v6 & DSL / Cable modems

David W. Hankins David_Hankins at isc.org
Fri Feb 6 18:15:45 UTC 2009

On Fri, Feb 06, 2009 at 11:50:55AM -0600, Jack Bates wrote:
> Two routers, 2 default routes. Support for shim6 or other multiple IP 

What most people do of course is VRRP.

Barring that, you just specify multiple default routers, and the
client will select the router that still responds to ARP.  But
support for this is not universal, so.

When that isn't available, what I have done in the past here is to use
the DHCP server to give out a default router option that is sorted
according to the DHCP relay agent that was used to reach the server
(keyed on giaddr contents).

The net result is that the client uses the default gateway which it
used to reach the DHCP server, and so will automatically receive an
updated value if that router fails, as part of DHCP lease rebinding
(it will reach the server via the alternate router).

No need to take on 'routed -q' in the client, it can stay a simple
dumb host, with all configuration complexity in the DHCP server or
negotiated in HA by the routers.

P.S. You can also set the DHCP server-identifier on the opposition of
     the giaddr field contents; so when the client reaches renewal
     time, if will be able to use the surviving router if its default
     router has failed (and thus will not have to wait for rebinding).
     This has further config implications as the server(s) are no
     longer able to detect the difference in a client's renewal or
     rebinding, but it can be an effective optimization.

David W. Hankins	"If you don't do it right the first time,
Software Engineer		     you'll just have to do it again."
Internet Systems Consortium, Inc.		-- Jack T. Hankins
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