Enhanced Interior Gateway Routing Protocol - EIGRP Classification As A Distance-vector

EIGRP Classification As A Distance-vector

In the past, EIGRP was described in various Cisco marketing materials as a balanced hybrid routing protocol, allegedly combining the best features from link-state and distance-vector protocols. This description is not correct from a principal point of view. By definition:

• Distance-vector routing protocols are based on a distributed form of Bellman-Ford algorithm to find shortest paths. They work by exchanging a vector of distances to all destinations known to each node. No further topological information is ever exchanged. Thus, each node knows about all destinations present in the network and it knows the resulting distance to each destination via every of the node's neighbors. However, the node does not have any idea of the actual network topology, nor does the node need it.
• Link-state routing protocols are based on algorithms to find shortest paths in a graph (the most often used algorithm is Dijkstra's algorithm). They work by exchanging a description of each node and its exact connections to its neighbors (in essence, each node describes its adjacencies to neighboring nodes and this information is flooded throughout the network). Therefore, each node knows the exact network topology, i.e. it has a graph representation of the network. Using this graph, each node computes the shortest paths from itself to each available destination.

The EIGRP routers exchange messages that contain information about bandwidth, delay, load, reliability and MTU of the path to each destination as known by the advertising router. Each router uses these parameters to compute the resulting distance to a destination. No further topological information is present in the messages. This principle fully corresponds to the operation of distance-vector protocols. Therefore, EIGRP is in essence a distance-vector protocol.

It is true that EIGRP uses a number of techniques not present in naive distance-vector protocols, notably

• the use of explicit hello packets to discover and maintain adjacencies between routers;
• the use of a reliable protocol to transport routing updates;
• the use of a feasibility condition to select a loop-free path;
• the use of diffusing computations to involve the affected part of network into computing a new shortest path.

None of these techniques, however, makes any difference to the basic principles of EIGRP, which exchanges a vector of distances to each known destination network without full knowledge of the network topology, and, as a matter of fact, similar techniques have been used in other distance-vector protocols (notably DSDV, AODV and Babel). While EIGRP is indeed an advanced distance-vector routing protocol, it is not a hybrid protocol.