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Link-state routing protocols offer greater
scalability and faster convergence than distance-vector protocols such as RIP
and IGRP. Unfortunately, these advantages come at a price. Link-state protocols
require more memory and processing power from the router, and more knowledge and
expertise from the administrator than do distance-vector protocols.
Link-state protocols are based on the Dijkstra
algorithm, sometimes referred to as the Shortest Path First (SPF) algorithm. The
most common link-state routing protocol, Open Shortest Path First (OSPF), is
examined in Chapter 4, OSPF in a Single Area, and Chapter 5, Multiarea OSPF.
Routers running a link-state protocol, such as OSPF, are concerned with the
states (for example, up or down) of links (interfaces on other routers) in the
network. A link-state router builds a complete database of all the link states
of every router in its area. In other words, a link-state router gathers enough
information to create its own map of the network. Each router then individually
runs the SPF algorithm on its own map, or link-state database, to identify the
best paths to be installed in the routing table. These paths to other networks
form a tree with the local router as its root. 
Instead of learning routes and then broadcasting the routes with incremented
distances to neighbors, link-state routers advertise the states of their links
to all other routers in the area so that each router can build a complete
link-state database. These advertisements are called link-state advertisements (LSAs).
Unlike distance-vector routers, link-state routers can form special
relationships with their neighbors and other link-state routers, to ensure that
the LSA information is properly and efficiently exchanged. 
After an initial flood of LSAs provides routers
with the information that they need to build a link-state database, routing
updates occur only when a link-state changes, or, if no changes have occurred,
after a specific interval. If a link state changes, a partial update is sent
immediately. The partial update contains only link states that have changed, not
a complete routing table. An administrator concerned about WAN link utilization
will find these partial and infrequent updates an efficient alternative to
distance-vector routing, which sends out a complete routing table every minute
or so. Moreover, when a change occurs, link-state routers are all notified
immediately by the partial update. Distance-vector routers have to wait for
neighbors to note the change, increment the change, and then pass it on to the
next neighbor down the line.
The benefits of link-state routing include faster
convergence and improved bandwidth utilization over distance-vector protocols.
Link-state protocols support Classless Inter-Domain Routing (CIDR), VLSM, and
supernetting. This makes them a good choice for complex, scalable networks. In
fact, link-state protocols generally outperform distance-vector protocols on any
size network. So why are not link-state protocols used exclusively for routing?
Link-state protocols have two major disadvantages:
- Link-state routing may overtax low-end
hardware. Link-state routers require more memory and processing power than
distance-vector routers, which potentially makes link-state routing
cost-prohibitive for organizations with tight budgets and legacy hardware.
- Link-state protocols require complex
administration. Configuring link-state routing can be a daunting task, and
many administrators prefer to avoid its complexity and stick to
distance-vector routing. Even capable administrators may opt for a
straightforward distance-vector protocol on simple networks.
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