20 minute leases

John Wobus jw354 at cornell.edu
Fri Oct 30 20:24:02 UTC 2015

On Oct 26, 2015, at 10:08 AM, Friesen, Don MTIC:EX <Don.Friesen at gov.bc.ca> wrote:
>    Hopefully a quick question. We migrated some sites from a few old DHCP servers running 4.1.1 to some not as old servers running 4.2.5.  The users with laptops began complaining about sporadic loss of IP connectivity.  They noticed they were getting 20 minutes leases instead of 24 hour leases.  I watched the traffic and it seems all initial leases to unknown MAC addresses get a 20 minute lease and on renewal get the 24 hour lease.  This is not a complaint, I like the idea of a trial lease.  I just want to verify that the 4.1.1 version did not have this behavior without having to recreate that environment.  I’d like to explain the behaviour to my users with a degree of confidence.
> Don Friesen

As others have pointed out, this is a feature of failover,
and I’ve been observing that for at least a decade.
The actual interval is configured (“mclt"), so a configuration
change could have changed the behavior.

In our own case, we saw sporadic renewal-failures when our
DHCP servers became overloaded.  If that is the case
or packets are being lost for other reasons, IP addresses
can be lost.  Correct client behavior tries renewals
multiple times so the underlying problem can grow a
while and remain unnoticed.

DHCP service is disk-bound so a new server setup lacking
previous disk performance (e.g. previous tuning or performance
features) could affect the DHCP service reliability.  As could
disk contention such as using syncing syslog for the DHCP log.

John Wobus
Cornell U IT

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