North American Network Operators Group

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Re: oh k can you see

  • From: Chris Woodfield
  • Date: Sat Nov 05 13:12:37 2005

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