nanog mailing list archives
Re: How Not to Multihome
From: "Patrick W. Gilmore" <patrick () ianai net>
Date: Mon, 8 Oct 2007 22:15:45 -0400
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
Current thread:
- Re: How Not to Multihome, (continued)
- Re: How Not to Multihome Keegan . Holley (Oct 08)
- Re: How Not to Multihome Joel Jaeggli (Oct 08)
- Re: How Not to Multihome Justin M. Streiner (Oct 08)
- Re: IPv6 routes, was: How Not to Multihome Mike Leber (Oct 08)
- Re: IPv6 routes, was: How Not to Multihome Keegan . Holley (Oct 08)
- Re: IPv6 routes, was: How Not to Multihome William Herrin (Oct 08)
- Re: How Not to Multihome Justin M. Streiner (Oct 08)
- Re: How Not to Multihome Patrick W. Gilmore (Oct 08)
- Re: How Not to Multihome Justin M. Streiner (Oct 08)
- Re: How Not to Multihome Patrick W. Gilmore (Oct 08)
- Re: How Not to Multihome Brian Wallingford (Oct 08)
- Re: How Not to Multihome Keegan . Holley (Oct 08)
- Re: How Not to Multihome Justin M. Streiner (Oct 08)
- Re: How Not to Multihome Patrick W. Gilmore (Oct 08)
- RE: How Not to Multihome Jamie Bowden (Oct 09)
- Re: How Not to Multihome Patrick W. Gilmore (Oct 09)
- Re: How Not to Multihome Justin M. Streiner (Oct 09)
- Re: How Not to Multihome Stephen Satchell (Oct 10)