Because the DHCP client's MAC address is embedded in the DHCP message (the Chaddr field), the Network Bridge provides special handing for broadcast DHCP messages so that clients on Layer 3-bridged LAN segments can receive these messages for automatic TCP/IP configuration. When the Network Bridge computer receives a broadcast DHCPDiscover or DHCPRequest message on a port that is providing Layer 3 bridging, it performs the following steps:
1. If an entry does not already exist, add an entry to a DHCP Pending table that includes the XID of the DHCP message, the original value of Chaddr, and the port on which the broadcast message was received.
2. Broadcast the DHCP message out all ports except the one on which it was received, changing the Chaddr field to the MAC address of the sending port.
When the response is sent by the DHCP server or relay agent, it is unicast to the Network Bridge computer. When the Network Bridge computer receives the DHCP response, it performs the following steps:
1. Look up the XID of the DHCP response in the DHCP Pending table. If a match is found, the table entry is used to determine the DHCP client's MAC address and the port on which to forward the DHCP response. If a match is not found and the Network Bridge is not performing its own DHCP configuration, silently discard the DHCP response.
2. Change the Chaddr field to the value found in the DHCP Pending table (the DHCP client's MAC address).
3. Unicast the DHCP message to the client using the appropriate port.