Mikrotik RouterOS – How to Forbid DHCP Server from Leasing Specific IP

You should consider replacing the DHCP server, as it is obviously broken. DHCP servers should keep lease information between restarts and preferably also probe addresses before releasing them into the pool to avoid address duplication.

If that is not an option you can drop the lease length. As long as the DHCP server can handle the churn it should work, but short leases will cause a small increase the amount of broadcast traffic on your network.

Short leases are primarily a problem when you have clients disconnecting and reconnecting a lot, for example in WiFi networks. Very short leases (less than 1 minute) can cause weird problems with some DHCP clients that have time-outs longer than the lease.

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.