Inconsistent renews from F/O peers

[Mark Sandrock]

Our ISC DHCP version is 4.2.4-P2.

It may be that 'dhcp-cache-threshold'
is not yet supported in that version,
which would help account for the
different behavior of the FO peers.

Thank you.
Mark

Sent from my iPhone

> On May 11, 2015, at 11:20 AM, Shawn Routhier <sar at isc.org> wrote:
> 
> 
>> On May 11, 2015, at 9:02 AM, Simon Hobson <dhcp1 at thehobsons.co.uk> wrote:
>> 
>> Mark Sandrock <sandrock at mac.com> wrote:
>> 
>>>    it sometimes happens that shortly
>>> after obtaining an initial lease of MCLT,
>>> (3600 seconds), some Windows clients
>>> send a broadcast renew request that
>>> is responded to differently by the two
>>> failover peers.
>> 
>>> Although this seems incorrect behavior
>>> on the switch'es part, the pathological
>>> behavior of Windows renewing a lease
>>> only 3 seconds into it, also seems wrong.
>> 
>> When this happens, do you have any indication how long it took to get the reply back to the client ?
>> I'm wondering if a combination of factors (delay by the server, delay by the switch (I assume that the DHCP packets are being handled by the management CPU)) are leading to a delay at the client which is long enough for it to retry - hence the second renew after 3 seconds.
>> 
>> You may need to leave a packet capture running until you capture an event. IIRC there is a field (who's name escapes me at the moment) in the packet which indicates how long the client has been trying - if the first packet has 0 and the second has 3, then this would seem to support the hypothesis.
> 
> It is the “secs” field.
> 
> **
> 
> Another item to check is if dhcp-cache-threshold is enabled on both servers.
> This is a feature to limit the number of times the servers touch the lease file.
> If a client requests a renew early instead of handing out a full lease the server
> simply hands out the previous lease again, correcting the lease time it sends 
> as necessary.  So when renewing a lease of 3600 seconds 3 seconds after
> it was handed out the server would hand out a time period of 3597 seconds.
> 
> 
> 
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