Lots of prepends - AS20912 case

Ivan Pepelnjak ivan.pepelnjak at zaplana.net
Fri Feb 20 15:35:31 UTC 2009

>From the end-user perspective, it makes sense to make the "prepend"
parameter an integer. The only thing an end-user really needs is routing
policy (primary/backup selection) and sometimes AS path prepending is the
only solution. Allowing them to insert third-party AS numbers into the AS
path increases their confusion (assuming they were never exposed to Cisco
IOS). Obviously, the number of prepends has to be limited to something
sensible (10 seems a good number, and it looks like Mikrotik has implemented
that restriction).

The "set as-path prepend last-as" is a completely different story; it's used
to do proxy prepending for your customers.

Ivan Pepelnjak

> -----Original Message-----
> From: Mikael Abrahamsson [mailto:swmike at swm.pp.se] 
> Sent: Friday, February 20, 2009 3:06 PM
> To: nanog at nanog.org
> Subject: Re: Lots of prepends - AS20912 case
> On Fri, 20 Feb 2009, Dorn Hetzel wrote:
> > Replacing what is conventially thought to be a string with 
> an integer 
> > multiplier seems a massive violation of the principle of 
> least astonishment.
> On a Cisco running 12.0S:
> route-map test1
> set as-path prepend last-as ?
>    <1-10>  number of last-AS prepends
> Cisco seems to be doing more sensible limits, but I do agree 
> that the feature makes sense.
> There are two ways of handling when someone puts in a very 
> high number to number of prepends:
> 1. Say "out of limit" and disallow it in the config checker.
> 2. Actually prepend the number of times specified.
> The option done here:
> 3. Prepend number of times entered modulo 256, is just broken.
> -- 
> Mikael Abrahamsson    email: swmike at swm.pp.se

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