Inconsistent renews from F/O peers

Mark Sandrock sandrock at
Tue May 12 15:46:57 UTC 2015

> On May 12, 2015, at 10:09 AM, Bob Harold <rharolde at> wrote:
>> On Tue, May 12, 2015 at 9:25 AM, Mark Sandrock <sandrock at> wrote:
>> > On May 11, 2015, at 11:20 AM, Shawn Routhier <sar at> wrote:
>> ...
>> >
>> > 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, dhcpd.conf(5) gives a default of
>> 25% for 'dhcp-cache-threshold',
>> which contradicts what I am seeing.
>> The issuing F/O (primary) peer indeed
>> ACKs a 3597 in response to the renew,
>> but at the same time, (as the 3-second
>> renew is broadcast), the secondary
>> F/O peer ACKs an 86,400 value,
>> i.e., extends the MCLT lease time.
>> This seems like incorrect behavior
>> on the part of the F/O peer.
>> Thank you.
>> Mark
> As I understand it...
> As soon as both servers have a copy of the lease information, then they should return the full lease time.  The MCLT time is only returned while the lease information has not been synchronized.  So I think the peer is giving the correct response.  The primary might not have gotten confirmation back from the peer, so it might also be giving the 'correct' response from what it knows.  But you would need to trace the traffic or watch debug logs to know if the primary had gotten a response in those three seconds, in which case it gave the wrong response.

It seems possible that the issuing DHCP
server does not yet know if its F/O peer
knows about the lease... But your
suggestion requires that the stated
default of 25% for 'dhcp-cache-threshold'
does not apply if that parameter is not
present in the 'dhcpd.conf' file -- which
it is not on the F/O peers concerned.

I take it otherwise, that the default applies
in all cases, unless it is over-ridden.

But I don't know that for a fact.

Thank you.

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