multihoming without BGP
Robert E. Seastrom
rs at bifrost.seastrom.com
Wed Jun 11 03:47:39 UTC 1997
Of course, the downsides of using the interface-default hack are:
1) it does not guarantee shortest path for the packets (unless someone
has hacked together an lbnamed version that talks to gated and sees
which interface has a shorter path to customer <x> based on number of
AS hops before it answers the DNS query).
2) It uses a separate address for each interface (not important for a
single box, but a room full of boxes, say, 50 of them, 3-way homed at
a single site... hmm, that's 100 extra addresses you didn't want to
use). I suspect that upstream providers will not be thrilled to hand
out more address space if they discover it is being put to such
3) I have not looked at the code, but if it is on a per-interface
basis, based on the addresses in the packets, that would seem to
suggest that it might not like BSDI 3.0's virtual host scheme (adding
IP addresses to the loopback port and then proxy-arping them onto the
wire). If this is correct, that would mean you would have to use a
different physical machine for each customer. Of course, on this
point I'm purely speculating.
4) It puts the onus for fail-over on the DNS server, which means one
is going to be using very short TTL.
5) Unless (#1), (#4) implies that fail-over will be manual. Is your
Emacs ready to rock and roll on 50 zone files?
I admire Paul's hack; it is spiffy for what it is, but I would hardly
promulgate it as an advised way to multihome without using BGP.
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