North American Network Operators Group

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Re: Confussion over multi-homing

  • From: Brantley Jones
  • Date: Thu Sep 14 17:21:17 2000

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.