Re: DHCP failover and “secs” value

Chris Buxton chris.p.buxton at
Tue Oct 23 17:49:04 UTC 2012

On Oct 23, 2012, at 12:34 AM, Abu Abdulla alhanbali wrote:
> - For new user: DHCPDISCOVER (broadcast) will reach both servers.
> Hashing will be done on the MAC and one server will respond based on
> “split” value as far “secs” is less than the configured value. If
> “secs” is more, then both servers will provide DHCPOFFER to the
> client. User will choose one and send DHCPREQUEST (broadcast). If
> “secs” is less than the configured value, only the server identified
> in the DHCPREQUEST will respond with DHCPACK. If “secs” is more, both
> will respond. How will the two servers sync this MAC between
> themselves?

Using the failover protocol. Each server has a record of every active lease; leases handed out by one server are then synchronized with the other server over a dedicated, persistent TCP connection between them.

> - For a current user and in some cases, DHCP renew is sent to both
> servers (broadcast). will the behavior be the same as above i.e.
> depends on “secs” value.

I'm not sure, but I would expect that a broadcast renewal would be treated that same way. Since the MAC address will hash the same way, as long as the split value hasn't been changed between the last contact and this renewal, it should be answered by the same server as before.

> - In many cases, one server will end up with more active leases than
> the other one, balancing is done in a configurable way. How the lease
> ownership is shifting from one server to the other one since the user
> will continue to request from the same server he got the IP from.

As Simon pointed out, what's rebalanced is the pool of "free leases", or inactive IP addresses.

Chris Buxton
BlueCat Networks

More information about the dhcp-users mailing list