Networks ignoring prepends?

Tue Jan 23 18:47:26 UTC 2024

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. 

						Jay B.