Some deployments require that the remote IP address and prefix range starts delegating new IP addresses and prefixes upon the failure of the intercommunication link, without waiting for the intercommunication link to transition from the COMM-INT state into the PARTNER-DOWN state and the MCLT to expire while in PARTNER-DOWN state.
In other words, the takeover of the remote IP address-range and prefix should follow the failure of the intercommunication link, without any significant delays.
This can be achieved by configuring both of the following two items under the dhcp failover CLI hierarchy:
The partner-down-delay must be set to 0. This causes the intercommunication link to bypass the COMM-INT state upon the failure and transition straight into the PARTNER-DOWN state. The remote IP address-range and prefix can be taken over only in PARTNER-DOWN state, when the MCLT expires.
The ignore-mclt-on-takeover flag must be enabled. With this flag enabled, the remote IP address and prefix can be taken over immediately upon entering the PARTNER-DOWN state of the intercommunication link, without having to wait for the MCLT to expire. By setting this flag, the lease times of the existing DHCP clients, while the intercommunication link is in the PARTNER-DOWN state, are reduced to the MCLT over time and all new lease times are set to MCLT. This behavior remain the same as originally intended for MCLT. this functionality must be exercised with caution. Be mindful that the partner-down-delay and MCLT timers were originally introduced to prevent IP address duplication in cases where DHCP redundant nodes transition out-of-sync because of the failure of intercommunication link. These timers (partner-down-delay and MCLT) would ensure that during their duration, the new IP addresses and prefixes are delegated only from one node, the one with local IP address-range and prefix. The drawback is that the new IP address delegation is delayed and service is impacted.
If the intercommunication link could be guaranteed to always available, then the DHCP nodes would stay in sync and the two timers would not be needed. Therefore it is important that in this mode of operation, the intercommunication link is well protected by providing multiple paths between the two DHCP nodes. The only event that should cause intercommunication link to fail is the entire nodal failure. This failure is acceptable because in this case only one DHCP node is available to provide new IP addresses and prefixes.