Distance-Vector Protocol Scalability Issues

Distance-Vector Protocol Scalability Issues
In small networks—meaning those with fewer than 100 routers and an environment that’s much
more forgiving of routing updates and calculations—distance-vector protocols perform fairly
well. However, you’ll run into several problems when attempting to scale a distance-vector protocol
to a larger network—convergence time, router overhead (CPU and memory utilization), and
bandwidth utilization all become factors that hinder scalability.
A network’s convergence time is determined by the ability of the protocol to propagate
changes within the network topology. Distance-vector protocols don’t use formal neighbor
relationships between routers. A router using distance-vector algorithms becomes aware of a
topology change in two ways:

When a router fails to receive a routing update from a directly connected router

When a router receives an update from a neighbor notifying it of a topology change somewhere
in the network
Routing updates are sent out on a default or specified time interval. When a topology change
occurs, it could take up to 90 seconds before a neighboring router realizes the change. When the
router finally recognizes the change, it recalculates its routing table and sends the whole routing
table out all physical interfaces.
Not only does this cause significant network convergence delay, it also devours bandwidth—
just think about 100 routers all sending out their entire routing table and imagine the impact on
your bandwidth. It’s not exactly a sweet scenario, and the larger the network, the worse it gets,
because a greater percentage of bandwidth is needed for routing updates.
As the size of the routing table increases, so does CPU utilization, because it takes more processing
power to calculate the effects of topology changes and then converge using the new
information. Also, as more routes populate a routing table, it becomes increasingly complex to
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Chapter 4 
IGRP and EIGRP
determine the best path and next hop for a given destination. The following list summarizes the
scalability limitations inherent in distance-vector algorithms:

Network convergence delay

Increased CPU utilization

Increased bandwidth utilization