DHCP lease time /renewal concept

Posted on 2003-03-12
Medium Priority
Last Modified: 2007-12-19

we have some 13,000 PC using Novell 5.1 servers for DHCP.(2 servers)
our lease time is 7 days. our network got 15 VLAN and a common VLAN (where DHCP servers are in)& WAN .All VLAN got IP forwaders (to forward BOOTP to DHCP server).everything working fine.

As per theory if PC get a IP from DHCP it shd renew by 1/2 of lease.(T1)
If particular DHCP not avilable it should send out (T2) to any server by
87.5 % of lease.(T2).

say I got a IP by 01/03/2003 08:00 am ( lease obtained)
expiry is  08/03/2003 08:00 am (7 days )

now the CLient should attempt to renew after  3.5 days(mean 04/03/2003 08:00 pm).
but actually what is happening is client is renewing it's IP for every restart.
say if i restart by 01/03/2003 evening (say 08:00 pm)now hte new settting in ipconfig will be like following

lease obtained 01/03/2003 08:00 pm
expiry is  08/03/2003 08:00 pm (7 days - from the time i restart the PC )

I checked with so many source including novell site. but what novell telling it is NORMAL. see the attached TID from novell.But i dont feel DHCP working shd be this way. I am doing anything wrong in config on server side ? If server side config wrong also this DHCP REQUEST from client which is win2k,winNT,XP .why this is happening  ? anything due to IP forwaders (nortel products)? if is due to IP forwarders why it is happening across the WAN also (cisco routers forwarding client request to DHCP)?

Frnds thanks in advance for you valuable comments...



Every time the DHCP workstation is rebooted, it renews the lease. This happens even though
1) the DHCP lease has not expired.
2) the T1 (renewal) time has not been reached.
Each time the TCP/IP stack is unloaded and reloaded, or if the workstation is rebooted, the DHCP workstation sends a DHCPREQUEST packet to the DHCP server, which causes a reset of the renewal (T1), rebinding (T2), and the lease time. This is normal.

See RFC 2131 for more information.
Question by:moorthy_kulumani
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Accepted Solution

suhasjs earned 375 total points
ID: 8126026
moorthy, what Novell has told you makes complete sense. Let us understand the concept of leasing IP addressed. The idea is to dynamically & OPTIMALLY use the available block IP addresses. So, if a machine is dynamically allocated an IP address (using BOOTP, say), and you then permanantly allocate that IP address to that machine, you would have permanently lost the address from the pool even if the machine is later taken out of your network.
DHCP introduces the concept to lease, whereby, the lease of the address needs to be renewed periodically, failing which, the address to returned to the pool.

As the machine has be configured to be assigned an IP address by DHCP, it requests for an address on each reboot. The DHCP server then knows that this IP address is definately in use, and reserves it on behalf of the machine for a furthur period of, in your case, 7 days.

Only if there is no request/renewal request for 7 days, the address is returned to pool.

The concept of T1 and T2 times are to ensure that: if the machine is kept continuously on for days, there will not be a situation that the machine is running but it's IP address lease has lapsed. So, T1 and T2 are kept at figures less that 100%, and also the reason that T2 > T1.

Expert Comment

ID: 8126594
The client is making the decision to renew/get a lease each time that it starts up.  This is normal and common in the land of DHCP.  What varies is what happens if a new lease is not obtained.  Some devices will attempt to use the previously obtained IP information and most will have some alternative.  Some systems expect to be able to test for a host using the DHCP assigned IP address.  This fails if the machine is down when it is tested and backup with that IP address when it is restarted.  I am not aware of any device that determines there is time left on the lease and just uses the IP address without performing the lease process.

Author Comment

ID: 8127573
hi friends thanks a lot for your comments...

Yes i have seen devices (Free BSD appliance) which won't go and ask/renew ip address for every restart. these devices are DVB routers given by ISP. say if u purchease  one year WAN link from ISP then they will give you IP with lease one year via DHCP. i have seen whenever i restart my device it won't go and ask the IP  provider or other DHCP server .the lease obtain time also never vary after so many restart... What my concern in our setup is our management dont want the 13000 "DHCP request" coming to the DHCP server everyday. they just want the request to rised by client when least reach 1/2 time of lease.or from new dhcp client not all the time u restart your PC. Eventhough we expalined them about the concept they want solution to aviod the "DHCP request" from DHCP client. my friends also telling these info stored in registry e can control like that but i dont know how to go about this..
hoping a good solution to stop the DHCP request from PCs for every restart...

Once again thanks FYC...
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Expert Comment

ID: 8132196
>now the CLient should attempt to renew after  3.5 days(mean 04/03/2003 08:00 pm).

ok. I remember Novell doing that

> but actually what is happening is client is renewing it's IP for every restart.

Well, I remember early Win9x doing that, but for later OS more people ask to reverse default to what you have.

So I think you could look for MS patch, or, upgrade OS, or, review client. Namely, are you using the Microsoft or the Novell client.  As I recollect, Client 32 was the #1 download (free) from Novell website. You do have to review a Win9x order for Networking clients, to not use MS' Client first, and it may be even better to remove it.

The MS client once had a habit of renewing IP address for every power up.

> Only if there is no request/renewal request for 7 days

goo. while many go partial days, even under an hour, I think two weeks is fine, so each has same leased address after a weekend.

Author Comment

ID: 8133113
I have done the following testing..

 work without novell client ( now we have 4.83 sp1)& different version of client..

 network access order first is novell....

 No MS client..

 As of my seaching no patching available for..NT /2k/xp

 Checked with novell about any setting to do with Novell client related to DHCP ( anyhow without novell client it is working same way)

 to cross chk  i just stop & restart DHCP service will renew the IP address.

 so anyother way i can control this...



Expert Comment

ID: 8141149
whenever you boot your pc it sends a DHCPREQUEST message to make sure your ip address wasnt given to somebody else while your pc was sleeping.

That counts as a new lease.


Author Comment

ID: 8150042
Dear suhasjs,

  Thanks a lot for your commnets.

  I tested some more. WINNT this concept is ok...

  but for WIN2000 & WIN XP clients DHCP server should be up and running always. It won't operate with OLD leased IP (if DHCP server not available within lease expiry time).

 i disabled APIPA (automatic IP address assignment). but say i got IP today for 7 days lease....
i am restarting my PC next day with APIPA disabled
tis time if my DHCP server down my PC won't be having IP address at all...

  i will lose my connection to network....

  need solution for this...


LVL 24

Expert Comment

ID: 8169783

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