Hi,

Please explain DHCP Spilt scope working mechanism.

Is there any resemblance in DHCP Failover.

Thanks

Hi,

DHCP split-scope:
Multi DHCP servers for fault tolerance. If one of the DHCP servers down, others will continue to provide service for clients.  For example, 2 DHCP servers configured with split-scope(80/20), scopes with the same subnet address and subnet mask are configured on both servers. The two scopes have complementary exclusion ranges, and they will not serve the same address to different clients.

Link below has described the detailed steps of lease renew. suppose that, DHCP server which provides lease to the client has down. At 87.5 percent of clients lease time elapses, the client enters a rebinding state, broadcast a DHCPDiscover message. At this time, DHCP server which has configured split-scope will to answer the client and provide it available IP address lease.
https://technet.microsoft.com/en-us/library/cc958919.aspx

DHCP split-scope has limitations, such as, it does not provide IP address continuity. DHCP failover has similar mechanism as split-scope, breaks the limitations and improves performance. You may reference the link below for detailed information about DHCP failover:
https://technet.microsoft.com/en-us/library/hh831385.aspx

Best Regards,
Eve Wang      

This is documented here too: https://technet.microsoft.com/en-gb/library/cc770535.aspx

50/50 DHCP rule is a common practice too.

With DHCP failover, you will have an active node and a passive one. If one node fails then the other one become active. Here, the DHCP cluster nodes already know the leases that were assigned and the ones that are available. Since this is known then the failover won't cause a duplicate IP address problem.

However, without failovering, you need to make sure that your standalone DHCP servers do not have IP scopes which are common. The reason is that, since they standalone, they do communicate or know which leases were already granted. That is why you need to split the DHCP scope between both and you cannot simply configure the second DHCP server with the same scopes.

There is almost no use case of this application. use DHCP Failover instead, it provides 100% resiliency.

HTH

Manouchehr 

Thanks Eve. :-)