Wednesday, February 24, 2016

Quasi-Failover DHCP 2008 R2

I was given a client recently that was running out of IP addresses on their DHCP server. 

After some investigation I discovered that they had 2 DHCP server on the same AD and on the same network. Let say the network is, it had a subnet mask of No really it does have that subnet mask. Anyways, the 2 DHCP servers are giving out the same scope of - DHCP1 was giving an exclusion of - and DHCP2 had an exclusion of 192.168.170 - 254. Of course there were other exclusions for servers and printers but you get the point. 

After looking a little bit deeper I noticed that the DHCP1 server was not giving out any leases. This didn't make sense until I fully thought about what was happening here. 

The reason DHCP2 server was the only one giving out IP addresses is because it is, for lack of a better term, faster. The reason is because once a machine has made contact with a DHCP server, even if it’s out of addresses, it will NOT try a different DHCP server. It will keep  trying that same DHCP server for an address. This is why the DHCP1 is not giving out leases, its to slow to answer. 

So, since DHCP2 is always the fastest, it always replies first, even if its full. The one way of testing, and verifying, this is to turn off the DHCP2 services on DHCP2, momentarily, and do a request for an address from a device. Then look at DHCP1 to see if it gave out the address to that device. Of course we tested this and it worked as I expected. 

Doing an “ipconfig /release” and then an “ipconfig /renew” on a computer would give you the request needed for testing. I would not do this on a computer that already has an address that you need to use to turn the DHCP services back on, maybe do this on a temp PC.

So basically in this configuration there is a quasi-failover DHCP system in place. If DHCP2 is offline then DHCP1 would pick up and start handing out addresses. Of course this is not the best way to setup a failover DHCP environment, but, it kinda works. Here is the correct way:

I hope this helps someone in their endeavour of trying to figure out why a DHCP might not be giving out leases. 

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