MCLT
简单的说就是，决定了当dhcp failover因为failover通信中断或其它原因导致其中某台，如B机出故障时，由A机为B机的客户暂时提供服务时分配给客户的租约时间。文档中建议配置为3600秒，但也不是说3600秒是最合适的。
这个参数仅能在dhcp failover集群的主服务器上设置，次服务器上不要设定这个参数。
-------------以下是man dhcpd.conf中关于mclt的描述--------------
The mclt statement
mclt seconds;
The mclt statement defines the Maximum Client Lead Time. It must be specified on the primary, and may not be specified on the secondary. This is the length of time for which a lease may be renewed by either failover peer without contacting the other. The longer you set this, the longer it will take for the running server to recover IP addresses after moving into PARTNER-DOWN state. The shorter you set it, the more load your servers will experience when they are not communicating. A value of something like 3600 is probably reasonable, but again bear in mind that we have no real operational experience with this.
=======关于failover 运行的描述=================
When a server starts that has not previously communicated with its failover peer, it must establish communications with its failover peer and synchronize with it before it can serve clients. This can happen either because you have just configured your DHCP servers to perform failover for the first time, or because one of your failover servers has failed catastrophically and lost its database.
The initial recovery process is designed to ensure that when one failover peer loses its database and then resynchronizes, any leases that the failed server gave out before it failed will be honored. When the failed server starts up, it notices that it has no saved failover state, and attempts to contact its peer.
When it has established contact, it asks the peer for a complete copy its peer lease database. The peer then sends its complete database, and sends a message indicating that it is done. The failed server then waits until MCLT has passed, and once MCLT has passed both servers make the transition back into normal operation. This waiting period ensures that any leases the failed server may have given out while out of contact with its partner will have expired.
While the failed server is recovering, its partner remains in the partner-down state, which means that it is serving all clients. The failed server provides no service at all to DHCP clients until it has made the transition into normal operation.
In the case where both servers detect that they have never before communicated with their partner, they both come up in this recovery state and follow the procedure we have just described. In this case, no service will be provided to DHCP clients until MCLT has expired.
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