Common DHCP Question

Irish ManIrish Man Member Posts: 72 ■■□□□□□□□□
Hey Guys...

This is my very first post......

Can someone please tell me the config for Load Balancing\Fault Tolerance when running two DHCP servers for two different subnets.

I believe its known as the famous 80/20 Rule.

Ohh and a second question regarding the exam. Is the Microsoft Scientific Calculator available when doing any of the subnetting questions ?



  • royalroyal Member Posts: 3,353
    First let me answer your second question. I have been to 2 different testing centers. When I went to Prometric, they did NOT have a scientific calculator. When I went to Pearson Vue, they DID have a scientific calculator. Do not rely on calculators for subnetting. You should know how to do subnetting by hand.

    Now for the 80/20 rule.

    Subnet 1 Dhcp:
    Scope 1
    Scope 2 (Backup Scope for Subnet2) -

    Subnet 2 Dhcp:
    Scope 1 -
    Scope 2 (Backup Scope for Subnet 1) -

    I know the numbers aren't 80/20, but the concept is the exact same. The math wasn't done.

    Basically, what this means, is that if a dhcp server goes down, it can pull dhcp information from a dhcp server in another domain. Of course you'll need a router capable of bootp forwarding or a dhcp relay agent on the subnet that needs to forward to the subnet with the dhcp server.

    One server has its own main scope that takes care of all ip address that end in 20-200. Lets say this accounts of 80% of all the addresses available for that IP Address Class. Now if you had given that scope 100% of the available addresses, and it went down, you are out of luck! Now lets say you change that so the dhcp server only services 80% of the available addresses. You then have another server in another subnet as the backup. It has a scope for other other subnet that services only 20% of the available ip addresses for subnet 1 ips. This ensures that if the main dhcp server fails, the backup dhcp server in the other subnet with 20% of available address will still be able to hand out addresses and clients will be able to work as normal.

    Now you may think. Why not just put the backup dhcp server in the same subnet? Well, it's pretty simple. If both dhcp servers were in the same subnet, both dhcp servers would be fighting to hand out addresses and both dhcp servers would be giving out Offers. This increases network traffic. Also, both dhcp servers would constantly have leased addresses. If the backup dhcp server is in another subnet, the dhcp server that is local will just about always be the one handing out the leases since it has better/faster connectivity. The backup server will then, in essence, be a backup server, not load balanced. Of course if you want load balancing, you can put 2 dhcp servers in the same subnet and just do 50/50.

    Hope this helps and is not too confusing.
    “For success, attitude is equally as important as ability.” - Harry F. Banks
  • Irish ManIrish Man Member Posts: 72 ■■□□□□□□□□
    Thanks icroyal

    This is an excellent explanation.

    Cheers :)
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