lots of prepends

Thu Feb 19 21:54:47 UTC 2009

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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