DHCP Lease Time - 150s

[Glenn Satchell]

Yes, 5 minutes would be appropriate if you're doing 5 minute leases.

If you have a lot of unknown clients using 5 minute leases could 
generate a high load on your server with the frequent renewals. Remember 
that most clients renew at around half the lease time, so you'd be 
getting a request and sending an ack every 2.5 minutes. Changing the 5 
minute lease to 10 or 15 minutes will drop the number of requests to 
half or one third of the current number. If there's only a few unknown 
clients then the 5 minute lease could be ok.

regards,
-glenn

On 05/27/10 01:48, Colantuoni, Robert wrote:
>
> Glenn,
>
> Thank you!
>
> Yes it looks like MCLT is set to 150. I hadn't caught this b/c it's in a file that doesn't change. I recall that it should be less than the lease time correct?  If I'm handing out either a 5 min lease or 7 day lease, would it be appropriate to do 5 minutes?
>
>
> ----------------------------------
> Robert G Colantuoni
> CIT - Operational Support Services
> University at Buffalo
> rgc at buffalo.edu
> 716.645.3552
>
>
>
>> -----Original Message-----
>> From: dhcp-users-bounces+rgc=buffalo.edu at lists.isc.org [mailto:dhcp-users-
>> bounces+rgc=buffalo.edu at lists.isc.org] On Behalf Of Glenn Satchell
>> Sent: Wednesday, May 26, 2010 11:43 AM
>> To: Users of ISC DHCP
>> Subject: Re: DHCP Lease Time - 150s
>>
>> Hi ROb
>>
>> What is mclt set to in the primary failover configuration?
>>
>> mclt is used as an initial lease time to offer clients in a failover
>> configuration. If the client accepts the lease then the server transfers
>> this information to the partner and offers the requested lease time the
>> next time the client renews.
>>
>> 150 is probably a bit low, so something like in the range 5 to 30
>> minutes might be better, although setting this to the best value is a
>> bit of a black art.
>>
>> regards,
>> -glenn
>>
>> On 05/27/10 01:31, Colantuoni, Robert wrote:
>>> I have an issue with short lease times being issued, mostly with Cisco VoIP
>> phones. We have a pretty vanilla config, using failover, that boots known
>> hosts from one pool of addresses and unknown hosts from another pool. We also
>> hand out different lease times for known and unknown hosts.
>>>
>>> ISC DHCP 4.0.0
>>>
>>> Relevant Config Sections:
>>>
>>> -- cut --
>>>
>>> # LEASE OPTIONS - GLOBAL
>>> default-lease-time 604800;      #  7 days.
>>> max-lease-time 1209600;         #  14 days.
>>> min-lease-time 604800;          #  7 days.
>>>
>>> -- cut --
>>>
>>> if not known {
>>>           default-lease-time 300;         #  5 minutes
>>>           max-lease-time 300;             #  5 minutes
>>>           min-lease-time 300;             #  5 minutes
>>> }
>>>
>>> -- cut --
>>>
>>> class "active"   {
>>>           match hardware;
>>>           default-lease-time 604800;      #  7 days.
>>>           max-lease-time 1209600;         # 14 days.
>>>           min-lease-time 604800;          #  7 days.
>>> 	 -- cut --
>>> }
>>>
>>> We then declare all hosts we know about like this:
>>>
>>> subclass "active" 1:00:1e:f7:c2:3e:7e;
>>> host voip-6222 {
>>>        hardware ethernet 00:1e:f7:c2:3e:7e;
>>>        -- cut --
>>> }
>>>
>>> This has worked well for us for many years, but I've recently seen an issue
>> crop up where the DHCP server is handing out leases of 150 seconds to known
>> hosts that should be receiving 7 day leases.  I thought perhaps this was
>> related to the 300s default above, so I changed it to 400s but the short
>> lease time didn't increase relatively to 200s.
>>>
>>> There is nowhere in my config that specifies anything like 150 seconds for
>> a lease! I'm pretty sure the defaults are higher than that also. I've grepped
>> through the source and I didn't see anything obvious. Even more puzzling is
>> that the tstp (the date the DHCP server tells it's failover peer that the
>> lease expires) is correctly 7 days:
>>>
>>> lease 10.7.6.61 {
>>>     starts 3 2010/05/26 14:31:33;
>>>     ends 3 2010/05/26 14:34:03;
>>>     tstp 3 2010/06/02 14:32:48;
>>>     tsfp 3 2010/06/02 14:32:48;
>>>     atsfp 3 2010/06/02 14:32:48;
>>>     cltt 3 2010/05/26 14:31:33;
>>>     binding state active;
>>>     next binding state expired;
>>>     hardware ethernet 9c:af:ca:ff:ca:95;
>>>     -- cut --
>>> }
>>>
>>> The phones seem to eventually get a 7 day lease, but it's intermittent.
>>>
>>> I hope the email provides enough detail... Thanks in advance,
>>>
>>> Rob
>>>
>>>
>>>
>>> ----------------------------------
>>> Robert G Colantuoni
>>> CIT - Operational Support Services
>>> University at Buffalo
>>> rgc at buffalo.edu
>>> 716.645.3552
>>>