From the blog.

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Complexity – My Friend, My Enemy
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BGP Well-Known Communities

400 Words. Plan about 2 minute(s) to read this.

I’m still making slow progress through Narbik’s advanced lab workbooks. I was scheduled to have the BGP book done last Saturday. (Ha ha. I’m funny.) I’m only halfway through the BGP volume at this point, not because it’s harder than I expected, but because I just haven’t had the hours to deal with it. The last BGP lab I did dealt with communities, including well-known communities. It seems that if they are “well-known” communities…well, I should know them and what they are used for.

The names are fairly intuitive, but here’s a handy chart I stole from here that describes them in a tidy way.

Q. What are the well known communities of the BGP community attribute?

A. The community attribute is a transitive, optional attribute designed to group destinations in a certain community and apply certain policies (such as accept, prefer, or redistribute). This table shows the well known BGP communities.

Community Description
Local-AS Use in confederation scenarios to prevent sending packets outside the local autonomous system (AS).
no-export Do not advertise to external BGP (eBGP) peers. Keep this route within an AS.
no-advertise Do not advertise this route to any peer, internal or external.
none Apply no community attribute when you want to clear the communities associated with a route.
internet Advertise this route to the internet community, and any router that belongs to it.

This chart makes a distinction that I should lab out to verify, but still…it’s the distinction between “local-AS” and “no-export”. At a glance, they both do the same thing: routes tagged with BGP community of “local-as” or “no-export” shouldn’t advertise the route to an eBGP peer. But this table implies that “local-as” will keep a route inside of a sub-AS in a confederation scenario, whereas “no-export” would send the route to all sub-AS’s in the confederation. Maybe I’ll lab that up and give it whirl a bit later on. I was just happy to find a chart that explained the difference.

Heh. As I was blogging this, I did a search on my own blog articles from the Official Exam Certification Guide, and discovered some info from here that confirms what I’m thinking.

LOCAL_AS (Cisco) aka NO_EXPORT_SUBCONFED (RFC1997) – FFFF:FF03 – don’t advertise outside the local confederation sub-AS.

I still might lab it at some point, but I feel less concerned about it than I did.