Maybe I'm missing something, but the core issue is that the NO-
EXPORT'ed anycast instance has a higher localpref inside the AS it's
being advertised to, and as such supressing the non-NO_EXPORT'ed
prefix. The "exportable" prefix gets suppressed at a point on the
network such that the peering routers never see it, so it never makes
it out of that AS.
Advertising the NO-EXPORT as a /25 (or two /25s) would serve the same
purpose as tagging it NO-EXPORT, because as you say most peers filter
on the /25. Incidentally it would obviate the need to assign it a
higher localpref because it's a shorter prefix. However, unless I'm
misunderstanding something, the /25 would not preempt the /24
advertisement, so the /24 would still make it out of the AS.
Just make sure you don't have anything numbered x.x.x.127 on the
block...
On Nov 2, 2005, at 8:42 AM, Randy Bush wrote:
Is it an idea to have anycasted instances using NO_EXPORT
announce /25's instead of /24's?
many many folk filter on /24, so the /25 would not be seen.
Isn't that the point? The existing /24 is tagged NO-EXPORT after all...
Another possibility is for $LARGE_ISP to localpref the
NO_EXPORTED down to $LOW value
and then how will the down-preffed prefix be seen within
$large_isp? local-pref is a very heavy hammer.
randy, at the clue edge i guess
-C
