MCLT

簡(jiǎn)單的說(shuō)就是，決定了當(dāng)dhcp failover因?yàn)閒ailover通信中斷或其它原因?qū)е缕渲心撑_(tái)，如B機(jī)出故障時(shí)，由A機(jī)為B機(jī)的客戶暫時(shí)提供服務(wù)時(shí)分配給客戶的租約時(shí)間。文檔中建議配置為3600秒，但也不是說(shuō)3600秒是最合適的。

這個(gè)參數(shù)僅能在dhcp failover集群的主服務(wù)器上設(shè)置，次服務(wù)器上不要設(shè)定這個(gè)參數(shù)。

-------------以下是man dhcpd.conf中關(guān)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.

=======關(guān)于failover 運(yùn)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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