Good Article on DHCP

kryollakryolla Member Posts: 785
Everyone know that configuring a router as a DHCP server on the same subnet is easy. Hopefully this thread will make it easy for people like me that was not clear on how to set up a central DHCP server to serve multiple subnets. The important part of the DHCP header is the giaddr field which is the address of the Interface of ip helper address command

Here is from Cisco Docs

DHCP defines a process by which the DHCP Server knows the IP subnet in which the DHCP client resides, and it can assign an IP address from a pool of valid IP addresses in that subnet.

The DHCP Server identifies which DHCP address pool to use to service a client request as follows:

•If the client is not directly connected (the giaddr field of the DHCPDISCOVER broadcast message is non-zero), the DHCP Server matches the DHCPDISCOVER with a DHCP pool that has the subnet that contains the IP address in the giaddr field.

•If the client is directly connected (the giaddr field is zero), the DHCP Server matches the DHCPDISCOVER with DHCP pool(s) that contain the subnet(s) configured on the receiving interface. If the interface has secondary IP addresses, the subnets associated with the secondary IP addresses are examined for possible allocation only after the subnet associated with the primary IP address (on the interface) is exhausted.
Studying for CCIE and drinking Home Brew


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