The EIGRP No Auto-Summary Command | EIGRP Summarization Lab

The “no auto-summary” command is one of the most important commands that you shouldn’t neglect in configuring EIGRP. Honestly, during my first exam relating to EIGRP in the Cisco Networking Academy, I just put this command to all routers under the EIGRP process. Well, it works! Without delving into the importance, I got a passing score. Yeah!

But just putting the command without the “ifs”, it made me realize that there is something I need to understand about this.

The “no auto-summary” command is configured under the EIGRP process. It prevents the auto-summarization of networks. Without enabling this command, the routes from its interfaces will be advertised as classful A, B or C networks to its neighbours.

Let’s say we have three routers: Trapdoor, Ebo and Huntsman. Ebo and Huntsman are connected to Trapdoor via serial links and they are connected to each other via FastEthernet links with 192.168.2.0/24 network.

Looking at the “show ip interface brief” of Trapdoor, loopback has been configured. These loopbacks should be accessible by Huntsman and Ebo.

After configuring basic eigrp configuration:

Trapdoor(config)#router eigrp 10
Trapdoor(config-router)#network 172.32.0.0
Trapdoor(config-router)#network 192.168.32.0
Trapdoor(config-router)#end
Trapdoor#

Let’s take a look at what happens to the routing table of each router after advertising the 172.32.0.0 network and 192.168.32.0 networks:

Ebo and Huntsman do not learn the loopbacks of Trapdoor.
Let’s try to enable the “no auto-summary” command:

Trapdoor(config)#router eigrp 10
Trapdoor(config-router)#no auto-summary
Trapdoor(config-router)#end

Take a closer look at Ebo and Huntsman’s routing table:

Both Ebo and Huntsman learned Trapdoor’s loopback address via EIGRP. The pings are also successful on all routers:

To make sure that “no auto-summary” command is configured, you can do “show ip protocols.”

Using the show ip protocol, we can see that automatic network summarization is not in effect.