EIGRP Summarization

EIGRP Summarization


Summarization is used to reduce the size of a routing table thus reducing the load on CPU and memory.

There are two types of summarization:

-         Auto summarization

-         Manual summarization

Auto summarization: EIGRP supports auto summarization and it happens ate network major boundary.  Auto summarized routes are advertised as normal EIGRP routes.  A summary route that has the next hop as Null0 interface is added to the routing table. This interface is helpful to avoid routing loops.

The command to enable auto summarization is:

Router(config-router)#auto-summary

To disable auto summarization the command is:

Router(config-router)#no auto-summary

Manual Summary: needs to be configured by the administrator depending on the requirement. It is done on major network boundaries and the subnet will be summarized into one Classful network.

It can be configured on per-interface basis

A route pointing to null is created once summarization is configured for an interface

The command used for summarization is :

Router(config)# int fa0/0

Router(config-if)# ip summary-address EIGRP <as-number> <address> <subnet_mask>

Router

router a

Router_B

router b

Router_C

router c

After manual summarization, using the command

Router_A(config)#int fa0/0

Router_A(config-if)#ip summary-address eigrp 10 12.0.0.0 255.255.248.0

Router_A

Router_A

Router_a show ip route

We see that the route table has the summarized address in Router_B and Router_C

Router_B

Router_B show ip route

Router_C

Router_C show ip route

Thus the 3 entries

D       12.0.0.0 [90/158720] via 1.1.1.1, 00:02:39, FastEthernet0/0

D       12.0.1.0 [90/158720] via 1.1.1.1, 00:02:39, FastEthernet0/0

D       12.0.2.0 [90/158720] via 1.1.1.1, 00:02:39, FastEthernet0/0

Is summarized to

D       12.0.0.0 [90/156160] via 2.2.2.1, 00:00:19, FastEthernet0/0

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