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Chapter 2
Routing Principles
complete routing table entries out all active interfaces at periodic time inter-
vals. This can cause congestion on the slower WAN links.
Classless Routing
lassless routing protocols include the subnet mask information when
an update is sent. This allows different length subnet masks to be used on the
network, called Variable Length Subnet Masks (VLSM). You must use a
classless routing protocol if you want to have a network design like the one
shown in Figure 2.2.
F I G U R E 2 . 2
Classless network using VLSM
What the classless protocol allows is a subnet mask of
on the LANs and a subnet mask of on the WANs, which
saves address space.
VLSM is not the only benefit of classless routing protocols. Classless rout-
ing protocols allow summarization at non-major network boundaries,
unlike classful routing protocols, which allow summarization only at major
network boundaries.
Another benefit of classless routing is that less bandwidth is consumed
since no periodic updates are sent out the routers' interfaces. Updates are
sent only when a change occurs, and then only the change is sent, not the
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