North American Network Operators Group
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Re: How Not to Multihome

  • From: Justin M. Streiner
  • Date: Mon Oct 08 21:54:31 2007

On Mon, 8 Oct 2007, Patrick W. Gilmore wrote:

If you went ahead and did this, the more specific route being announced by you on behalf of your customer would be more likely to attract traffic back to you. Prefix length is checked in the BGP route selection process before AS path length. This would work in normal "everything works fine" situations, but when things break, troubleshooting the source of the customer's reachabilit woes will get very interesting.

You have made an assumption that the original upstream would not originate a prefix equivalent to the one you are originating.

Internally or externally? A /24 would exist in the provider's IGP to point traffic to that customer.

Off the top of my head, I don't see why the provider who holds the parent block would do this externally. If the provider has, say, a /18 and they assign a /24 of that to this customer, there would be no legitimate reason to originate that /24 and propagate it out to the rest of the Internet. Note that I don't consider breaking that /18 up into 64 /24s and announcing them all separately to accomplish some sort of poor-man's traffic engineering to be a legitimate reason :)

jms