Downstream Usage-BGP Communites

Tue May 10 22:27:27 UTC 2011

On Tue, 10 May 2011, Nick Olsen wrote:

> Was hoping to gain some insight into common practice with using BGP
> Communities downstream.

Generally, the transitive BGP attribute you have the most direct control 
over is AS_PATH, though it's not impossible for a provider to munge the 
AS_PATH on routes they receive from their transits and peers, beyond your 
control.

Some providers might have communities that let you pass things along to 
their transit providers and peers, or influence traffic patterns / route 
propagation.

For example, if I buy transit from ISP X, and they get transit from 
Level3 and Sprint, they might offer a community that lets me selectively 
prepend to Sprint (or Level3), I can affect how traffic flows to my 
network.  In your example, AS100 might have a community that you can 
set on your announcements that will cause them to set 4323:666 on that 
prefix when it's passed to TWTC.  If they don't offer a community, then 
doing what you're looking for would require one of their network people to 
put something manual in place.  Many large networks don't like to (or 
won't) do that because one-off requests don't scale very well, and it can 
add complexity when troubleshooting a connectivity problem, or when 
someone fat-fingers an access-list/distribute-list/prefix-list.

This varies greatly, based on the level of control your direct BGP 
neighbors are willing or able to offer to you.  Also, in general, the 
farther away a network is from you (in terms of AS hops), the less likely 
you are to have control over how they propagate and act upon your 
announcements.

jms