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List:       dhcp-users
Subject:    Re: Recovery protocol - minimize outage time
From:       Simon Hobson <dhcp1 () thehobsons ! co ! uk>
Date:       2014-06-20 12:55:10
Message-ID: FBE5D718-BA6D-4DF9-8130-B2FAB8D8415A () thehobsons ! co ! uk
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Amanda Edades <ampaed1@gmail.com> wrote:

> ... I am not sure if a single server can handle potentially up to 350,000 leases \
> being renewed every 60 seconds for 16+ minutes

To which the obvious response is "don't set a short lease time" !
Setting very short lease times is a recipe for problems - it artificially increases \
the load on your server (typically just when you don't need it), and it also \
increases the effect of high server load.

Eg, say you load up your server to the point where it misses some requests. With a \
lease of (say) 2 hours, a client will start trying to renew at 1 hour into the lease \
(ie 1 hour left). It will try harder and harder as time goes on but it will only drop \
off the network after a full hour. That means you could take the DHCP servers offline \
completely for up to an hour and the only devices affected would be those newly \
joining without a current lease.

Set your lease to 60 seconds and clients will attempt to renew at 30s. After just \
another 30 seconds the device will drop off the network. That doesn't give you much \
time between a problem occurring and the helldesk phones going into meltdown !

Plugging your numbers in, 350,000 leases means roughly 350,000 renewals/hour with a 2 \
hours lease - or just under 100/second. With a 60 second lease, that makes it around \
700,000 renewals per *minute*, or heading on for 12,000 renewals/second. That latter \
figure will take some serious server capacity and performance tweaking to handle. But \
to make it worse, if you don't have some very high spec server and it lags behind, \
it'll have to handle several times that load as clients will probably not wait long \
before they send another renewal request, then a shorter time, then a shorter time \
before they broadcast a Discover packet (which means even more server load). You \
don't need much of a backlog and the load rockets, the backlog increases, and your \
network falls over as devices fall off it.

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