Networks ignoring prepends?

Mon Jan 22 13:23:39 UTC 2024

On Mon, 22 Jan 2024, William Herrin wrote:

> Howdy,
>
> Does anyone have suggestions for dealing with networks who ignore my
> BGP route prepends?
>
> I have a primary ingress with no prepends and then several distant
> backups with multiple prepends of my own AS number. My intention, of
> course, is that folks take the short path to me whenever it's
> reachable.
>
> A few years ago, Comcast decided it would prefer the 5000 mile,
> five-prepend loop to the short 10 mile path. I was able to cure that
> with a community telling my ISP along that path to not advertise my
> route to Comcast. Today it's Centurylink. Same story; they'd rather
> send the packets 5000 miles to the other coast and back than 10 miles
> across town. I know they have the correct route because when I
> withdraw the distant ones entirely, they see and use it. But this time
> it's not just one path; they prefer any other path except the one I
> want them to use. And Centurylink is not a peer of those ISPs, so
> there doesn't appear to be any community I can use to tell them not to
> use the route.
>
> I hate to litter the table with a batch of more-specifics that only
> originate from the short, preferred link but I'm at a loss as to what
> else to do.

In my experience, it's pretty common for service providers to use 
localpref to differentiate paid/free/customer routes (with LP increasing 
in this order).  Since LP trumps as-path length, no amount of prepending 
will get around this.

You may be limited to seeing if your backup providers have community 
controls that would let you tell them "don't share with Centurylink" or 
seeing if your primary has similar controls that would let you advertise 
both the aggregate and more specifics, but have them not propagate the 
more specifics except to those networks (i.e. Centurylink) that you need 
to see them to get them off your backup paths.

----------------------------------------------------------------------
  Jon Lewis, MCP :)              |  I route
  Blue Stream Fiber, Sr. Neteng  |  therefore you are
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