EIGRP summarization

By default,Auto summarizing is enabled in EIGRP.Automatic summarization can be disabled using the following command:     
Router(config)#  router eigrp 100
Router(config-router)#  no auto-summary
"No auto-summary" command is needed because by default EIGRP will behave like a classfull routing protocol which means it won’t advertise the subnet mask along the routing information. For example 1.1.1.0/24 and 2.2.2.0/24 will be advertised as 1.0.0.0/8 and 2.0.0.0/8. Disabling auto-summary will ensure EIGRP sends the subnet mask along

Summaries can also be produced manually on any interface
Syntax:
Router(config)# interface < interface >
Router(config-if)# ip summary-address eigrp < as-no> <summarized_network_ip> <summarized_subnet_mask>
Example:

Router(config)#  interface fa 0/0
Router(config-if)#  ip summary-address eigrp 100 79.0.0.0 255.0.0.0
The manually-created summary route is not advertised as an internal EIGRP route, but instead is classified as an EIGRP summary route. An EIGRP summary route has an Administrative Distance of 5.

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