Migration of leases from one failover pair to another without outage

Chris Fabri fabric at northwestern.edu
Wed Feb 27 15:17:45 UTC 2008

On 2/27/08 12:03 AM, Nick Urbanik wrote:
> Or indeed, I'd love to hear any other suggestions for migrating leases
> from one failover pair to another without an outage.

How about this, it's a little daring, and you'll need to up your lease 
time probably somewhat significantly, so I you can't to that, it's 
probably not an option. Nor if you need the lease time to be short again 

Increase the lease time to something fairly significant.  I'm think like 
a week, but a couple days might suffice.   The goal here is to get the 
leases file to be essentially static for the duration of the switchover. 
  By making the leases file so long, you should nearly eliminate the 
possibility of clients doing a DHCPREQUEST at half lease time. 
Unfortunately you'll probably cause problems for new client requests, so 
  you'll want to make the change when the least number of new clients 
are coming online.

Once you've established this very low turnover leases file, you can 
simply shut down secondary a, and then transfer the leases file from 
secondary a to secondary b, as well as update the config.  REmove 
secondary A from the helper addresses.   Startup secondary b.  I'm 
assuming the IPs on secondary a and be are different, in which cases 
they won't negotiate the failover peering, but should both be able to 
answer queries.  Oonce secondary B is started up, you can update the 
helper addresses to be only secondary B, and shutdown primary A.    Now 
update primary A's config to point at peer secondary B.   Start it up, 
and once it's up and running, re-add the helper address for primary A.

That's pretty convoluted, but it should really minimize downtime, and 
only impact a small number of clients.  I"m not sure you can do this 
with 0 downtime and no client issues, but the few clients should recover 
fairly quickly, but there could possibly be some fallout.

If you just do an IP swap of secondary a and secondary b, I bet this 
could be done much more easily.  Just shutdown secondary a and remove 
it's IP from the helper addresses.   Swap IPs, restart the server that 
was formerly secondary b, and let DHCP sync up the files.  Primary A 
should continute to answer queries in a peer-down state.    Once the now 
seconday A box is back online, add the helper addresses back in. 
That's probably what I'd do.   chris

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