Long AS Path

Stephen Satchell list at satchell.net
Thu Jun 22 13:29:33 CST 2017


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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