Failover Clarification

Benjamin Wiechman benw at
Wed May 9 13:44:43 UTC 2007

> The two servers communicate with each other frequently, and go through
> various pool balancing exercises so that they tend to keep an equal
> number of free leases on each server.

So in theory if I have a couple of free leases it *should* work. In practice
how often do the servers work to balance the leases? I'm not seeing a
configuration directive that would define this time. I was reading that if
the free address are within about 10% it the peers don't really work to
balance the addresses, is this still true?

> When you're down to the last free IP one server will not reply because
> it has no lease to offer. The other server should offer the available
> lease that it has.

OK. That is reasonable. So if I'm running a dhcp relay through my switches I
would need to be doing a directed broadcast to allow the servers to perform
their own load balancing, rather than a relay by IP.

> About a year ago I did some work for a client to set up a script which
> ran on each dhcp server. When communication was lost for a certain
> amount of time (30 or 60 minutes I think) it used the omshell interface
> to switch the server to partner down. When the partner communications
> returns dhcpd automatically return to failover mode by itself.

> The details should be in the archives. It was a pretty simple script,
> and it only got used once in a DR test where we manually switched off
> the network between sites. This client had redundant networks, etc, so
> we never had a real failure to test it out.

Thanks for the pointer. I did see that yesterday and have been playing with
it. I was just curious if anyone had tried this in a similar circumstance.
Most people I speak with don't have a similar configuration.

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