On the same subnet, you cannot run two DHCP servers that assign addresses from the same range.
However, on the same subnet, you can have two DHCP servers that assign addresses from different ranges. Suppose you have a class C network, 192.168.1.0, and DHCP will give out addresses 192.168.1.10 through 192.168.1.150. You can run two DHCP servers on the subnet and let one distribute addresses .10 through .100 while letting the other pass out .101 through .150.
thanks, i understand what u mean , but i wanna ask if there is two DHCP server that assign addresses from different ranges, when a new comer come into the subnet ,which server will assign a ip for the new comer
The DHCP request goes out to each server you have identified with the ip helper-address. The client negotiates with the first server to respond. I have seen only one instance of a problem configuring the DHCP in this manner and it was resolved by removing TCP/IP from the client interface and then configuring it again to use a DHCP server.
When started, each DHCP client broadcasts a DHCP discover message (DHCPDISCOVER) to its local subnet to attempt to find a DHCP server. Because DHCP clients use broadcasts during their initial startup, you cannot predict which server will respond to a client's DHCP discover request if more than one DHCP server is active on the same subnet.
For example, if two DHCP servers service the same subnet and its clients, clients can be leased at either server. Actual leases distributed to clients can depend on which server responds first to any given client. Later, the server first selected by the client to obtain its lease might be unavailable when the client attempts to renew.
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