Inconsistent renews from F/O peers

Shawn Routhier sar at
Mon May 11 16:20:32 UTC 2015

> On May 11, 2015, at 9:02 AM, Simon Hobson <dhcp1 at> wrote:
> Mark Sandrock <sandrock at> 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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