DHCP failover is a feature that allows two DHCP servers to share the responsibility of assigning IP addresses to clients in the same scope. This way, if one server fails, the other can take over and prevent service disruption. But how do you choose between the two modes of DHCP failover: hot standby or load balance?
In hot standby mode, one server is the primary and the other is the secondary. The primary server handles all the DHCP requests and leases, while the secondary server only acts as a backup. The primary server periodically replicates its database to the secondary server, so that the secondary server can take over if the primary server fails. The advantage of this mode is that it provides a simple and reliable failover mechanism, but the disadvantage is that it underutilizes the resources of the secondary server.
How to choose between DHCP failover modes depends on your network design and requirements. Some factors to consider are: ? The location and connectivity of the DHCP servers. If they are on different subnets or have unreliable network connections, hot standby mode may be more suitable. If they are on the same subnet and have reliable network connections, load balance mode may be more efficient. ? The load and performance of the DHCP servers. If one server has more resources and capacity than the other, hot standby mode may be more appropriate. If both servers have similar resources and capacity, load balance mode may be more balanced.
A major disadvantage of the hot standby mode is that there can be some uncertainty that the standby server will function properly in the event of a failure. This article talks about testing DHCP failover lower in the article for this reason. Tests are critical so you don't find out your standby server is not working at the worst possible time. Load balance mode, on the other hand, provides a level of comfort knowing both servers are actively handling requests.
In load balance mode, both servers are active and share the load of DHCP requests and leases. Each server has a percentage of the load assigned to it, and each server maintains a copy of the entire scope database. The advantage of this mode is that it maximizes the performance and availability of both servers, but the disadvantage is that it requires more synchronization and communication between the servers.
Be careful with load balance mode as it is more prone to having issues, than hot-standby. A lot of companies add unwarranted complexities when their environment is not large enough to justify it. There are times when both are preferred, but for general rule of thumb, keep it simple wherever possible.
When deciding between hot standby and load balance modes, there are several factors to consider. For example, if you have a large or dispersed scope with many clients, load balance mode may be more efficient than hot standby mode. Additionally, if you have a low-bandwidth or high-latency connection between the servers, load balance mode could cause more traffic and delays. Furthermore, if you have servers with different hardware or software specifications, load balance mode may cause compatibility or performance issues. Finally, if you have a strict service level agreement or a high-availability requirement, load balance mode may offer more redundancy and reliability than hot standby mode.
To configure DHCP failover, you need to have two DHCP servers running Windows Server 2012 or later, and both servers must have access to the same scope. You can use the DHCP console or PowerShell to create a failover relationship between the servers, and specify the mode, load balance percentage, state switchover interval, and other settings. You can also monitor and manage the failover status and statistics using the DHCP console or PowerShell.
Testing DHCP failover can be done through various methods, such as simulating a server failure by stopping the DHCP service or disconnecting the network cable on one server, simulating a network failure by blocking or filtering the traffic between the servers, and simulating a client failure by changing the IP configuration or releasing the IP address on a client. By doing these tests, you can check if the other server can handle DHCP requests and leases, if the servers can detect and resolve communication problems, and if the client can obtain a new IP address from either server.
In order to ensure a smooth and successful DHCP failover, it is important to plan and design your DHCP scope and failover settings carefully. It is also essential to use consistent hardware and software configurations for both servers, as well as keeping them updated and patched. Additionally, secure and reliable network connections between the servers should be used in order to avoid any network congestion or interference. Lastly, monitoring and troubleshooting your DHCP failover regularly is necessary in order to resolve any issues or errors promptly.
Hot Standby Mode from experience tends to have less issues versus Load-balanced when scopes remain near full capacity. Load balanced has a tendency for bad addresses to show up and drop off. This is not bad if your scopes have several addresses free to offer. However, if your scopes only have one or two free addresses in them the bad addresses can be a problem requiring you to manually delete the bad address reservation in windows.
Changes to a scope's reservations, options, and (probably?) other settings don't automatically replicate to the second server other than during the initial setup. The only thing replicating is the lease information. You need to manually maintain these other changes, or have a scheduled task taking care of that for you. It can either run at a normal interval or be triggered by specific events in the windows event log. If you don't do this, you're going to end up with a server potentially misconfigured or missing information. Server options (not scope options) are never replicated. Be sure to manually maintain these so they are consistent on both servers. They likely don't change often (or ever) but don't forget to keep them aligned.