Saturday, 14 March 2015

Force DHCP Nearest Site - Windows 7 Pointing To The Wrong DHCP Server

How to force DHCP to the nearest site?

This is when you have both a primary and a backup DHCP server or maybe you just have two DHCP servers with equal roles.

In our case we had primary on the main site and backup server in another country. Something happened to the primary server but it was fixed. But the clients kept connecting to the backup one.

The way it worked normally was that clients should seek from the DHCP server that gives the quickest response i.e. the nearest one. So why wasn't it working now?

Simple, because once clients have a lease, they always try to go back to the DHCP server where they got their most recent lease to try to obtain a new one.

The solution was to deactivate the scope of the backup DHCP server for the length of the lease time (I forget what the default is in Windows but I think it's 8 days). As a result, the clients couldn't obtain a new lease and looked for a new DHCP server and reverted back the the primary one (which also happens to be the nearest). After about a week, I reactivated the scope on the backup DHCP server.

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