RIP and VLSM
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Simply stated, don’t use VLSM with RIP. You can do it, but it won’t
work and it can cause a lot of head scratching if you don’t realize what is
happening. |
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If
you look back at the RIP message format in Figure 5-8, you will notice that a
very important piece of information is missing, the subnet mask! When RIP is
constructing the routing message for an interface, RIP only includes those
networks that have the same subnet mask as the interface on which the message is
to be transmitted. In Figure 5-10, we have a router with four interfaces. Two of
the interfaces use a /20 subnet mask and two of the interfaces use a /24 subnet
mask. |
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Figure 5-10: RIP and VLSM |
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Downstream routers on interfaces 1 and 2 would never learn about
networks 1.0 and 2.0, and routers downstream of interfaces 3 and 4 would never
learn about networks 16 and 32. If all the subnet masks are equal, then there is
not a problem. Without transmitting the sub net mask, RIP cannot take advantage
of the properties of VLSM, yet another limitation. |
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