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We have a network setup for a demo, which lasts about 15mn. Our DHCP server is configured to assign ~ 100 addresses (max number of simultaneous connections or our AP) ... but since people might come and go very quickly we need to keep the lease time very short in order to free the IP addresses and allow other people to connect.

Initially I wanted to go for a lease time as short as 25 seconds, considering that the demo is quite short, and to be sure that no IP will be "abusively" reserved by the DHCP server ... However, I am afraid of several things.

First, the impact on the load of the network.

Second, I have read here and there that there might be some "weird" issues with time leases below 1 minute (e.g. What is a good DHCP lease timeout configuration).

Does somebody know what can be the different problems with using such a short time lease? What is the impact on the network? What would be a short but safe lease duration to use?

The client must renew its IP configuration data before the lease period expires. ... ADHCP client automatically attempts to renew its lease as soon as 50 percent of thelease duration has expired. The DHCP client will also attempt to renew its IP address lease each time that the computer restarts.

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I got a small problem with my WNDR3700V5.

My ISP is using a very short dhcp lease time (2 minutes)

It looks like the router is not requesting a new ip-adress once the lease expires.

So after 2 minutes the connection is lost.

I did some tests with an additional router inbetween the ISP WAN connection.

The lowest lease time I could enter on this router was 30 min and I noticed that the WNDR3700V5 requested a new ip-address about 15 min before the actual lease expired.

So it looks like the problem might be that the lease time is too short, the router requests a new ip-address 15 min before it expires but since the lease time is only 2 min (< 15 min) the router actually never requests a new ip-address.

Does anybody experience the same problem of does anybody know how I can modify the 15 min and change it into 1 min.

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With a very low lease time you will see an increase of network traffic, particularly broadcast traffic as the "discover" and "offer" phases of DHCP are layer 2 broadcasts. How much of an issue this is depends on many factors such as the size and complexity of the network, latency, performance of the DHCP server, etc. Keep in mind DHCP clients do not wait until their lease is expired to try to renew it. So if you gave me a 60-second lease I'll be talking to the DHCP server (potentially) every 30 seconds to renew it.

As for "weird" issues, anything goes. Different DHCP clients will behave differently. Some may handle it fine, some may have problems renewing so often and fail. Perhaps there are clients which get a lease and simply sleep for a certain period of time then check if they need to renew or toss the address if it expired. If the sleep is longer than the lease then the system will keep the IP longer than it is allowed to. I haven't seen that issue before but I have seen things like the IP a client requests in the "request" phase being different than the one the server gave it in the "offer" phase but the server actually gave the client the "request" IP, which was already in use. Never under-estimate how poorly software can be written.

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