[clue-tech] multihomed ISP and BGP

[Nate Duehr]

Angelo Bertolli wrote:
> Nate Duehr wrote:
>> One carrier had pre-pended their own AS at multiple hops, another 
>> carrier had their entire network looking like one AS to us, and the 
>> other had a mixture of all of the AS's of the companies they'd bought 
>> and acquired over the years.
> 
> Uhh, isn't there supposed to be some kind of standardization for this?  
> Anyone can just decide how they want to weight things?

I can only speak from back then when I heard the router guys cussing 
regularly... (GRIN).  Are there probably some standards now?  Perhaps.

I could ask my router-head friends and see if the world got smarter, but 
I know I haven't seen any new versions of BGP and I am pretty sure I've 
never seen any announcements on NANOG or other places where the 
networking folks hang out, that there's been any breakthrough technology 
in routing tables that would stop such things above from happening.

There definitely is always a balancing act between allowing this and 
"aggregating" entries in a core/backbone router due to limited RAM 
(think about how big the AS table in a central core backbone router is 
today!) and CPU/ASIC resources.

Paying the penalty of looking up lots of numbers to route packets when 
people want ping times less than 100ms, and no major addition of latency 
and no packet loss on a world-wide scale... it's a pretty big 
engineering task.

Since we're really only talking about the BGP "edge" routers, how the AS 
numbers are handled upstream, is very carrier-dependent.  You usually 
have to demonstrate some level of "clue" before they start lowering 
restrictions like them aggregating your AS hops, even if you were trying 
to use AS pre-pending to make a particular route "look" worse to the 
rest of the world.  But if you're calling on the phone asking for it, 
generally they know you have enough clue to shoot yourself in the foot, 
and then fix it... so they'll usually remove the aggregation they're doing.

The "interesting" part of all this is that you can't SEE this, unless 
you're on the "other end"... so there have been these things called 
"looking glass" routers available to the public for years and years... 
routers that just sit there and have special code on them or are 
locked-down enough that the owner trusts people to log into them, look 
at the "looking glass" view of the Net routing from that location, and 
not do bad things with it... the only way to see if your changes here in 
Denver actually showed up in say, the UK.  Or whereever...

Routing experts are a different breed, that's for sure.  Many of them 
personality-wise are perfectionists, and they work on a network that was 
DESIGNED to fail in strange ways.  Tough gig!

Nate