Problem obtaining IP from a cisco router

David W. Hankins David_Hankins at isc.org
Thu Feb 8 21:46:43 UTC 2007


Sorry I missed your original message.

On Thu, Feb 08, 2007 at 11:48:08AM -0700, Jeremy Bessee wrote:
> Brad Stockdale wrote:
> 
> >172.16.208.15       00d0.b7a9.26c2          Infinite                Manual
> >172.16.208.129      00d0.b7a9.26c2          May 03 1993 08:06 PM    Automatic

ISC dhclient will 'remember' the most recent lease it had.

If dhclient had gotten this address from your Cisco before you assigned
the static, it will store a record of this in 'dhclient.leases' somewhere
on the client system (where depends on the system).

When it reboots, or is forcibly restarted, it will attempt to request
this address (using the requested-address option, #50).

So it may be that the Cisco is allowing the client to request an
address that is not statically configured for it (pretty reasonable
behaviour).  Because the address is available, it lets the client
have it, even though it would give out a static normally.

That's my theory, anyway.  ISC dhcpd does the same thing by default.

It's a good default.

In ISC dhcpd, we combat this "client stickiness" to leases by
providing explicit allow/deny syntax.  In dhcpd.conf, you would
put "deny known;" or "deny members of classname;" in the relevant
dynamic pool, so that the client would not be offered leases from
the dynamic pool (it would be NAKed, and later OFFER'ed the fixed
address).

I don't know anything about Cisco DHCP config syntax, much less
enough to provide guidance on how to procede.

For dhclient's end, a simple fix would be to kill dhclient, remove
dhclient.leases, and start dhclient.  It would accept whatever the
server OFFER'ed in that case then.

-- 
David W. Hankins	"If you don't do it right the first time,
Software Engineer		you'll just have to do it again."
Internet Systems Consortium, Inc.	-- Jack T. Hankins


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