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Re: T1 vs. T2 [WAS: Apology: [Tier-2 reachability and multihoming]]

  • From: Leo Bicknell
  • Date: Wed Mar 30 10:34:28 2005

In a message written on Tue, Mar 29, 2005 at 02:27:56PM -0600, John Dupuy wrote:
> I was looking at it from a route announcement point of view. Transit is 
> where AS A advertises full routes to AS B. Thus, AS B is getting transit 
> from A. Peering is where A & B only advertise their network and, possibly, 
> the networks that stub or purchase transit from them.

This is oversimplistic.  Transit does not have to be full routes.
Don't confuse the business case with the technical configuration.
That is, all combinations of:

{paid,settlement free}-{customer routes only, full routes, no routes,
you leak mine, I leak yours}

exist.  Some are more common than others.  Sometimes multiple
combinations exist between the same two parties.

> It is my understanding that the top ISPs "trade transit". They provide full 
> routes to each other without payment, regardless of how or where the route 
> was learned from. They are willing to pass some traffic without 
> compensation because it makes for better connectivity. From an announcement 
> POV they are not peering.

The top of the food chain is a full mesh of customer routes only.
I have never seen anyone at the top of the food chain trade full
routing tables, something that would likely be obvious from time
to time in various outage scenarios.  There is no business case to
provide free transit on that level.  It would be too easily abused.

That's not limited to "top" ISP's either.  Full tables are not done
on a peering level, ever.  If anything wonky is being done it's
done with selective leaking of routes in one or both directions,
never ever ever with a full table.

       Leo Bicknell - - CCIE 3440
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