3com 5500-SI Manual Do Utilizador

DHCP Configuration  283

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.

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Option: A length-variable field in DHCP packets, carrying information such as part 
of the lease information and packet type. It includes at least one option and at 
most 255 options.

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Option 82: Also known as relay agent information option. This option is a part 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, sub-option 2 and sub-option 5.

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Sub-option 1: A sub-option of option 82. Sub-option 1 represents the agent circuit 
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.

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Sub-option 2: A sub-option of option 82. Sub-option 2 represents the remote 
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.

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Sub-option 5: A sub-option of option 82. Sub-option 5 represents link selection. It 
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.

1 Option 82 format

A DHCP packet contains a field named options, which can be empty or contain the 
option for a feature (such as option 82). Option 82 can contain multiple sub-options, 
as illustrated in Figure 67

Figure 67   Option 82 format

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Code: Identifies the relay agent information option number. As for packets 
containing option 82, the value of this field is 82. Option 82 lies before option 255 
and after the other options.