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:
To disable auto summarization the command is:
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>
After manual summarization, using the command
Router_A(config-if)#ip summary-address eigrp 10 22.214.171.124 255.255.248.0
We see that the route table has the summarized address in Router_B and Router_C
Thus the 3 entries
D 126.96.36.199 [90/158720] via 188.8.131.52, 00:02:39, FastEthernet0/0
D 184.108.40.206 [90/158720] via 220.127.116.11, 00:02:39, FastEthernet0/0
D 18.104.22.168 [90/158720] via 22.214.171.124, 00:02:39, FastEthernet0/0
Is summarized to
D 126.96.36.199 [90/156160] via 188.8.131.52, 00:00:19, FastEthernet0/0