[99919] in North American Network Operators' Group
Re: How Not to Multihome
daemon@ATHENA.MIT.EDU (Patrick W. Gilmore)
Mon Oct 8 22:23:04 2007
In-Reply-To: <Pine.LNX.4.64.0710082130040.29005@whammy.cluebyfour.org>
Cc: "Patrick W. Gilmore" <patrick@ianai.net>
From: "Patrick W. Gilmore" <patrick@ianai.net>
Date: Mon, 8 Oct 2007 22:15:45 -0400
To: nanog <nanog@merit.edu>
Errors-To: owner-nanog@merit.edu
On Oct 8, 2007, at 9:46 PM, Justin M. Streiner wrote:
> 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.
Well, "internally" is kinda useless to this discussion, wouldn't you
think?
I get the feeling that you are trying to ask a clever question there,
but it didn't come across that way.
> 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 :)
Interesting. Did you not read the first paragraph in this e-mail?
In fact, I seem to recall that you wrote it (attribution is missing,
so I can't be 100% certain).
Personally, I'd call that a "legitimate reason".
To be clear, I am not suggesting de-aggregating every CIDR down to /
24s. But the global table doesn't grow any more whether the customer
announces the /24 from their own ASN, or if you muti-originate it
from two upstreams - or just one upstream for that matter. So there
is no "legitimate reason" to _not_ announce it, but there is a reason
to announce it.
--
TTFN,
patrick
