DHCP server failover states

The DHCP server, when paired in redundant fashion, can transition through several states:

• TRANSITION

• SHUTDOWN

• INIT

• STARTUP

  In this state, the DHCP server recovers leases from the MCS database and does not respond to any unicast and broadcast messages.

• NORMAL

  In this state, the 7750 SR and 7450 ESS DHCP server is serving all IP leases from the local and access-driven IP addresses-ranges and prefixes (assigning new leases and extending existing ones). Remote IP addresses-ranges and prefixes are not served (new or existing ones).

• P RENORMAL

  In this state, the DHCP recovers leases from the MCS database after a resolved communication failure. The DHCP server responds to unicast and broadcast messages for addresses in the local ranges, and to unicast messages for addresses in the remote ranges.

• COMMUNICATION INTERUPTED

  IP addresses and prefixes under the local and access-driven IP address-range and prefix are served in the same fashion as in the NORMAL state. The IP addresses and prefixes under the remote IP address-range and prefix are renewed. However, the new address and prefix from the remote address-range and prefix are not allocated until the partner-down timer expires, the failover state consequently transitions into PARTNER DOWN and the MCLT timer while in the PARTNER-DOWN state expires. This is necessary in case that the failure occurred only on the intercommunication link while the access link is still operational (DHCP server nodes become isolated).

  COMMUNICATION INTERUPTED state indicates that there is a failure of some kind, but it cannot be determined whether an entire node failed or only the inter-chassis link. The access layer may still be fully operational, but the new leases (including RENEWS/REBINDS) cannot be synchronized between the two peers.

• PARTNER DOWN

  After the DHCP Server reaches this state, the remote IP address-range and prefix is taken over and after an additional period of MCLT (Maximum Client Lead Time), the new IP addresses and prefixes from it can be delegated to clients. PARTNER-DOWN state is an indication (and assumption at the same time) that the remote node is truly down.

  Otherwise, the IP address duplication may occur when all the following conditions are met:

  • The DHCP server nodes become isolated.

  • The failover state is PARTNER-DOWN.

  • DHCP/PPPoX clients have simultaneous access to the same IP address-range and prefix on both DHCP servers.