The Atomic Aggregate Attribute in BGP

[Mitrixsen]

Hello, everyone!

I am trying to understand how the Atomic Aggregate attribute works in BGP.

So if my understanding is correct, once we configure route summarization, the path attributes of those routes that we've summarized are lost. The AS Path is lost, the MED is lost and the communities are lost.

This Atomic Aggregate attribute basically indicates that for the summarized route, a loss of path information has occured? Does this have any uses, can we or the routers work with this somehow or is it just purely informative?

And if I was to decide to configure AS SET when summarizing networks, since the AS Path information is preserved, this attribute is no longer used? Since I didn't find it in BGP UPDATE packets for summarized routes that had AS SET configured.

Thank you in advance for your help.

Kind regards,
David

[M02@rt37 · Accepted solution]

@Mitrixsen,

Atomic Aggregate attribute is used to indicate the loss of specific path attributes during route summarization. It is informative in nature and helps routers make routing decisions when receiving summarized routes. If you use AS SET during summarization, the AS Path information is preserved, and the Atomic Aggregate attribute is not needed.

The AS SET attribute allows multiple AS numbers to be represented in the AS Path, preserving the information about the contributing ASes. In this case, the Atomic Aggregate attribute is not necessary because the AS Path attribute still provides the required information.