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Our wireless in our NYC office has a guest SSID that has no auth. Understandably, we get a LOT of users on this network. Lately, our IP space has been filling very fast at events and I'd like to step down our DHCP lease time so that older addresses are aged out faster.

Our current lease time is 8 hours, but I'm thinking of dropping that down to 30 minutes. My concern is that doing that will prompt the server to begin handing out IP addresses that the clients still believe they're entitled to, since DHCP T1 timeout hasn't elapsed.

What would be the proper procedure for making this change so as to ensure minimal network interruption? Should I progressively halve the lease time so that the T1 timer coincides with maximum lease age?

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  • Is the guest SSID on it's own network? I'm assuming you are using a 192.168.x.x/24 network. Can you change to something larger? Also, what is the average amount of time someone is connected to your guest network?
    – pooter03
    Apr 1, 2015 at 15:35
  • The mean-time-for-use is lower than 8 hours, so lease time is the low-hanging fruit to fix at this point. I'd prefer to have an answer to the question rather than entertain swapping out IP space. Apr 1, 2015 at 15:49
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    Well, the DHCP lease includes the T1 (usually half the time of the lease) and T2 (usually 87.5% percent of the lease time)timers. When the T1 timer is up, the client tries to renew with the same server. If it hits the T2 timer, the client tries to renew with any DHCP server via broadcast. If the lease runs out, the DHCP server should put the address into a grace period before it puts it back in the pool. As far as leasing duplicate IPs, Windows clients, in theory, check to see if the IP address offered is already in use via ARP. If it is, it declines and restarts the DHCP process.
    – pooter03
    Apr 1, 2015 at 19:11
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    Your biggest concern with too short of a lease period is flooding the network with DHCP traffic and bombarding the DHCP server with requests. In your case, try a shorter lease time, maybe 4 hours. You can use wireshark to detect the amount of DHCP broadcasts. Depending on your DHCP server, you may be able to get information and warnings from the logs. If 4 hours isn't causing any problems with the DHCP server, cut it down again and do the same checks until you find a happy medium. Also look here
    – pooter03
    Apr 1, 2015 at 19:15

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