Network Statements, Part 5

Let’s now take a look at the interplay between network statements and auto-summary with EIGRP. Refer to the example topology:

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

router eigrp 1

network 172.16.0.0

network 10.0.0.0

Like RIPv2, EIGRP is a classless protocol that performs automatic route summarization by default. Thus, 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 – 10.2.2.0/24 and 172.16.0.0/16

This was exactly the same result was with RIPv2. Likewise, we can disable auto-summary under EIGRP:

router eigrp

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

Notice that in all cases the router is advertising the actual prefixes as configured, again just like RIPv2. You can see the status of EIGRP auto-summary with show ip protocols.

What if we now use EIGRP in conjunction with VLSM?

Note that the network 10.0.0.0 subnets now have different masks. Let’s go back to our original EIGRP configuration, with automatic route summarization enabled (the default):

router eigrp 1

network 172.16.0.0

network 10.0.0.0

As before, when using EIGRP 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 – 10.2.2.0/24 and 172.16.0.0/16

Once again, the results were the same as with RIPv2. Now, let’s disable automatic route summarization under EIGRP:

router eigrp 1

network 172.16.0.0

network 10.0.0.0

no auto-summary

The router advertises 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

In this case the effects were again just like for RIPv2.

Next time, we’ll further examine the behavior of EIGRP’s network statements.

Author: Al Friebe

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