Of course, peering agreements may speak to route filtering at that particular interface, but can't assure global routability. For all intents and purposes, /24s are globally routable, but their are several meaningful exceptions - Verio and legacy class B space come to mind. Daniel Golding Director of R&D "I'm not evil. I'm just drawn that way" NetRail, Inc. 1-888-NetRail On Thu, 14 Sep 2000, Brantley Jones wrote:

At 01:23 PM 9/14/2000 -0500, you wrote:

...

Wouldn't one of the ISPs have to advertise a longer prefix? I would think that the address space would come from only one of the providers, in which case the other provider would have to advertise this space on top of its own /20. It is irrelevant whether the two ISPs advertise one another, the longer prefix would be the first choice for the backbone traffic. If the longer prefix route goes down, traffic would still go to the /20 the other provider is advertising. The ISP who is advertising the route on top of its own /20 can't aggregate said route as it only can route to that portion of the address space defined in the longer prefix.

Geoff Zinderdine

The problem is GETTING a /20 from anybody. We recently tried and could only get a /23 (being a small start-up). BUT, that /23 is (apparently) globally routable because of peering agreements with L3 and UUNET. Our /23 prefix has yet to be filtered by anybody.

Brantley