oh k can you see

Sat Nov 5 18:11:21 UTC 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
>

-C