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Distance Vector Protocols
All routing protocols use an algorithm or set of algorithms to calculate
anddistribute routing information. A distance vector protocol (DVP) is
a dynamic routing protocol that uses an algorithm based on the work
of Bellman, Ford, and Fulkerson known as vector distance or the
Bellman-Ford algorithm. RIPv1, RIPv2, and Cisco's Interior Gateway
Routing Protocol (IGRP) are considered DVPs.
The idea behind the DVP algorithm is that each router on the network
(routing domain or process) compiles a list of the networks it can
reach and sends the list to their directly connected neighbors. The
routers then create routing tables based on what they can reach
directly and indirectly, using their neighbor routers as gateways. If
multiple paths exist, the router only keeps the best one. This route is
chosen based on the protocol's particular metric for determining the
best route. Routing protocols each use different route metrics to
choose the best route.
Link State Protocols
Link state protocols (LSPs) are based on a different algorithm than
DVPs. LSPs are based on a graph theory algorithm called Dijkstra's
algorithm, named after its creator E.W. Dijkstra. The algorithm works
like this: The premise is that a collection of points is seen as a tree;
one point is the base and the shortest path to all other points as the
pathway. The cost of the path is the sum of the path costs between
the source point and the terminus point. Whereas DVPs are interested
in the shortest route, LSPs are interested in the operational status of
all of the links in the network. Each host compiles a "map" of the
routing domain from its own point of view.
From a practical perspective, LSPs and DVPs have some similarities.
In both protocols, the router comes online and finds other routers as
"neighbors" that are directly accessible from its connected network
interfaces and part of the same process. The router then sends out a
message known as a link state announcement (LSA) from each of its
network interfaces. The LSA contains information about the router's
interfaces: their cost metric, operational state, and which routers are
reachable from them. The directly accessible routers then forward the
LSA to other neighbors. LSAs are saved together as a link state
database. The router uses the database to compute its own routing
table.