3Com 5500 SI - Switch - Stackable, Руководство по настройке

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DHCP Configuration 283
Option 82 supporting 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
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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.
Format of the option 82 field
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.