3Com 8807 Configuration Manual page 705

configurations for maintenance convenience. You can select to use the bridge
MAC address of the Relay, the device name (configured through sysname), or
user-defined strings.
The identifier format of sub-option1 in the 3Com fixed network mode of Option
82 is:
AccessNodeIdentifier eth frame/slot/subslot/port: vlan
The following section describes the meaning of each filed in the formula above:
AccessNodeIdentifier: Access node identifier, which is a string no longer than 50
characters. It is the bridge MAC address by default
frame: Frame number, which is 0 if the frame is not supported
eth: Ethernet port type
slot: Slot number
subslot: Subslot number
port: Port number
vlan: VLAN Identifier
related protocols and specifications
The protocols and specifications related to Option 82 support on DHCP relay are:
RFC 2131 Dynamic Host Configuration Protocol
■
RFC 3046 DHCP Relay Agent Information Option
■
Working Mechanism of Option 82 Support on DHCP Relay
The process for a DHCP client to acquire an IP address from a DHCP server through
a DHCP relay is the same as that for a DHCP client to acquire an IP address directly
from the DHCP server in the same network segment. Both the processes have four
phases: discovery, offer, selection and acknowledgement. For the details, refer to
the DHCP section in "Network Layer Protocols" in this manual. The following only
introduces the working mechanism of Option 82 support on DHCP relay.
1 A DHCP client broadcasts a request packet during initialization.
2 If a DHCP server exists in the local network, the DHCP client acquires an IP address
from this server directly. If not, the broadcast packet is processed by the DHCP
relay device that is connected to the local network. The DHCP relay device will
check whether Option 82 exists in the packet.
3 If Option 82 exists in the packet, the relay processes the packet according to the
configured strategy. The relay may drop the packet, replace the original Option 82
with its own Option 82, or keep the original Option 82 unchanged. Then, the relay
forwards the packet (if not dropped) to a DHCP server.
4 If Option 82 does not exist in the request packet, the DHCP relay device adds
Option 82 into the packet and then forwards it to a DHCP server. In this way, the
packet contains the MAC address and VLAN ID of the port of the switch that the
DHCP client is connected to, and the MAC address of the DHCP relay itself.
DHCP Option 82 Configuration 705