Why RIP Doesn t Support Variable-Length Subnet Masking

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Why RIP Doesn't Support Variable-Length Subnet Masking

The capability to specify a different subnet mask for the same network number is called variable-length subnet masking (VLSM). RIP and IGRP are classful protocols and are incapable of carrying subnet mask information in their updates. Before RIP or IGRP sends an update, it performs a check against the subnet mask of the network that is about to be advertised, with the subnet mask of the interface sourcing the update. If the two subnet masks don't match, the update gets dropped.

The following example demonstrates this concept. In Figure 2-8, Router 1 has three subnets with two different masks (/24 and /30).

Figure 2-8. An Example of a VLSM Network

Router 1 goes through the following steps before sending an update to Router 2:

  1. Router 1 checks to see if 131.108.5.0/24 is part of the same major network as 131.108.6.0/30, which is the network assigned to the interface that is sourcing the update.

  2. It is part of the same major network, so Router 1 determines whether 131.108.5.0/24 has the same subnet mask as 131.108.6.0/30.

  3. Because the subnet masks are not the same, Router 1 drops the network and doesn't advertise the route.

  4. Router 1 now determines whether 131.108.7.0/30 is part of the same major network as 131.108.6.0/30, which is the network assigned to the interface that is sourcing the update.

  5. It is part of the same major network, so Router 1 next determines whether 131.108.7.0/30 has the same subnet mask as 131.108.6.0/30.

  6. Because the two subnet masks are the same, Router 1 advertises the network.

The preceding procedure determined that Router 1 includes only 131.108.7.0 in its update that is sent to Router 2. The debug ip rip command in Example 2-7 actually shows the update sent by Router 1.

Example 2-7 debug ip rip Command Output Reveals RIP Update Information Sent by Router 1 to Router 2, as Illustrated in Figure 2-8
 RIP: sending v1 update to 255.255.255.255 via Serial0 (131.108.6.2)             subnet 131.108.7.0, metric 1 

Notice in the output in Example 2-7 that the only subnet included in the update is 131.108.7.0. The subnet 131.108.5.0 is not included because it has a different subnet mask.

This results in the following entry in Router 2's routing table displayed by the show ip route command (see Example 2-8).

Example 2-8 show ip route Command Output Reveals That the Subnet 131.108.5.0/25 Is Missing from Router 2's Routing Table
 Router2#  show ip route  131.108.0.0/30 is subnetted, 3 subnets      R       131.108.7.0 [120/1] via 131.108.2.2, 00:00:08, Serial0      C       131.108.6.0 is directly connected, Serial0      C       131.108.2.0 is directly connected, Ethernet0 

To avoid eliminating subnets from routing updates, either use the same subnet mask over the entire RIP network or use static routes for networks with different subnet masks.

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Troubleshooting IP Routing Protocols
Troubleshooting IP Routing Protocols (CCIE Professional Development Series)
ISBN: 1587050196
EAN: 2147483647
Year: 2002
Pages: 260

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