The issue

The current setup is this: the gateway acts as an arp proxy from the subnet that ul uses to the subnet that skynet uses.

This has the following problem: if we add a second gateway, it'll also be in the subnet, and therefore the first gateway will still try to do proxy arp for it, causing general badness.

If we want to have a redundant gateway, we have two choices:

  1. have the backup gateway not be addressible

  2. re-subnet our subnet to work around this

(A) is undesirable as we lose the ability to access the backup gateway externally, which will be needed if the primary gateway falls over and the failover fails. Therefore, (B) is the way forward.

The proposal

Move the gateways to the subnet (host IPs -> for their external interfaces. Specifically, the active gateway should be, the backup gateway, with the static IPs for the gateways being and The subnet mask will be

All other machines will be moved to the subnet (host ips -> This has the advantage that they won't need to change their IPs, just their subnet masks and gateways (i.e. we don't have to change dns for a start).

The gateways internal interfaces will also be moved to the subnet. The active gateway should be, the backup gateway will be, with the static IPs for the gateways being and

How/Why it works

There is no longer any problem with having multiple gateways. They are only trying to do proxy arp for the internal IPs, which are in a different subnet to the gateway IPs.

last edited 2006-07-08 21:21:07 by 87