Lots of prepends - AS20912 case

Fri Feb 20 15:56:56 UTC 2009

If we really want bgp for idiots, perhaps a checkbox for "make this
(slightly,more,greatly) less preferred for incoming traffic" would do the
job :)

Then again, perhaps people who want the results of their local configuration
distributed to the ends of the earth should at least read a book or two...

-Dorn

On Fri, Feb 20, 2009 at 10:35 AM, Ivan Pepelnjak <ivan.pepelnjak at zaplana.net
> wrote:

> >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
> http://blog.ioshints.info
>
> > -----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
>
>
>