FAQ-A backup solution of DHCP server and VRRP

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When VRRP hot standby group is used as the gateway, DHCP servers may not back up each other. Because the VRRP group does not support backup of the DHCP address pool and address allocation status, the master and slave devices are independent DHCP servers. There are problems as follows:

1. If the DHCP server and address pool are configured only on the master, the DHCP server goes down when the master is down. After the slave upgrades to the master, it cannot assign addresses to clients.

2. If the same DHCP server and address pool are configured on both master and slave devices, address conflict may occur.

You can use the following methods to solve the problem:

1. Configure CSS or iStack to virtualize two devices into one, and configure it as the DHCP server. This solution implements backup while eliminating conflicts.

2. Both master and slave devices are configured as DHCP servers, but their address pools cannot be the same. You can divide an IP address segment into two parts, with the equal number of addresses assigned to master and slave devices. This solution prevents address conflict. When the master goes down, some devices need to apply for addresses from the slave and their IP addresses may change. Ensure that there are sufficient addresses for clients to apply for.

Other related questions:
Does the DHCP server support VRRP
In V100R006C00 and later versions, the master device in the VRRP group assigns IP addresses. The IP address pool information on the master router cannot be synchronized to the slave routers in the VRRP group. Therefore, you are not advised to configure the DHCP server for VRRP groups.

Can both the DHCP server and VRRP be configured on an S series switch
For the S series switch excluding the S1700 starting from V100R006C00, the DHCP server assigns addresses only to the master device in a VRRP group. The IP address pool information on the master device cannot be synchronized to the backup device in the VRRP group. Therefore, you are advised not to configure the DHCP server and VRRP.

Configure active and standby DHCP servers for S series switches
The active and standby DHCP servers can be configured on S series switches (except the S1700). To improve network reliability, you can deploy two DHCP servers, one as the master server and the other as the backup server. VRRP can be configured on the two servers. When the master server is faulty, the backup server dynamically assigns IP addresses to hosts, ensuring that host services are not interrupted. In normal cases, the master device dynamically allocates IP address to clients. When the master device is faulty, VRRP triggers a master/backup switchover in which the backup device takes over the role of the master device and dynamically allocates IP addresses to clients. Real-time data backup cannot be implemented between master and backup devices in the VRRP group. To prevent IP address conflict after the master/backup switchover, the IP address ranges configured on master and backup devices must be different. In this way, after the master/backup switchover, the backup device assigns network configuration parameters such as IP addresses to newly online DHCP clients. Some clients have obtained IP addresses from the master device, and send Request messages to the DHCP server to request lease renewal. Because their IP addresses do not exist on the backup device, the backup device returns DHCP NAK messages so that these clients will apply for new IP addresses. When the original master device recovers, users can decide whether to switch service traffic back to the master device depending on the actual requirements. When configuring the DHCP server in the VRRP network, pay attention to the following items: - To implement this function, the system version is required to be upgraded to V100R006C00SPC800 or a later version. - In spite of different IP address ranges of master and backup devices in the VRRP group, both of IP address pools use the virtual IP address as the gateway. - In the two address pool views, run the exclude command to exclude real IP addresses of master and backup devices in the VRRP group to prevent address conflict. - Only a half of planned IP address segments can be used simultaneously. Therefore, the same number of extra IP addresses need to be planned, consuming large amounts of IP address resources. For configuration details about the master and backup DHCP servers, click S系列交换机:VRRP组网下同网段内配置基于全局地址池的DHCP服务器示�?.

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