This section briefly explains auto summarization in EIGRP and how it works. Auto summarization is a feature that is used to summarize the routes of EIGRP to its class full networks automatically. It is a method of summarizing multiple networks into a single summary address. For example, suppose we are planning to take eight subnets of class B, the default network used is 172.16.0.0/16, and are submitted using 3-bit subnetting. To know more about What is Auto Summarization in EIGRP, keep reading this article till the end.
The eight subnet’s of class B are as follows:
Router R1 contains 4 subnets and are 172.16.0.0/19, 172.16.32.0/19, 172.16.64.0/19, 172.16.96.0/19 and remaining four are contained by R2 router 172.16.128.0/19, 172.16.160.0/19, 172.16.192.0/19, 172.16.224.0/19.
After configuring EIGRP, view the R1 routing table, using the ‘show IP route’ command.
The output of the R2 route is shown below.
The routing table summarizes the routers and it creates a confusion in-network because they are summarizing their subnets to default class B. Router summarizes the same two networks.
We can use the “no auto-summary” command from Router Configuration mode to turn off the auto summarization feature.
To disable the auto summarization in R1 following steps are followed:
R1#(config)#router eigrp 65500
To disable the auto summarization in R2 following steps are followed:
R2#(config)#router eigrp 65500
After disabling the auto summarization check the output to confirm whether the two routers are getting separated individually.
TYPES OF SUMMARIZATION
Two types of summarization are supported by EIGRP, they are:
In auto summarization, it automatically provides the summary routes to neighbor routers. The working principle of the auto summarization was when the auto summarization gets enabled, EIGRP checks for direct connection if so, it automatically summarised to their classful mask. The summarization only happens to the subnets of routes on its interface.
Consider the following example:
Here for the process, two routers are used. They are R1 and R2. Router R1 is connected with the local subnet that is 10.0.1.0/24 and the router R2 is advertised by R1. By using the auto-summary feature the R1 summarizes the network by sending R2 the route. And auto-summary works happens when the auto-summary was enabled R1 send 10.0.0.0/8 instead of 10.0.1.0/24.
Manual summarization works only on interface level by the command named ip summary-address eigrp. It is easy and can summarize anywhere on the network.
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