Long AS Path

Ryan L ryan.nsplist at gmail.com
Fri Jun 23 19:59:35 CST 2017


I didn't see anyone answer (sorry if I missed it and this is redundant) ...

In the path selection algorithm, local preference is processed before
AS-PATH.

Within your provider's AS, your prefixes could be a default localpref of
100, and learned prefixes from their peers 85, for example. In this case,
Intra-AS will always be preferred due to higher lpref.

You may prepend a handful of times out of your connection that is within
the provider's AS, thus making the overall AS-PATH longer, but localpref
still remains 100 vs. peer 85, so intra-AS still preferred.

Some providers allow you to send community attributes with your prefixes to
modify the localpref within the provider AS and make it lower than their
peer localpref. Solves this particular conundrum.

Couple examples:

https://onestep.net/communities/as3356/
https://onestep.net/communities/as1299/

Depending on your allocation size and your needs, if you wanted to force
all traffic over the "fast" link and use the "slow" link as an emergency
backup, it could be easier to just announce more specific routes out the
faster connection and send an aggregate out the backup one. No communities
needed in that scenario. All depends on what you need to do, of course.

HTH,
Ryan


On Thu, Jun 22, 2017 at 9:29 AM, Stephen Satchell <list at satchell.net> wrote:

> On 06/22/2017 04:27 AM, Jon Lewis wrote:
> >
> > You do have to wonder, what was the thought process that resulted in 35
> > being the right number of prepends "accomplish" whatever TE they were
> > shooting for?
> >
> > AS path: 10026 9498 55644 55644 55644 55644 55644 55644 55644 55644 55644
> > 55644 55644 55644 55644 55644 55644 55644 55644 55644 55644 55644 55644
> > 55644 55644 55644 55644 55644 55644 55644 55644 55644 55644 55644 55644
> > 55644 55644 55644 45271
> >
> > I don't mean to single out 55644.  It's just the first/most obnoxiously
> > long as-path I found when looking for long ones.
> >
> > I seriously doubt provider/customer/customer-of-customer relationships
> > ever get much deeper than a handful or so of ASNs...so if prepending a
> > few times doesn't get it done, 10-20-30 prepends are unlikely to help.
> >
> > In the above case, that long path is actually our best path.  We
> > localpref peering above transit.  So, it doesn't matter how many
> > prepends, they add, we're never going to not use this path if its
> > available.  We have transit paths to these routes that are only a single
> > handful of ASNs long.
>
>
> I think I understand the problem, and now I understand why prepends
> didn't do much for me.  Over the years, I tended two multi-homed sites.
> In both cases, the two uplinks had different speeds.  When I used
> prepend to try to get the outside world to prefer the faster link, some
> traffic was stubborn about coming in the slow one.
>
> Difference in speeds?  In the first network it was 45 mbps and 10 mbps.
> In the second network it was 16 mbps and 1.5 mbps.  Both network owners
> were too stingy at the time to opt for harmonized rates.
>
> Question:  how could communities be used to "force" preference for one
> uplink over another by the peers?  I'm long past needing this, but
> others might benefit.  (And when you stop learning, you're dead.)
>


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