[99918] in North American Network Operators' Group
Re: How Not to Multihome
daemon@ATHENA.MIT.EDU (Justin M. Streiner)
Mon Oct 8 21:52:46 2007
Date: Mon, 8 Oct 2007 21:46:11 -0400 (EDT)
From: "Justin M. Streiner" <streiner@cluebyfour.org>
To: nanog <nanog@merit.edu>
In-Reply-To: <47E4D668-8676-4FDF-A001-E4481EE339C4@ianai.net>
Errors-To: owner-nanog@merit.edu
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
