lots of prepends

Tomas Caslavsky tomas at caslavsky.cz
Thu Feb 19 16:29:08 CST 2009


Hi all,

I am writing on behalf of AS8928.
We have changed our BGP policy against  AS 20912 to allow maximum of 20 
AS prepends.

Our NOC will communicate this issue to customer and when I will have 
some news why this happened  I will update NANOG list.

Best Regards
Tomas Caslavsky

+---------------------------------------------------------------+
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Paul Stewart wrote:
> The only ill effect is if set it too low.... we tested it a bit at 20-30 AS path length range figuring we shouldn't see *much* and it was staggering over time.  The unfortunate thing more related to your question is that we found some AS's that were prepending 40-50 times to ALL their upstreams so with max-as set too low we had no routing to them at all!
>
> We've had it set to 100 for quite a while now and no side effects....
>
> Paul
>
>
> -----Original Message-----
> From: Seth Mattinen [mailto:sethm at rollernet.us] 
> Sent: Thursday, February 19, 2009 4:50 PM
> To: nanog at nanog.org
> Subject: Re: lots of prepends
>
> Mikael Abrahamsson wrote:
>   
>> Today 85.119.176.0/21 was announced by AS20912 with 177 prepends. I
>> noticed 20912 modulo 256 is 176. AS47868 modulo 256 is 252 which matches
>> this mondays prepend-incident.
>>
>> So, what router OS will put 20912 into a byte and thus end up with 176
>> in something like "set as-path prepend last-as <no of prepends>" ? It
>> needs to be fixed.
>>
>> Has anyone noticed any ill effects with IOS and using "bgp max-as"? Will
>> it just drop any prefixes with long as-paths and no other ill
>> operational effects?
>>
>>     
>
> No ill effects here, but I never saw the others before this one, and I'm
> only seeing it via 3561.
>
> 010308: Feb 19 13:08:13.455 PDT: %BGP-6-ASPATH: Long AS path 3561 3257
> 8928 20912 20912 20912 20912 20912 20912 20912 20912 20912 20912 20912
> 20912 20912 20912 20912 20912 20912 20912 20912 20912 20912 20912 20912
> 20912 20912 20912 20912 20912 20912 20912 20912 20912 20912 20912 20912
> 20912 20912 20912 20912 20912 20912 20912 20912 20912 20912 20912 20912
> 20912 20912 20912 20912 20912 20912 20912 20912 20912 20912 20912 20912
> 20912 20912 20912 20912 20912 20912 20912 20912 20912 20912 20912 20912
> 20912 20912 20912 20912 20912 20912 received from 216.88.158.93: More
> than configured MAXAS-LIMIT
>
> ~Seth
>
>
>
>  
>
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