3Com 7757 Configuration Manual page 612

612 C 58: DHCP R
HAPTER
Option 82 Support
A C
ELAY GENT ONFIGURATION
forwarding process of the DHCP relay agent. For the interaction process of the
packets, see "Obtaining IP Addresses Dynamically" on page 590.
1 The DHCP client broadcasts the DHCP-DISCOVER packet.
2 After receiving the packets, the network device providing the DHCP relay agent
function unicasts the packet to the designated DHCP server based on the
configuration.
3 The DHCP server assigns IP addresses and transmits the configuration information
to the clients through the DHCP relay agent so that the clients can be configured
dynamically. The transmission mode depends on the flag field in the
DHCP-DISCOVER packet. For details, see "DHCP Packet Format" on page 590.
Introduction to option 82 support
Option 82 is a relay agent information option in DHCP packets. When a request
packet from a DHCP client travels through a DHCP relay agent on its way to the
DHCP server, the DHCP relay agent adds option 82 into the request packet. Option
82 includes many sub-options, but the DHCP server supports only sub-option 1
and sub-option 2 at present. Sub-option 1 defines agent circuit ID (that is, Circuit
ID) and sub-option 2 defines remote agent ID (that is, Remote ID).
Option 82 enables a DHCP server to track the address information of DHCP relay
agents, through which and other proper software, you can achieve the DHCP
assignment limitation and accounting functions.
Primary terminologies
Option: A length-variable field in DHCP packets, carrying information such as
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part of the lease information and packet type. It includes at least one option
and at most 255 options.
Option 82: Also known as relay agent information option. This option is a part
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of the Option field in DHCP packet. According to RFC3046, option 82 lies
before option 255 and after the other options. Option 82 includes at least one
sub-option and at most 255 sub-options. Currently, the commonly used
sub-options in option 82 are sub-option 1 and sub-option 2.
Sub-option 1: A sub-option of option 82. Sub-option 1 represents the agent
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circuit ID, namely Circuit ID. It holds the port number and VLAN-ID of the
switch port connected to the DHCP client, and is usually configured on the
DHCP relay agent. Generally, sub-option 1 and sub-option 2 must be used
together to identify information about a DHCP source.
Sub-option 2: A sub-option of option 82. Sub-option 2 represents the remote
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agent ID, namely Remote ID. It holds the MAC address of the DHCP relay
agent, and is usually configured on the DHCP relay agent. Generally,
sub-option 1 and sub-option 2 must be used together to identify information
about a DHCP source.
Related specification
The specifications concerning option 82 supporting are as follows:
RFC2131 Dynamic Host Configuration Protocol
RFC3046 DHCP Relay Agent Information Option