-----Original Message----- From: Steven Bakker [mailto:steven@icoe.att.com] Sent: Wednesday, October 10, 2001 11:08 AM To: Nipper, Arnold Cc: Stephen Burley; Gert Doering; Dave Pratt; lir-wg@ripe.net; routing-wg@ripe.net Subject: Re: Multihoming - Resilience or Independence
Of course another prefix and another AS is added to the routing table but thereby you are witdrawing at least more than two (both
On Wed, 2001-10-10 at 15:24, Nipper, Arnold wrote: prefs as well as
AS). ==> table gets smaller.
Your math is confusing... or maybe I'm misunderstanding the obvious. Yes, multihoming adds another AS and (at least one) (longish) prefix to the routing table. But in your reasoning, which two prefixes and one AS are being withdrawn? If I resiliently multi-home to one ISP, and I get my address space from its block, I do not add any prefix or AS to the (global) routing table. Even if I multi-home to two ISPs and selectively NAT depending on the outgoing connection, I'll be NAT-ing into the respective ISP's address space (which I presume is properly aggregated), adding no prefix or AS.
Your prefixes (let's say /24) has to be announced by two ISPs so the packets will come back to you. If ISP A's link failed then that /24 with ISP A in the ASPATH will be withdrawn. And ISP B's /24 will become active. So in the global routing table there will be TWO /24 with different ASPATH. If this /24 is in ISP A's PA space then it saves one but the one from ISP B's one NEEDs to be in the global routing table. The worse case: ISP A only announces aggregate block even if the link failed then the traffic will still go thru ISP A then get dropped. Ping Lu Cable & Wireless USA Network Tools and Analysis Group W: +1-703-292-2359 E: plu@cw.net