Re: Understanding the impact of a DHCP server outage

You are correct.

At 50% of the lease time the DHCP client attempts to renew its lease from the DHCP server that it leased the address from via unicast. If this is not successful the DHCP client waits until 87.5% of the lease time has expired and then re-attempts to renew it address, again via unicast to the DHCP server. If this is unsuccessful the client attempts to obtain a new lease from any DHCP server via the standard broadcast method (DHCP DISCOVER). If no DHCP server responds the client falls back (assuming XP) to the alternate configuration, which is APIPA by default.


"Hugh" <Hugh@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message news:FD888088-3673-48A9-8E02-289B399F0709@xxxxxxxxxxxxxxxx
We want to make sure we fully understand the impact to DHCP clients whenever
we have a DHCP server outage. After doing to testing a tracing, it seems the
breaking point for a DHCP client (at least in our environment) is controlled
by the value of the Rebinding Time Value parameter in the DHCP Offer packet.

If my testing was accurate, DHCP clients (Win XP SP2 clients, specifically)
will continue to use their previously obtained IP address until they reach
the value of the Rebinding Time Value (which is set automatically to 87.5% of
the lease value). So at 50% of the lease time (set to 14 days in our
environment), the client passes the renewal threshold. However, in my
testing, the client still used its IP address. However, once I reached the
Rebinding Time Value, the client would send DHCP Discover packets and, when
no response was received from a DHCP server, the client would then revert to
an APIPA address.

The conclusion from our testing is that DHCP clients must meet two criteria
before they will fail to obtain (or use an existing) IP address:

First, prior to or during the failure, the DHCP client exceeds its Rebinding
Time Value.
Second, during the failure (and after exceeding the Rebinding Time Value),
the DHCP client is restarted.

So my question is simple: Are our conclusions correct? Thanks.
--
Hugh

.

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