North American Network Operators Group

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Re: How Not to Multihome

  • From: Patrick W. Gilmore
  • Date: Mon Oct 08 22:23:53 2007

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.