3Com 4200G 12-Port Configuration Manual page 408

394 C 46: DHCP R
HAPTER
Option 82 supporting
C
ELAY ONFIGURATION
Note that such an interacting process may be repeated several times for a DHCP client
to be successfully configured.
Actually, a DHCP relay enables DHCP clients and DHCP servers on different networks
to communicate with each other by forwarding the DHCP broadcasting packets
transparently between them.
Introduction to option 82 supporting
Option 82 is a relay agent information option in DHCP packets. When a request
packet from a DHCP client travels through a DHCP relay on its way to the DHCP
server, the DHCP relay 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 clients
and DHCP relays, 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 part
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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 of
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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, sub-option 2 and sub-option 5.
Sub-option 1: A sub-option of option 82. Sub-option 1 represents the agent circuit
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ID, namely Circuit ID. It holds the VLAN-ID and MAC address of the switch port
connected to the DHCP client, and is usually configured on the DHCP relay.
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, and is
usually configured on the DHCP relay. Generally, sub-option 1 and sub-option 2
must be used together to identify information about a DHCP source.
Sub-option 5: A sub-option of option 82. Sub-option 5 represents link selection. It
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holds the IP address added by the DHCP relay, so that the DHCP server can assign
an IP address on the same segment to the DHCP client.
Mechanism of option 82 supporting on DHCP relay
The procedure for a DHCP client to obtain an IP address from a DHCP server through
a DHCP relay is exactly the same as that for the client to obtain an IP address from a
DHCP server directly. The following are the mechanism of option 82 supporting on
DHCP relay.
1 A DHCP client broadcasts a request packet when it initiates.
2 If a DHCP server exists in the local network, it assigns an IP address to the DHCP client
directly. Otherwise, the DHCP relay on this network receives and processes the
request packet. The DHCP relay checks whether the packet contains option 82 and
processes the packet accordingly.