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Chapter 6
number of routes it can store in its table, or even the inability to communi-
cate with other protocols, then you have a protocol that will likely hinder the
growth of your network.
All the issues we've brought up so far are general scalability consider-
ations. Before we look at IGRP and EIGRP, let's take another look at the dif-
ferences between link-state routing protocols and distance-vector protocols
and the scalability issues of each.
Link-state routing and distance-vector protocols are discussed in detail in
Chapter 2, and are discussed in Chapter 7 as they relate to BGP.
Distance-Vector Protocol Scalability Issues
In small networks--meaning those with fewer than 100 routers and an envi-
ronment that's much more forgiving of routing updates and calculations--
distance-vector protocols perform fairly well. However, you'll run into sev-
eral problems when attempting to scale a distance-vector protocol to a larger
network--convergence time, router overhead (CPU utilization), and band-
width 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 con-
nected 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. So
when a topology change occurs, it could take up to 90 seconds before a
neighboring router realizes what's happened. When the router finally recog-
nizes the change, it recalculates its routing table and sends the whole thing
out to all its neighbors.
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
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