Aggregate-Address command

kafifi13

Guys,

I was hoping someone could help me understand this command a little better. I'm a little confused on why and how it's used.

Thanks

Cucumber

Short Answer:
Use it when wanting to reduce the number of advertised routes.

Long Answer:

This command will make your BGP router summarize routes into a single route. hence, instead of advertising many routes that could be summarized, your BGP router will only advertise one summaized route.

To enforce this behaviour to include the networks that are being injected in the same router you add the parameter summary-only, otherwise your router may still advertise more specific routes. You know for the case where, in the same router you are injecting specific routes with commands like network 1.1.1.0, network 1.1.2.0, network 1.1.3.0 that would be included inside a aggregate-address 1.1.0.0 255.255.0.0, you would advertise 4 routes unless you used summary-only.

When you use this command, the summarized route being advertised will have inside its AS_PATH attribute, only the Autonomous System number of the BGP router that summarized the routes. Also, the ATOMIC_AGGREGATE attribute gets added to the advertised route. The ATOMIC_AGGREGATE will tell everybody who summarized the route. Its like a warning for further BGP routers that somebody summarized this route.

In other words, the summarized route advertisement will no longer tell the list and order of previous Autonomous Systems that have been visited, but only what BGP router summarized the route and in which AS the route was summarized.

This brings the possibility of a routing loop, therefore, if you include the as_set parameter in the command, the AS_SET atrribute will get appended to the summarized route, including a list of unordered visited Autonomous Systems in the advertising.
When using the option, triggered updates regarding routes that are being summarized will get created by the BGP router in which you are summarizing the routes.

kafifi13

Thanks for the response...that was great.