exceedingly long time in assigning ip addresses

Simon Hobson dhcp1 at thehobsons.co.uk
Mon Dec 13 17:39:09 UTC 2010


Massimo Manghi wrote:

>on this network another dhcp server exists, they manage a very small 
>group of clients belonging to a different ip zone (though they share 
>the same lan with my zone) and they too don't serve free leases.
>
>not every client has this problem, this seem restricted to windows 
>clients located in a specific section of the building. This suggests 
>an interference between the 2 dhcp servers exists and the problem to 
>be matter of timing (being a server faster to respond than the other 
>on that section of the lan). Are there time parameters to be set 
>that can be adjusted to make the negotiation tolerant of these 
>interferences?

What I suspect may be happening is this (S1 is your server, S2 is the 
other one)

C > Discover
S1 > Offer
S2 > Offer
C > Request
S1 > Ack
S2 > Nack

In this case, your server will "win". But if the timing is different 
then you may have this (as seen by the client) :

C > Discover
S2 > Offer
S1 > Offer
C > Request
S2 > Nack
S1 > Ack

In this case, you lose because the other server has Nack'd the request.

of course, the other server may be configured to only make offers to 
certain clients, in which case the exchange may go :

C > Discover
S1 > Offer
C > Request
S1 > Ack
S2 > Nack

and again, the timing of which server response gets to the client 
first will determine if it gets a usable address.

I'd suggest two things :

1) Sniff traffic on the lan segment in question looking for DHCP packets.
2) "Discuss" the matter with the admin for the other server.

Even though you aren't having anything to do with the other IP subnet 
- you MUST define it alongside yours in a shared network. If you 
don't then your DHCP server will treat requests for the other network 
as "wrong" and NACK them. If this isn't happening then either you 
have authoritative turned off (not generally a good idea), or the 
clients for the other server are not behaving properly.

-- 
Simon Hobson

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
author Gladys Hobson. Novels - poetry - short stories - ideal as
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