DHCP failover peers operate within server states. These states tell the DHCP failover server how to interact or not interact with its peer server, manage normal server operations, and manage operations when the two servers can't communicate.
You can force the DHCP failover state of the DNS/DHCP Server in the event of connectivity errors.
To force the DHCP failover state:
- From Main Session mode, type configure additional and press ENTER.
Type dhcp force-<failover state> and press
Where the <failover state> is one of the following failover states:
- startup-state—When a DHCP server configured for failover first initialized, it assumes that failover is operation and goes into STARTUP while attempting to contact its failover peer. During this short time, the server is unresponsive to DHCP client requests. This delay provides the server with an opportunity to determine whether or not its peer’s state has changed and to react to that changed state prior to responding to DHCP client requests.
- normal-state—This is the standard operational state for DHCP failover servers. In this state, both servers can communicate with each other.
- comm-int-state—In this state, the servers can no longer
communicate with each other and neither server is aware of the state of
its peer. In this state, all operations assume that the other DHCP
server could also be live and issuing address leases.
After a server has entered the communication-interrupted state, it changes the way that it assigns address leases. Clients initially attempting to renew existing leases receive a new lease for the remainder of their regular lease time with the maximum client lead time (MCLT) value added. Subsequent leases are only handed out for the MCLT and clients are never given a lease renewal; they always receive a lease for a new address instead. If a client releases an address lease manually, then that address is abandoned until the server returns to the normal state.
The disadvantage of the communication-interrupted state is immediately apparent. If clients are given short (MCLT) lease times and their leases aren't renewed, the address pool could quickly become depleted and there would be an increased level of network traffic.
- partn-down-state—When a failover peer is down, you need to manually change the working peer to the Partner-Down State. The remaining server then becomes the Primary server, whether or not it was the Primary before you use the force partn-down-state command. The working server continues to hand out leases for the MCLT, but renews the leases. This server also reclaims all of the expired, reset, and released leases and is able to use the entire free address pool for allocations. When its partner comes back online, this server remains the Primary server.
- recover-state—The RECOVER state allows a DHCP server to get a complete update from its peer when it starts up and discovers that its peer has been in PARTNER-DOWN. It will also enter this state when it does not have a record of ever communicating with its peer. When entering this state, the server stops providing DHCP services while it establishes communications with its peer. Once the server has received an update from its peer, the server will move into the RECOVER-WAIT state.
- shutdown-state—This is an administrative state that a failover server can be placed into during a planned outage. In this state, the server sends a state message to its peer informing it that it will be in the SHUTDOWN state and out of service. The failover peer then automatically goes into the PARTNER-DOWN state.