Failover Association Operations

When a host broadcasts a DHCPDISCOVER message, it includes its MAC address. Both the primary and secondary peers receive this message. To determine which server should allocate an IP address to the host, they each extract the MAC address from the DHCPDISCOVER message and perform a hash operation. Each server then compares the result of its hash operation with the configured load balancing split. The split is set to 50% by default to ensure an even split between the two servers. When the split is 50%, the primary server allocates the IP address if the hash result is between 1 and 127, and the secondary server allocates the IP address if the hash result is between 128 and
255. As a server allocates an IP address, it updates its peer so their databases remain synchronized.
As shown in Figure 30.1, when a host broadcasts a DHCPDISCOVER message, both the primary and secondary servers receive the message. They perform a hash operation on the MAC address in the DHCPDISCOVER message, and the result is 250. Since the load balancing split is 50% and the hash result is 250, the secondary server responds to the host with a DHCPOFFER message. The secondary peer allocates an IP address from its assigned pool of IP addresses. It then sends a lease update message to the primary server so that the primary server knows how the address is assigned and can properly take over if the secondary server fails.


Figure 30.1 Load Balancing and IP Addresses Allocation