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Network Statements, Part 2

Author: Al Friebe 8 September 2010 904 views No Comments
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Last time, we looked at the basic operation of RIP network statements. Now, let’s look at that topic in more detail.

To get RIPv1 running on the Fa0/1, Fa0/2 and Fa0/3 interfaces (but not on Fa0/0), we used the following configuration:

router rip
network 172.16.0.0
network 10.0.0.0

As a result, the router advertised the following on its interfaces:

  • Fa0/0 – nothing (this interface is not running the protocol)
  • Fa0/1 – 10.1.1.0 and 172.16.0.0
  • Fa0/2 – 10.0.0.0
  • Fa0/3 – 10.2.2.0 and 172.16.0.0

Note that RIPv1, being a classful protocol, does not advertise the masks with the updates, and it behaves as follows:

  • When the subnet to be advertised belongs to a different classful network than that of the advertising interface, advertise the classful network (this is known as “automatic route summarization”).
  • When the subnet to be advertised belongs to the same classful network as that of the advertising interface and the masks are the same, advertise the subnet.

Note that with RIPv1, automatic route summarization always occurs at the boundary between two classful networks. Now let’s modify things slightly.

The logical networks that are now connected to the router are:

  • 192.168.1.0/24
  • 172.16.1.0/24
  • 10.1.1.0/24
  • 10.2.2.0/28

Note that the subnet mask on the Fa0/1 interface is now “/28” (not “/24”), and with classful protocols such as RIPv1, this makes a crucial difference. Let’s get RIPv1 running on the Fa0/1, Fa0/2 and Fa0/3 interfaces (but not on Fa0/0), using the same configuration as before. In this case, the router will advertise the following prefixes:

  • Fa0/0 – nothing (this interface is not running the protocol)
  • Fa0/1 – 172.16.0.0
  • Fa0/2 – 10.0.0.0
  • Fa0/3 – 172.16.0.0

Compare this to the previous case, where the masks on the network 10.0.0.0 interfaces were consistent (both were “/24”), and you’ll notice that although the router is still advertising the 172.16.0.0 network on Fa0/1 and Fa0/3, and network 10.0.0.0 on Fa0/2, it is no longer advertising network 10.0.0.0 subnets on either Fa0/1 and Fa0/3. This is because a classful protocol such as RIPv1 will verify that when advertising a subnet via an interface that is on that same classful network, the masks of the advertising interface and the subnet to be advertised are the same. Thus, when it comes to classful protocols, we have a third rule:

  • When the subnet to be advertised belongs to the same classful network as that of the advertising interface and the masks are different, suppress the advertisement.

In other words, the router is enforcing the classful routing protocol rule prohibiting VLSM (Variable-Length Subnet Masking).

Next time, we’ll look at the actions of network statements under RIPv2, a classless routing protocol.

Author: Al Friebe

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