Maximum Client Lead Time (MCLT) is the maximum time that the 7750 SR and 7450 ESS DHCP server can extend the lease time to its clients beyond the lease time currently know by the 7750 SR and 7450 ESS DHCP partner node. By default, this time is relatively short (10 minutes).
The purpose of the MCLT is described in the following scenario:
The local DHCP server assigns a new IP lease to the client but it crashes before it sends a sync update to the partner server. Because of the local DHCP server failure, the remote DHCP server is not aware of the IP address and prefix that was allocated on the local DHCP server. This condition creates the possibility that the remote DHCP server allocates the same address and prefix to another client. This would cause IP address and prefix duplication. MCLT is put in place to prevent this scenario.
MCLT based solution is shown in Figure: Potential expiration time.
The sequence of events is the following:
DHCP server 1 is the local DHCP server (with the local address-range and prefix) that creates the IP lease state for a new client. The initial lease-time assigned to the client is MCLT which is normally shorter than the requested lease time.
This DHCP server fails before it gets a chance to synchronize the lease state with the DHCP Server 2 (remote DHCP server with the remote address-range and prefix).
The remote DHCP server transitions into the PARTNER-DOWN state (if the partner-down timer is 0). In this state the remote DHCP server can extend the lease time to the existing clients but it cannot assign a new lease for a period of MCLT. In MCLT/2 a new RENEW request is sent directly to the local DHCP server. This server is DOWN and therefore it cannot reply.
The client broadcasts a REBIND request that reaches the remote DHCP server. The remote DHCP server has no knowledge of the requested lease and therefore it does not reply.
The lease for the client expires and the client must reinitiate the IP address and prefix assignment process.
Because the remote DHCP server is not aware of the lease state that was assigned by the local DHCP server, there is a chance that the remote DHCP server assigns to the new client the same IP address and prefix already allocated by the local DHCP server just before it crashed. Therefore the remote DHCP server needs to wait for the MCLT time to expire so that the IP addresses and prefixes allocated (but never synchronized) by the local DHCP server can time out.
When the communication channel between the chassis is interrupted, two scenarios are possible:
The entire node becomes unavailable. In this case the redundant node takes over and it starts reducing the lease time until the lease time reaches MCLT.
COMMUNICATION-INTERUPTED
The remote DHCP server only renews the leases but does not delegate new ones (primary is DOWN).
The local DHCP server renews the leases (which eventually trickle down to MCLT) and delegates the new ones with the lease time of MCLT (secondary is down).
PARTNER-DOWN
The remote DHCP server starts delegating new IP addresses and prefixes from the remote address-range and prefix after MCLT (primary is down). The lease time of the new clients is MCLT. A lease cannot be assigned for a period longer than what is agreed with the peer incremented with the MCLT. As for a ‟new” lease nothing is agreed yet so the sum falls back to the MCLT itself.
The communication channel is down but the remote DHCP server is not (meaning that the clients have still access to both servers). The behavior in this case is following:
COMMUNNICATION-INTERUPTED
The remote DHCP server keeps renewing existing leases but it does not delegate the new leases. There is little chance this could happen as the clients continue to send RENEWS by unicast to the local DHCP server which is still active. The non-synched leases in the remote DHCP server times out. The local DHCP server starts ticking down the lease time to MCLT.
The local DHCP server continues to delegate the new leases, although with the MCLT lease time.
PARTNER-DOWN State
The local DHCP server continues to extend the existing leases but also starts delegating new IP leases after the initial MCLT elapses.
Both local and remote DHCP server delegates new leases in the PARTNER-DOWN state (although the remote DHCP server in PARTNER-DOWN state must wait an additional MCLT period before it can start delegating new leases).