When you deploy your configuration to each server in the DHCP failover pair, DHCP begins to use a failover configuration. Failover is configured with default values based on best practices.
You can modify the default settings using DHCP service deployment options at the server level. If you need to modify any of them, modifications should only occur on the primary server. For more information, refer to Adding DHCPv4 service deployment options.
Configure the following failover parameters at the server level to modify default values:
- Load Balance Override—Determines when the Primary server or secondary server bypasses load balancing and responds to the client even if the client is supposed to be served by the peer server. Every message from a DHCP client has a secs field, which indicates for how long the client has been trying to contact a DHCP server. If the value in the secs field is greater than the load balance override parameter, the DHCP server always tries to respond to the client, regardless of the load balance split. The default setting is 3 seconds.
- Load Balance Split—Informs the Primary
server what portion of all clients it should serve. A value of 128
(active-active) indicates that both servers split the load evenly and that both
should respond to client requests using a load balancing algorithm to decide
which server should assign the address in each case. If the Load Balance Split
is set to 256 (active-passive), then only the Primary server responds to client
requests. Despite the load balance split setting, the Primary server generally
still only holds 50% of the available addresses for any failover pool and the
secondary server holds the other half, even though the secondary server doesn't
typically respond to requests. The default setting is 128.Note: The value of 256 isn't valid for earlier versions of the DNS/DHCP Server. For earlier versions of the DNS/DHCP Server, configure this option with the value of 255.
- MCLT (Maximum Client Lead Time)—Determines the maximum amount of time by which either server can extend a lease assigned by its peer without contacting the other server. The MCLT is also the recovery interval for the server that failed. As such, it increases the length of time to return to normal failover operations after a server failure. The default setting is 1800 seconds.
- Max Response Delay—Defines how long a peer server waits without receiving any messages from its partner until it assumes that the partner has failed. It should be long enough for a server to notice that its peer isn't responding, but prevents a temporary failure from breaking the failover pair. The default setting is 60 seconds.
- Maximum Unacked Updates—This setting defines how many binding updates a DHCP server can send without receiving acknowledgments. The default setting is 10 updates.
Note: The primary server uses TCP port 647 to contact the secondary server and the secondary server uses port 847 to contact the primary. These port numbers aren't configurable.