Enhanced Interior Gateway Routing Protocol (EIGRP) is a distance-vector routing protocol. While Routing Information Protocol (RIP) is a commonly used distance-vector routing protocol for small networks, it begins to scale off in larger networks as its convergence times increase and it has a maximum hop count of 15. Open Shortest Path First (OSPF) is another routing protocol, but it is a link-state routing protocol, meaning it is a more complex protocol than a distance-vector protocol. EIGRP combines the advantages of both OSPF and RIP into a routing protocol that supports multiple network protocols, uses neighbour discovery and has a maximum hop limit of 100 to 255.
EIGRP discovers routers around itself using EIGRP by sending out Hello packets. When routers receive Hello packets from each other, they will compare their Autonomous System (AS) number. Routers can only from an EIGRP adjacency with other routers with the same AS number. An AS number determines if a router is within a single administrative domain. As EIGRP inherits a link-state protocols ability to calculate the best path, EIGRP routers will transmit metrics, called K-values, to each other.
The AS number system allows EIGRP routers to be in different autonomous systems at the same time, allowing large networks to be split up into smaller, more manageable networks. Routers with different AS numbers will not form neighbour adjacencies, and non-neighbours do not send routing information to each other. This feature allows routers to send smaller routing tables to each other with only information concerning their small network, instead of sending large routing tables back and forth.
EIGRP routers will only send out his information when a new neighbour is discovered as periodic updates are not sent out. When a new router is added to the network, updates are sent to the multicast address 18.104.22.168. These updates are assigned a sequence number that neighbouring routers must acknowledge. Using the sequence numbers, the router will then be able to determine which routers have and have not responded with an acknowledgement. If a neighbouring router does not respond to the multicast, 16 unicast messages will be sent and if there is still no response, the neighbour will be declared dead.
EIGRP uses multiple metrics to calculate the best path for packets to travel. EIGRP uses bandwidth, delay, load and reliability to compute the cost of each path and chooses the lowest-cost path to send packets. If the best-route is no longer available, the EIGRP router will scan its topology table for a backup path and use that instead. If no backup path can be determined, the router will contact its neighbours to find an alternate path.
EIGRP is a routing protocol combining advantages of RIP and OSPF into a protocol that can find a best path to send packets with using metrics as well as reduce bandwidth and workload usage on a router and its neighbours. It also allows administrators to split large networks up into smaller groups to reduce the workload on resident routers.