170 for external routes.
EIGRP sends only hello packets over the stable network and only the changes in the routing table, instead of the entire routing table, to save the resources of a network. 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. It also supports route summarization and load balancing. We can also 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. 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.
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