To keep this simple lets say I have 100 IP address in my scope.

Most of the time no more than 20 are being used.  I reserve 10% for my hot spare. So I reserve 10 IP addresses for my spare.

Scenario 1: The DHCP server crashed the night before we had a group of guest come in.  There will be 30 guest.  We know the hot spare will renew the IPs that was in use and has 10 reserved but will it be able to assign the 20 added IP addresses for our guest?

Scenario 2:  I have reserved 10 of my IP address out of 100 for the hot spare.  One day we get group of visitors and we need 95 of our 100 address.  Can we get access to the additional DHCP address we will need?

Hi Lishron,

According to your description, I suppose you split the scope and configure hot stand by mode.

If yes, in scenario1, the DHCP server could only offer 10 IP addresses, the other 20 clients would faile to get IP addresses.

In scenario2, the clients would get IP addresses from the standby server. When there is no available address on the server, it would not respond to the DHCP discover packet. Then the standby server would respond and offer IP addresses.

Best Regards,

Leo

first thanks so much for this.  

I did do some research last night and did not find info on scenario 2 so thanks so much for that info.

I did find this on scenario one and if i am reading it correctly if i set the MCLT to something like 5 minutes then the stand by server would be able to issue out all IP addressees from that scope after 5 minutes of the primary DHCP being down.   I think.

Maximum client lead time (MCLT)

The maximum amount of time that one server can extend a lease for a DHCP client beyond the time known by the partner server. The MCLT defines the temporary lease period given by a failover partner server, and also determines the amount of time that a server in a failover relationship will wait in partner down state before assuming control over the entire IP address range.

The MCLT cannot be set to zero, and the default setting is 1 hour.

Hi Lishron,

Yes, that's right.

After the MCLT has expired, the standby server will be permitted to use the entire available IP address pool for new DHCP leases, provided the server is in PARTNER DOWN state. If the server is still in COMMUNICATIONS INTERRUPTED state, it will not use the entire available IP address pool for new DHCP leases.

DHCP Failover Modes:
https://technet.microsoft.com/en-us/library/dn338976.aspx

Best Regards,

Leo