DHCP Failover Hot-Standby Mode

In the previous blog on DHCP Failover, we discussed the DHCP failover load balance mode where both DHCP servers respond to client requests and load balance the requests between them based on an admin specified load distribution ratio.

In the other mode of a failover relationship, known as the Hot-Standby mode (Active-Passive), only one of the servers actively leases IP addresses and option configuration to clients in given subnet(s)/scope(s) while the other server (standby) is passive. The standby server services the clients, only in event of active server being down. The clients fallback to the active server once the active server becomes available again post the outage.

The Load balance mode is more suited for single site deployment where the 2 DHCP servers in a failover relationship are co-located with the subnets/scopes being served by them. As the servers are in network proximity with the clients, the clients do not experience any latency while acquiring or renewing an IP address.

Hot-Standby mode is more suited for multi-site deployment topologies. Each site would have a local DHCP server which is configured to provide the DHCP service to the clients on the local network and DHCP server at a remote site would be standby server. In a normal state of operation, computers and devices on a given site receive IP addresses and other network configuration from the DHCP server located at the same site as the clients. However, in the event of the local DHCP server being down, the DHCP server from the remote site would provide the service to the clients.

You could choose to deploy hot standby mode in a single-site deployment also if you need to.

Hot-Standby Mode Configuration

While configuring a failover relationship in Hot standby mode, there are 2 configuration parameters which are specific to the Hot Standby mode:

  • Role of the Server (Active/Standby)

The role of a server participating in hot standby failover relation can be set to Active / standby server as part of new relationship configuration.

  • Reserve Address percentage

As discussed in the blog on Load balanced mode, the free IP address pool of a scope which is part of a failover relationship in Load Balance mode, will be apportioned in the configured load distribution percentage to enable both DHCP servers to respond to client requests for new IP addresses. However, in a hot standby failover mode, free IP address pool is owned by the Active server entirely as it is serving all the client requests. In the event of an outage of the active server, the standby server needs to be able to renew existing IP address leases as well as give out new IP address leases to clients who request a new lease. For the later scenario – leasing new IP addresses to clients, the standby server needs a free IP address pool available to it from which it can give out new leases. The standby server will take over the free IP address pool of the active server only after it transitions into Partner Down state from Communicated Interrupted state and a time period of MCLT (Maximum Client Lead Time) has expired. This is as per the DHCP failover protocol. To enable the standby server to serve new IP address leases to clients during this interim period - i.e. till it transitions to Partner Down and takes over the entire free IP address pool of a scope – a percentage of free IP address pool needs to be available to standby server. This can be provided by the configuration parameter – reserve address percentage.

The percentage of addresses reserved for the hot standby server can be configured for a failover relationship configured for hot standby mode.  Free IP addresses in proportion to the percentage value configured would be assigned to the hot standby server. If address reserve percentage is set to 0, no addresses will be reserved for the hot standby server and new client leases cannot be granted by the hot standby server in case of outage of active server. The default value for reserve address percentage is 5%.  Since the reserve address percentage is meant for an interim period as discussed above, the value chosen for it can typically be small (5-10%).

Figure 1: Creating a Failover Relation in Hot Standby Mode

A new failover Relationship can be configured for Hot-standby mode and even an existing relationship in load balancing mode can be converted to hot-standby mode and vice-versa.

Operation in Hot Standby mode

Unlike the Load Balance mode, where the 2 DHCP servers compute a hash of the MAC address of the clients and decide whether to respond to the client or not, in hot standby mode, the servers do not compute hash of the MAC address of clients. The active server responds to all client requests and the standby server does not respond to any client requests at all while operating in NORMAL state. When active server goes down, the standby server transitions into Communication Interrupted state and starts responding to the clients. Once the active server is up, the standby server retreats into standby mode and stops responding to clients. This facilitates failback of the clients back to the active server.