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Network Statements, Part 3

Author: Al Friebe 20 September 2010 1,108 views No Comments
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This time, we’ll take a look at the interaction between network statements, subnet masks and automatic route summarization with RIPv2.

To get RIPv2 running on the Fa0/1, Fa0/2 and Fa0/3 interfaces, we’ll use the following configuration:

router rip

version 2

network 172.16.0.0

network 10.0.0.0

Note that Fa0/1 and Fa0/3 (the network 10.0.0.0 interfaces) are both using a “/24” mask. Under these conditions, the router will advertise the following prefixes:

  • Fa0/0 – nothing (this interface is not running the protocol)
  • Fa0/1 – 10.1.1.0/24 and 172.16.0.0/16
  • Fa0/2 – 10.0.0.0/8
  • Fa0/3 – 10.2.2.0/24 and 172.16.0.0/16

As we can see, RIPv2 (a classless protocol) advertises the masks with the updates. Also notice that when advertising a subnet of one network over an interface belonging to another classful network, RIPv2 does automatic route summarization by default.

To disable automatic route summarization under RIPv2, we add the “no auto-summary” command, like this:

router rip

version 2

network 172.16.0.0

network 10.0.0.0

no auto-summary

As a result, the router will now advertise the following prefixes:

  • Fa0/0 – nothing (this interface is not running the protocol)
  • Fa0/1 – 10.1.1.0/24 and 172.16.1.0/24
  • Fa0/2 – 10.1.1.0/24 and 10.2.2.0/24
  • Fa0/3 – 10.2.2.0/24 and 172.16.1.0/24

In all cases the router is now advertising the actual prefixes as configured. By the way, you can check the status of automatic route summarization for RIPv2 with the “show ip protocols” command.

What if we use RIPv2 along with VLSM?

Note that the two network 10.0.0.0 subnets now have different masks (“/24” and “/28”). Let’s go back to our original RIPv2 configuration, with automatic route summarization enabled (the default):

router rip

version 2

network 172.16.0.0

network 10.0.0.0

When using RIPv2 with auto summary enabled, the router will advertise the following prefixes on its interfaces:

  • Fa0/0 – nothing (this interface is not running the protocol)
  • Fa0/1 – 10.1.1.0/24 and 172.16.0.0/16
  • Fa0/2 – 10.0.0.0/8
  • Fa0/3 – 10.2.2.0/28 and 172.16.0.0/16

Now, let’s disable automatic route summarization under RIPv2, as follows:

router rip

version 2

network 172.16.0.0

network 10.0.0.0

no auto-summary

The router will now advertise the following prefixes:

  • Fa0/0 – nothing (this interface is not running the protocol)
  • Fa0/1 – 10.1.1.0/24 and 172.16.1.0/24
  • Fa0/2 – 10.1.1.0/24 and 10.2.2.0/28
  • Fa0/3 – 10.2.2.0/28 and 172.16.1.0/24

As you can see, RIPv2 is advertising the network 10.0.0.0 subnets with their actual prefixes, and thus supports VLSM. Note that although you can disable “auto-summary” under RIPv1 (and “show ip protocols” will reflect that), doing so doesn’t have any effect.

Next time, we’ll continue our investigation of network statements.

Author: Al Friebe

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