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Re: How Not to Multihome

  • From: Stephen Satchell
  • Date: Wed Oct 10 08:09:02 2007

Justin M. Streiner wrote:

On Tue, 9 Oct 2007, Patrick W. Gilmore wrote:

Justin, if Provider A _has_ permission from Provider B to announce a prefix, do you believe Provider A should be allowed to announce the prefix?

As long as all of the relevant parties know about it and are OK with it, that's fine. It's just not my first choice for solving the customer's multihoming dilemma, that's all :)


Back when I was a NOC monkey (that stopped a month ago), I had exactly that situation. I had MCI and SBC as upstreams. Before multihoming, my network was split in two segments, one for each substream. This made things like DNS interesting.

When I got my ASN, I got agreement from both MCI and SBC to announce my /21 allocations from them over both upstream circuits. As a result, I was able to go back to a single inside network, a single pair of DNS servers, and no more cross-router traffic via the Internet cloud.

I then got my ARIN allocation and went through the Fiscal Quarter From Hell renumbering everything into the new number block. I dropped MCI (long story) and lit up Idacomm, but kept SBC link and numbers.

When I left the ISP, my routers had been announcing my suballocation of SBC space for more than a year. With their permission. Their only requirement is that I have proper routing objects in a routing registry so SBC could see that the route I was announcing was valid. (What was VERY interesting was that I was using the ARIN registry, and SBC was not. The resulting bru-ha-ha uncovered a synchronization problem that ARIN had, and that ARIN fixed.)