DHCP Server Issue

billybob01

As we know when using 2 DHCP Servers it`s recommended to use the 80/20 rule, but i have noticed that on our 2 servers we have 8 scopes and 4 scopes have 20% and the others have 80% ip leases, this is the same on both servers! Is this correct? As i cant help in thinking that our IT Manager has set them up as 50/50.

blargoe

Should be OK, as long as you don't have the same address being leased by both servers.

billybob01

Thanks blargoe, what about the leases though, we have ours set at 1 day, so after a weekend break, should there still be leases shown within the scopes? or should it be clear? What troubleshooting can i do in order to check the replication between the DHCP Servers, so that i know all is well?

royal

There is no replication between DHCP servers.

Example:
192.168.1.1 not part of your scope and reserved for router
192.168.1.2 through 192.168.1.20 reserved for servers (this can be a part of your scope and reserved or just not added to the scope at all)

DHCP Server #1 (80%) - 192.168.1.21 - 192.168.1.220
DHCP Server #2 (20%) - 192.168.221 - 192.168.1.254

There is no replication needed as each DHCP server contain a scope that hands out different IP ranges.

You might have it set up where they hand out the same set of ranges, but it has different reservations. Hence why you think there might be some replication going on.

Example:
DHCP Server #1 (80%)
Scope - 192.168.1.21 - 192.168.1.254
Reservation - 192.168.1.221 - 192.168.1.254
Result - Only hands out 192.168.1.21 - 192.168.1.220

DHCP Server #2 (20%)
Scope - 192.168.1.21 - 192.168.1.254
Reservation - 192.168.1.21 - 192.168.1.220
Result - Only hands out 192.168.1.221 - 192.168.1.254

And ya, once a lease expires, it's opened up for anyone else and should be cleared. That's pretty much the whole point.

billybob01

Thanks for the info royal, i thought as much but i wanted to confirm my thoughts with someone. It`s just confusing me how my manager has set the dhcp servers up. i logged on before anyone got into work on monday and noticed loads of ip addresses just sitting there in the scopes, also the 8 scopes are divided up so that 4 have an 80% lease and the other 4 have a 20% lease! There are no reservations to be seen!!

royal

Sorry, it's exclusions, not reservations.

Anyways, using exclusions vs not using exclusions is merely just a preference. And btw, 80/20 is the book information. It's better to do 50/50 or 60/40 in real-world due to 1 server going down, the other can pick up a lot of the slack instead of only having a few addresses left. Regardless, it's easy to bring a new DHCP server up anyways.

dynamik

royal wrote:

And btw, 80/20 is the book information. It's better to do 50/50 or 60/40 in real-world due to 1 server going down, the other can pick up a lot of the slack instead of only having a few addresses left. Regardless, it's easy to bring a new DHCP server up anyways.

That's the way I've always done it, and it's nice to see someone more experienced back that up. The 80/20 rule can be applied to a lot of things, but I never thought it made sense for DHCP servers. Thanks.

sprkymrk

dynamik wrote:

royal wrote:

And btw, 80/20 is the book information. It's better to do 50/50 or 60/40 in real-world due to 1 server going down, the other can pick up a lot of the slack instead of only having a few addresses left. Regardless, it's easy to bring a new DHCP server up anyways.

That's the way I've always done it, and it's nice to see someone more experienced back that up. The 80/20 rule can be applied to a lot of things, but I never thought it made sense for DHCP servers. Thanks.

Depends on your lease times. If you have leases that last 5-7 days, you should have plenty of time to fix a downed server before clients start lining up for IP's from a backup server.

Personally I prefer 71/29. :P

blargoe

Just to clarify, since someone mentioned reservations... if you do use reservations to reserve an IP assignemnt to a specific MAC address (i.e., if you use DHCP reservations for your network printers), you certainly would need to have them set on both servers to be fully redundant. This would not cause a potential duplicate IP assignment, since the address would always go to the device with that particular MAC address.