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Routing protocols may be classified as either
distance-vector or link-state routing protocols. These classifications describe
the algorithm, or formula, that routers use to calculate and exchange routing
information. Distance-vector routing protocols are based on the Bellman-Ford
algorithm (also known as a distance-vector algorithm).
Routers configured to use a distance-vector
routing protocol typically send their complete routing table at regular
intervals to neighbor routers.
In fact, simple distance-vector protocols, such as RIP and IGRP, broadcast (or
multicast) their routing table out all configured interfaces, by default.
Routers that use these protocols do not actually identify their neighbors for
direct communication.
A neighbor router receiving the broadcast update examines it and compares the
information to its current routing table. Routes to new networks, or routes to
known networks with better metrics, are inserted in the table. The neighbor then
broadcasts its routing table, which includes any updated routes.
Distance-vector routing protocols are concerned
with the distance and vector (direction) of destination networks. Before sending
an update, each router adds its own distance value to the route's metric. When a
router receives an update, it maps the learned network to the receiving
interface. The router then uses that interface to reach those destinations. 
Simple distance-vector routing protocols enjoy
two major benefits over link-state protocols. They are relatively easy to
configure, and they generally use less memory and processing power. RIPv1 has
the added advantage of almost universal support among all routing software and
is often used as a common denominator in mixed-vendor or legacy routing
environments.
Simple distance-vector routing protocols do not
scale as well as their link-state counterparts. RIPv1 and IGRP are classful
routing protocols (they do not send subnet information in updates), so they can not
support scalability features such as Variable Length Subnet Masking (VLSM) or
supernetting. In general, simple distance-vector routing protocols converge more
slowly than link-state protocols. Because complex and scalable internetworks
demand routing protocols that are quick to achieve convergence (when all routers
agree on the state of the network's topology), distance-vector protocols often
are not appropriate. Finally, RIP restricts networks from growing beyond 15 hops
between any two destinations, a limitation that proves too stifling for today's
large networks. IGRP overcomes this limitation by supporting a 255-hop maximum.
However, IGRP is a Cisco-proprietary protocol and therefore cannot support a
multi-vendor routing environment.
Because of the limitations of simple
distance-vector routing protocols, network administrators often turn to
link-state routing in complex internetworks.
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