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Configuring lease time

Configuring lease time

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What should be my lease time or How do I select my lease time?

This is very common debate that I keep having with my collogues and I am sure this question must be haunting many dhcp admin’s mind as well. Lease time can be as low as 15 minutes( tech. it can be in order of secs) to INFINITE.   However there are few parameters that an admin must keep in mind while deciding on the lease time. Those parameters are mostly the following

1.      User load or  density or number of user to support vs. number of free Ip address

2.      User affinity to the site

3.      Mobility,

4.      frequency of network configuration changes

There may be additional parameters which can be added but I think considering the above will solve most of your issue wrt lease time

1.      User load or  density or number of user to support vs. number of available IP address

If the user load is high i.e. you have more number of users as compared to number of free ip address than to optimize the usage of Ip address you should keep the lease time short. May be in order of 1’s of hour. Recommended is 4 or 8 hours. If you have more number of free ip address then the number of users to support then you can have a long lease time in order of Days. Microsoft default is 8 days. You can safely use it

2.      User affinity to the site

In public location like coffer shop, airport, railway station, etc. where user keep visiting the lease time should  be short like 15 min. to avoid unnecessary blockage of ip address by stale user. By doing this you will make sure that at given point of time you have minimum blocked ip address that are not being used by active users and thus ensure optimum usage of your ip address scope

In offices where user location is permanent eg. Desktop it’s recommended to have a long lease time like  Microsoft dhcp server default 8 days.

3.      Mobility

If you need to support mobile users carrying notebooks , hand held devices ,etc then you should have short lease time in order of hours like 2 hrs or 4 hrs or 8 hrs

4.      frequency of network configuration changes

If the network configuration change happens quite frequently may be weekly or monthly then in that case its better to keep the lease time in order of 12 hrs like 12 hr or 1 day. As longer the lease time longer it takes for client configuration changes controlled by DHCP server to propagate.

 

Should I have INFINITE lease time?

It’s technically possible and most of the devices support it but the recommendation is never to have infinite lease time. The main reason is that any change in network configuration on dhcp server will not be updated on the client as the client will not trigger renew.  Also it’s reported on some site that few devices don’t behave properly with INFINITE lease time and result in service crash and other issues. So if you are dhcp admin and want to avoid unnecessary issues it’s recommended not to have INFINITE lease time

Neelmani

Windows Enterprise Networking

 

[This posting is provided "AS IS" with no warranties, and confers no rights.]

 

 

Comments
  • I neeed to change a infinite lease time for vista, cause allways when lease time is finished my PC get after shut down a free IP   adress, but I need a fix IP (defined 192.138.1.34 ) adress for this PC.

    In vista it should possible like in window 2000 to say for lease time INFINITE

    brdt regards   Peter

  • Can someone redirect me to an elaborated explaination of a DHCPINFORM packet structure?

  • How would an enduser get lease time increased?  According to AT&T who I connect BB@7.2mbps their DHCP servers are dynamic and they do not assign a finite lease.  However, I get 300 seconds and noone has been able to assist(Dell/AT&T/Errikson).  The value in the registry at hklm\system\currentcontrolset\services\tcpip\parameters\interfaces\{BA1DFEA2-5559-4080-978D-FBF7D82A7A95},Lease,REG_DWORD,0x0000012c(300) is uneditable.

    The value at hklm\currentcontrolset001\services\tcpip\parameters has the above adapter id with a long string binary variable.  I would bet this entry is uneditable as well.  

    Please help!

  • Hey,

    You cannot change lease time at the client side. The lease time information is sent by the DHCP server when leasing an address. Looks like AT&T is setting a lease time of 300 sec for you. However, this shouldn't be of concern to you as the DHCP protocol itself takes care of renewing/ extending the lease when it is about to expire.

    Raunak Pandya

  • Well, the concern is that every 2.5 minutes all packet travel stops while I suppose the ip is renewed.

    I would say 90% of the time it is un-noticeable.  However during large file downloads or during webex/gotomeeting presentations the pause can become quite disconcerting.

    Any idea what words could be used to get through the front line tech (who has no clue) to the 'network engineers' at AT&T to change the lease value from 300 to say 3600?

    Is there a .cfg file or the like an admin can look-up for a specific user/device connecting to the network?

    Ed

  • Did you try checking "ipconfig /all" output once the connectivity goes every 2.5 mins? Does your machine still has an IP address? What is the lease time.

    If the real problem is renewing the IP, check packets on the wire using netmon. You should be seeing DHCP Request and DHCP Ack packets at the time of renew which is usually @ half time of renew time.

    The settings on the server side for MS DHCP Server are stored seprately in a database and can be changed using DHCP Server management console. However, you are not sure what server AT&T are using.

    Raunak Pandya

  • Thanks Raunak,

    Yes to all the above, I get consistent reply with lease expires minus lease obtained times of 300 seconds which is 5 minutes and every 2.5 minutes renewal attempts occur.  I cannot trace a wire since the connection is wireless.  I suspect AT&T should be able to search for my device's IMEI or MAC address and identify the server(s) in question.

    I appreciate your help, I'll let you know if I succeed with AT&T.

  • The main reason is that any change in network configuration on dhcp server will not be updated on the client as the client will not trigger renew.

    ____________

    thomasjimmy

    <a href=”http://anycarfinance.com" rel=”dofollow”>Any Car Finance</a>

  • How about effects on DNS? If you significantly short the lease time, would you then need to take care of DNS aging as well? MS lease default for LAN is 8 days. If we want to drop it to 8 hours, should we set aging and scavening for 1+1 day?

  • Hi Yannara, yes - your assessment is correct. The DNS aging time should be longer than the lease time. If you set lease time to 8 hours, you can make DNS aging interval slightly longer than 8 hours.

  • Hi, if the lease duration is reduced, will it take effect on the IP addresses that were previously issued prior to the change? Or the lease expiration will stay as is for the already issued IPs and will take effect only on the new ones issued?

  • Hello EDLS, the changed lease duration takes effect only when the client renews the lease or when the client sends a new lease request. Till the client renews the lease, its expiry will be as per the previous lease duration.

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