Shortening lease on one subnetL: need reduce MCLT for all?

John Wobus jw354 at
Fri Dec 11 16:19:44 UTC 2015

> On an ISC DHCP server failover pair, version 4.2.5, I need to shorten
> the maximum DHCP lease time for one subnet from 2 hours to 20 minutes
> temporarily to allow the IP address ranges to be expanded.  This is
> together with a Cisco ASR9k that I am told is working as a caching
> relay for the DHCP server pair.
> Now it seems reasonable to reduce the MCLT to ten minutes during the
> work.  Is that correct?  From my understanding, there will be rather
> counterproductive behaviour if the MCLT is left longer than the
> maximum lease: is that not the case?

I think it’s reasonable to design around leases longer than
the MCLT for ordinary use but I don’t think violating that breaks
everything: I imagine it just makes failure recovery less than

We run a shorter lease time than the MCLT regularly, for
clients in our “walled garden”, who we want to force to another
non-walled pool after they register with us, so we give them a
very short lease time.  This approach has its own challenges, but
I don’t recall the MCLT discrepancy being any practical concern.

I don’t know anything about caching relays.

John Wobus
Cornell U IT

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