BOOTP Relay Agents Fail Using Multiple Logical IP Subnets (143127)

SUMMARY

BOOTP relay agents are not able to successfully service all DHCP client computers from a LAN segment using multiple logical IP subnets. Only one of the logical IP subnets is able to obtain DHCP IP addresses for its DHCP client computers.

For additional information on DHCP with multiple logical IP subnets when you are not using relay agents, please see the following article(s) in the Microsoft Knowledge Base:

ARTICLE-ID: 124026
TITLE : Requests Fail on Logically Multihomed Server (Single NIC)

MORE INFORMATION

A router implemented as a BOOTP relay agent must insert its IP address in the gateway IP address (giaddr) field of the DHCP Discover packet. A DHCP server uses the giaddr field to determine the subnet that sent the request for IP addresses and which IP addresses to offer. If the giaddr field is blank, the DHCP server assumes that the request is from the local subnet.

The following is an excerpt from RFC 1542:

If the relay agent does decide to relay the request, it MUST examine the "giaddr" ("gateway" IP address) field. If this field is zero, the relay agent MUST fill this field with the IP address of the interface on which the request was received. If the interface has more than one IP address logically associated with it, the relay agent SHOULD choose one IP address associated with that interface and use it consistently for all BOOTP messages it relays.

Because only one address is used in the giaddr field by the relay agent, only one of the logical subnets is serviced through DHCP.