Quagga router redundancy determinism

[Chris Caputo]

I just posted a patch to quagga-dev to add RFC 4191 "default router 
preference" support for IPv6 router advertisements to Quagga:

  http://lists.quagga.net/pipermail/quagga-dev/2009-May/006547.html

The cool thing about router preference support is that if your host 
supports it (*), you can get some determinism in the choice of gateway 
used by IPv6 hosts when multiple routers are sending out router 
advertisements.  This enables deterministic gateway redundancy similar to 
HSRP/VRRP setups.

For example, remove any static IPv6 default routes from your hosts (**) 
and configure two routers on the same LAN as follows:

 Primary:

   interface eth0
    no ipv6 nd suppress-ra
    ipv6 nd ra-interval 1
    ipv6 nd ra-lifetime 3
    ipv6 nd router-preference high

 Backup:

   interface eth0
    no ipv6 nd suppress-ra
    ipv6 nd ra-interval 1
    ipv6 nd ra-lifetime 3
    ipv6 nd router-preference low

With the above configs, hosts which support router preference decoding 
will use the primary gateway unless a router advertisement is not seen 
from it in 3 seconds, in which case it will use the backup gateway.  And 
when the primary gateway returns, it will be used again.

Chris

* On linux, the kernel config option is CONFIG_IPV6_ROUTER_PREF.  Not sure 
about other platforms.

** In practical use, I also keep a static default route in place, albeit 
with a higher metric than what router advertisements are set at.  On my 
linux boxes, router advertisements have a metric of 1024 and I set my 
static fallback/worst case scenario route to have a metric higher than 
that (ex. 6666).