DHCP server dual-device hot backup effectively implements rapid service switching by keeping user session information synchronized on the master and backup devices in real time on the control and forwarding planes. The user session information (including the IP address, MAC address, DHCP lease, and Option 82) generated during user access from the master device is synchronized to the backup device. When VRRP detects a link failure on the master device, a VRRP packet is sent to adjust the priority, triggering a master/backup VRRP switchover. After the master/backup VRRP switchover is performed, the original backup device takes over to assign addresses for new users or process lease renewal requests from online users. Users are not aware of DHCP server switching.
Figure 1 shows the typical network with a VRRP group deployed. DeviceA and DeviceB are the master and backup devices, respectively. Both DeviceA and DeviceB are DHCP servers that assign IP addresses to clients. In normal situations, DeviceA processes DHCP users' login and lease renewal requests. If DeviceA or the link between DeviceA and the switch fails, a master/backup VRRP switchover is performed. DeviceB then becomes the master. DeviceB can assign addresses to new users or process lease renewal requests from online users only after user session information on DeviceA has been synchronized to DeviceB.
If DeviceA or the link between DeviceA and the switch fails, new users cannot go online and the existing online users cannot renew their leases. To resolve this issue, configure DHCP server dual-device hot backup on DeviceA and DeviceB.
On the network shown in Figure 2, after DHCP server dual-device hot backup is configured on DeviceA and DeviceB, DeviceB synchronizes user session information from DeviceA in real time. If a master/backup VRRP switchover occurs, DeviceB can assign addresses to new users or process lease renewal requests from online users based on the user session information synchronized from DeviceA.