DHCP Reservations VS Static IP

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Course: Network +
Episode: Networking Services/Applications
Episode Time: 21:55

Hi everyone,
I first want to give props to Ronnie Wong and Daniel Lowrie for their ability to clearly elaborate on the covered topics in the Networking + series.
Thank you for that.

Questions: Would it be better to statically assign network configurations to mission critical machines on a network? If I understand correctly, when the DHCP Server goes down, the machines with statically assigned network configurations will still have access to the LAN/WAN.

For the purpose of this question, the DHCP Server is not on the same machine as the Domain Controller/DNS Server.

Thanks Everyone

Hey Marc,

Thank you for a great question. You are right in your understanding!

1- If you have statically assigned devices on your LAN, these devices will not be affected by the failure of your DHCP server.
2- If you have reservation assigned devices on your LAN and the DHCP Server fails then these devices do not necessarily stop communication with the LAN, however there are a couple of things to keep in mind:

• How long the IP address leases are set for.
  These leases are typically 8 days (but this is up to the server administrator)

• How much time is remaining on the reservation assigned leases.
  At the time of the DHCP server failure if a mission critical device has say 7 days left on the IP address lease and the DHCP server is restored in less than the remaining IP address lease duration then mission critical services will not be affected by the server failure. However if it takes longer than the lease duration to bring the DHCP server online then your mission critical devices and the services they provide will most definitely be affected.

When it comes to which is better well.....it depends (I hate answers like that...lol). Remember that DHCP is a Dynamic Host Configuration Protocol which means we can configure a lot more than just handing out an IP address such as default gateway, primary and secondary DNS server, domain names... and more. So in that context statically configuring say 25 servers would not be bad to reconfigure but if you have to statically configure 1000 servers then you are in for quite a task. So DHCP can help us with the management of these other configurations and avoid mistakes. Imagine a scenario in which you need to reconfigure the network's subnet mask, again 25 servers is not so bad to statically manage but 1000-2000 servers statically managed can be a bit complex and decentralized....insert a DHCP server, a simple reconfiguration of the DHCP options, it can hand out the new configuration to all those servers and all through centralized management.

Now this leads us back to your question still "What about a DHCP server failure," in a large environment they will plan for DHCP failure through DHCP clustering in which the DHCP servers can communicate with each other and become aware of a failure then respond accordingly.

Let's sum it up:

Smaller environments can benefit from static assignment in case of a DHCP server failure if money/scalability is an issue.
Larger environments can most certainly benefit from static IP assignment in case of a DHCP server failure, however the greater the amount of statically assigned devices the greater the complexity in managing configurations as well as reconfigurations. These networks can benefit from implementing DHCP server failover and pushing out configurations through DHCP.

So in the end both network types can benefit from static assignment each with their own considerations.

So I hope this helps, as there is not a one-size-fits-all answer!!

Wes Bryan

@Marc-Andre-Quesnel,

Wes gives some great comments to the question and I agree with him. I would especially emphasize the his mention of options that your servers may or may not need.

IF...those hosts need additional IP configurations that may change. I'm thinking of configurations for SIP phones or thin clients but you still want them to have the same IP addresses, then DHCP reservation is probably a great option, especially if those options change. I think of thin clients because that's what we added later to our network they needed the additional options that I could have manually configured across the 50 - 100 thin clients but, the options were the least intrusive and best consistent way to do so.
But if you're not needing additional IP configuration, then it would probably be better for static for the reasons that Wes mentioned.

Great points Wes and Ronnie, Thank you for that.

I hadn’t thought in a context of having thousands of servers to configure. When you put it like that, statically assigning network configurations seems much more complicated with all the decentralized management involved.

But in an enterprise environment (1000s servers), won’t we need to manually create a static entry in the DHCP server one by one?

Come to think of it, would there happen to be a feature in DHCP server that allows the admin to select a cluster of dynamically assigned IP addresses and to reserve them in one step.

Microsoft suggests creating a csv file with: *ScopeId,IPAddress,Name,ClientId,Description *
and to subsequently import these reservations via PowerShell.

Microsoft Technet Source

But wouldn’t it be much easier (if possible) to place these servers on a VLAN and assigning one specific DHCP scope to this VLAN (.2 to .1001). Once all servers have obtained a dynamic IP address, selecting all of these dynamically assigned addresses in DHCP server, and reserving them with one click.

Thank you again to you both. Really enjoying ITProTV.

@Marc-Andre-Quesnel said in DHCP Reservations VS Static IP:

Microsoft suggests creating a csv file with: *ScopeId,IPAddress,Name,ClientId,Description *
and to subsequently import these reservations via PowerShell.

Microsoft Technet Source

But wouldn’t it be much easier (if possible) to place these servers on a VLAN and assigning one specific DHCP scope to this VLAN (.2 to .1001). Once all servers have obtained a dynamic IP address, selecting all of these dynamically assigned addresses in DHCP server, and reserving them with one click.

Thank you again to you both. Really enjoying ITProTV.

Yes and no again. If you want to accept the randomly assigned address. Then yes, it would be easier. But If you want to assign particular IPs to particular servers or hosts, then no. Using the CSV file allows you to ultimately control who gets what address by telling DHCP how that reservation is to be handled.

Thanks Ronnie