Multihoming - Resilience or Independence

> -----Original Message-----
> From: Steven Bakker [mailto:steven at icoe.att.com]
> Sent: Wednesday, October 10, 2001 11:08 AM
> To: Nipper, Arnold
> Cc: Stephen Burley; Gert Doering; Dave Pratt; lir-wg at ripe.net;
> routing-wg at ripe.net
> Subject: Re: Multihoming - Resilience or Independence
> 
> 
> On Wed, 2001-10-10 at 15:24, Nipper, Arnold wrote:
> > Of course another prefix and another AS is added to the 
> routing table
> > but thereby you are witdrawing at least more than two (both 
> 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 are being withdrawn?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ASPATH will be withdrawn.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 at cw.net