Failback causes lost lease

[Sean McMurray]

In testing my dhcp failover, I pulled the ethernet cable on the primary 
server.

The secondary server acknowleged renewal requests as expected.

Then I plugged the cable back in. After both the primary and secondary 
had moved from communications-interrupted to normal, the secondary logs 
multiple dhcp requests from a client whose lease is owned by the primary 
server. The primary server does not log any of these but the last 
request, reporting that "lease in transition state expired".

Then the secondary server logs a DHCPDISCOVER from that client and 
records it load balancing to the primary server.

The primary server also sees the DHCPDISCOVER and offers a new lease 
that is not the same number as the previous lease. This despite the old 
number not having been reassigned.

The end result is that failback causes my clients to change their ip 
address.

Why does this happen and how can I prevent it?