Re: Networks ignoring prepends?

[Owen DeLong via NANOG <nanog () nanog org>]

> On Jan 23, 2024, at 10:47, Jay Borkenhagen <jayb () braeburn org> wrote:
>
> William Herrin writes:
> > The best path to me from Centurylink is: 3356 1299 20473 11875
> >
> > The path Centurylink chose is: 3356 47787 47787 47787 47787 53356
> > 11875 11875 11875
> >
> > Do you want to tell me again how that's a reasonable path selection,
> > or how I'm supposed to pass communities to either 20473 or 53356 which
> > tell 3356 to behave itself?
>
> What you want to do is pass communities to 3356 so they apply the same
> local-pref to routes from both paths, enabling as-path-length-based
> path selection to work.  That means lowering their local-pref on the
> currently-chosen customer path via 47787 to match the local-pref on
> the their 1299 peer path.
>
> as3356's TE communities are listed in their IRR aut-num: AS3356
> object: 
>
> remarks: ----------------------------------------------------
> remarks: customer traffic engineering communities - LocalPref
> remarks: ----------------------------------------------------
> remarks:        3356:70 - set local preference to 70
> remarks:        3356:80 - set local preference to 80
> remarks:        3356:90 - set local preference to 90
> remarks: ----------------------------------------------------
>
> Those communities look like RFC1998.  Thus presumably 3356's peer
> local-pref is 80, and you'll want to signal using 3356:80.  As you
> make signaling changes you should use as3356's looking glass to
> confirm.
>
> as47787 and as53356 should pass your 3356:80 community along to
> as3356.  If they don't do so, complain to them or vote with your
> feet.

The catch to all of that, however, is that he’s not directly peered with 3356 and many AS operators strip communities.

Owen