Summary-only Keyword in BGP Route Aggregation

[Mitrixsen]

Hello!

Just a quick question that I was curious about. When aggregating networks in BGP using the

aggregate-address

command, we have the option of including or not including the

summary-only

keyword a the end. Including it will only advertise the summary route and suppress the routes that are being summarized while not including it causes the local BGP speaker to advertise the

summary route + the routes

that are being summarized.

Are there any situations where not including the

summary-only

keyword would be useful?

Thank you in advance for your help.

Kind regards,

David

[Richard Burts]

David

Interesting question. Perhaps some neighbor might want to treat some components of the summary in one way and treat other components in a different way. Perhaps there is some attribute of the component (community perhaps) that would allow the neighbor to differentiate various components of the summary.

[M02@rt37]

Hello @Mitrixsen,

Good question!

Situation where not including the

summary-only

keyword can be useful ?

Path Selection??!! When the individual routes are advertised along with the summarized route, it provides more granularity for path selection. BGP can consider attributes of the individual routes, such as AS path length or MED, when deciding the best path. This allows for more control over traffic engineering and route preference within the summarized range.

[David Ruess · Accepted solution]

On a roll with these BGP questions I see! 

It does seem counter intuitive to advertise both specific routes and the summary route because the route will always take the more specific route. However as mentioned before BGP is a very diverse protocol and has to adapt to so many scenarios on the internet. Control is everything.

Lets say you have a BGP speaker (R1 in AS 100) advertising a summary route and the specific routes (no summary-only applied) to a few neighbors. Lets also say that those specific routes come from R2 AS 200.

What if R1 has a neighbor (R3 in AS 300) that is filtering routes from AS 200 for whatever reason. It will never learn the more specific routes.....however it WILL learn the summary address that contains all those routes since it will be seen as coming from AS 100 since the summary advertised takes the AS of the router doing the summarization (as long as the

as-set

command is not used). That scenario R3 will still have reachability to those networks through the summary while still blocking AS 100 routes. and the other neighbors are still receiving the more specific routes to make routing decisions.

That's just what I could think of but it still boils down to route manipulation. Same goes for individual route attributes as @Richard Burts and M02@rt37 mentioned too. If something is being blocked based off an attribute you can manipulate it with the

aggregate-address

command

-David