DHCP Failover

Two DHCP failover modes are available to use when you create a DHCP failover relationship: Hot standby mode: This mode provides redundancy for DHCP services. Load balance mode: This mode allocates DHCP client leases across two servers.

With DHCP failover, DHCPv4 scopes can be replicated from a primary DHCP server to a partner DHCP server, enabling redundancy and load balancing of DHCP services. DHCP servers that share a failover-enabled DHCP scope are called failover partners. Microsoft’s implementation of DHCP failover is based on the Internet Engineering Task Force (IETF) DHCP Failover Protocol draft.When two DHCP servers are configured for failover, they will share scope information, including all active leases. This enables both DHCP servers to provide leases to the same subnet for load balancing or redundancy purposes. Scope settings are replicated when you first configure DHCP failover, and can be replicated again later if configuration changes are made.Two DHCP failover modes are available to use when you create a DHCP failover relationship: Hot standby mode: This mode provides redundancy for DHCP services. Load balance mode: This mode allocates DHCP client leases across two servers.

The following specifications apply to DHCP failover: You cannot configure DHCP failover on a DHCP scope to include more than two DHCP servers. DHCP failover supports DHCPv4 scopes only. DHCPv6 scopes cannot be failover-enabled. DHCP failover partners must both be running Windows Server 2012 or a later operating system. DHCP failover can be configured, and settings can be modified without the need to pause, stop, or restart the DHCP Server service. If parameters of a failover-enabled scope are modified, these settings must be manually replicated to the partner DHCP server. Note: Automatic replication of scope settings is available if you use IP address management (IPAM) in Windows Server 2012 R2 Preview to modify failover-enabled scope settings. Replication of scope settings can be initiated from either DHCP server to its failover partner server. Clustered DHCP is supported in conjunction with DHCP failover. For purposes of failover, a DHCP cluster is considered a single DHCP server. DHCP clients must be able to communicate with both DHCP failover partner servers, either directly or using a DHCP relay. DHCP servers configured as failover partners can be located on different subnets, but this is not required. When DHCP failover is enabled on a DHCP scope, the DHCP server that renews a DHCP client lease can be different from the DHCP server that initially granted the lease. Two DHCP servers configured as failover partners will attempt to maintain a persistent TCP/IP connection. Two separate, synchronized client lease databases are maintained independently by each DHCP failover partner server. DHCP servers configured as failover partners are both aware of the status of the DHCP service on the other server, and are informed of any change in that status with minimal delay. If two DHCP servers configured as failover partners are unable to communicate, precautions are taken to avoid the same IP address lease being issued to two different DHCP clients. If a DHCP server becomes unavailable before it is able to successfully synchronize all DHCP client information with its failover partner, precautions are taken to ensure DHCP lease continuity for DHCP clients.

The following specifications apply to DHCP failover: You cannot configure DHCP failover on a DHCP scope to include more than two DHCP servers. DHCP failover supports DHCPv4 scopes only. DHCPv6 scopes cannot be failover-enabled. DHCP failover partners must both be running Windows Server 2012 or a later operating system. DHCP failover can be configured, and settings can be modified without the need to pause, stop, or restart the DHCP Server service. If parameters of a failover-enabled scope are modified, these settings must be manually replicated to the partner DHCP server. Note: Automatic replication of scope settings is available if you use IP address management (IPAM) in Windows Server 2012 R2 Preview to modify failover-enabled scope settings. Replication of scope settings can be initiated from either DHCP server to its failover partner server. Clustered DHCP is supported in conjunction with DHCP failover. For purposes of failover, a DHCP cluster is considered a single DHCP server. DHCP clients must be able to communicate with both DHCP failover partner servers, either directly or using a DHCP relay. DHCP servers configured as failover partners can be located on different subnets, but this is not required. When DHCP failover is enabled on a DHCP scope, the DHCP server that renews a DHCP client lease can be different from the DHCP server that initially granted the lease. Two DHCP servers configured as failover partners will attempt to maintain a persistent TCP/IP connection. Two separate, synchronized client lease databases are maintained independently by each DHCP failover partner server. DHCP servers configured as failover partners are both aware of the status of the DHCP service on the other server, and are informed of any change in that status with minimal delay. If two DHCP servers configured as failover partners are unable to communicate, precautions are taken to avoid the same IP address lease being issued to two different DHCP clients. If a DHCP server becomes unavailable before it is able to successfully synchronize all DHCP client information with its failover partner, precautions are taken to ensure DHCP lease continuity for DHCP clients.

The following specifications apply to DHCP failover: You cannot configure DHCP failover on a DHCP scope to include more than two DHCP servers. DHCP failover supports DHCPv4 scopes only. DHCPv6 scopes cannot be failover-enabled. DHCP failover partners must both be running Windows Server 2012 or a later operating system. DHCP failover can be configured, and settings can be modified without the need to pause, stop, or restart the DHCP Server service. If parameters of a failover-enabled scope are modified, these settings must be manually replicated to the partner DHCP server. Note: Automatic replication of scope settings is available if you use IP address management (IPAM) in Windows Server 2012 R2 Preview to modify failover-enabled scope settings. Replication of scope settings can be initiated from either DHCP server to its failover partner server. Clustered DHCP is supported in conjunction with DHCP failover. For purposes of failover, a DHCP cluster is considered a single DHCP server. DHCP clients must be able to communicate with both DHCP failover partner servers, either directly or using a DHCP relay. DHCP servers configured as failover partners can be located on different subnets, but this is not required. When DHCP failover is enabled on a DHCP scope, the DHCP server that renews a DHCP client lease can be different from the DHCP server that initially granted the lease. Two DHCP servers configured as failover partners will attempt to maintain a persistent TCP/IP connection. Two separate, synchronized client lease databases are maintained independently by each DHCP failover partner server. DHCP servers configured as failover partners are both aware of the status of the DHCP service on the other server, and are informed of any change in that status with minimal delay. If two DHCP servers configured as failover partners are unable to communicate, precautions are taken to avoid the same IP address lease being issued to two different DHCP clients. If a DHCP server becomes unavailable before it is able to successfully synchronize all DHCP client information with its failover partner, precautions are taken to ensure DHCP lease continuity for DHCP clients.

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