EIGRP

Today I am going to be talking about EIGRP which stands for Enhanced Interior Gateway Protocol. EIGRP is an dynamic IGP that is an advanced distance-vector protocol. EIGRP is viewed as a “hybrid” protocol because it has features of both distance-vector and link-state protocols. EIGRP has an AD of 90 and uses a composite metric formula that mostly considers bandwidth and delay, with optional considerations like load and reliability. 

Feasible/Reported Distance- EIGRP consists of a main route which is a successor route and a backup route which is a feasible successor route. In order to understand how the successor and the feasible successor routes are chosen we must first know what a feasible distance and a reported distance is. In EIGRP the feasible distance is the metric that has proven to be the best possible route to the destination according to EIGRP’s composite metric. Feasible distances are calculated with the neighbors reported distance which they share. So what is a Reported distance? Reported distance neighbors metric to reach a destination host. This is essentially the neighbors feasible distance to reach the destination host from its own perspective. When a neighbor calculates their path to reach a destination (reported distance), they share that information with the directly connected routers. As you can see this is the same thing a distance vector protocol does such as RIP but the difference is EIGRP only shares this metric instead of the whole routing table. When the directly connected routers receive this neighbor’s reported distance, they take that reported distance and calculate the cost of the link to reach that router that sent the reported distance. From there the local router adds the cost to reach that neighbor plus the reported distance (cost to reach neighbor + reported distance), and this will be the feasible distance for that local router. This process occurs hop-by-hop as routers learn the route from neighbors and calculate their own feasible distance. 

Successor- So now that we know what feasible distance and reported distance are, I can now talk about successors and feasible successors. A successor is the next hop router that provides the best path or metric to a destination network/host. The successor is determined by the feasible distance. Since the feasible distance is the best route based on EIGRP’s composite metric, the next hop router for that feasible distance route is the successor. Whenever a packet for a destination host reaches that router, it will do a routing table lookup, look at the destination network, and choose the successor (next hop router) to forward the traffic to. As you can see, this is the vector part of the distance-vector when it chooses the direction or next-hop of the packet.

Feasible Successor- A feasible successor on the other hand is the backup route calculated that satisfies the feasibility condition. It is a backup route that is stored in the topology table, whereas the successor route is stored in the routing table. This route is waiting on standby, so if anything on the successor route fails like a link going down, the feasible successor is already calculated to take over with a guarantee that it will not cause any routing loops. The feasible successor is basically the next-hop router that provides the backup path to a destination. In order for a router to become a feasible successor, it has to meet a certain rule which is called a “feasibility condition.” This is a rule in order to make sure that if this router was to become the successor, it will not cause any routing loops. The feasibility condition has one strict rule and that is that the reported distance must be less than the feasible distance (RD < FD). If this condition is not met then that router will not be able to become the feasible successor. This condition ensures that the feasible successor is closer to the destination than the local router, and if that is the case then routing loops will not occur. The feasible successor route allows EIGRP to have a convergence time of literally a couple seconds, as this backup route is on standby ready to take over if something fails. 

Tables- EIGRP’s choice of algorithm for calculating the best routes is DUAL (Diffusing Update Algorithm), this algorithm ensures loop-free paths and very fast convergence. EIGRP also shows similarities with OSPF (link-state protocol) by having 3 tables, a neighbor table, topology table (where feasible successor routes are), and routing table (where successor route is installed.)