3Com Switch 7750 Configuration Guide

Configuring DHCP Snooping 625
■ sub-option 1 (circuit ID sub-option): Padded with the port index (smaller than
the physical port number by 1) and VLAN ID of the port that received the
client’s request.
■ sub-option 2 (remote ID sub-option): Padded with the bridge MAC address of
the DHCP snooping device that received the client’s request.
By default, when Switch 7750s serve as DHCP snooping devices, Option 82 adopts
the extended format. Refer to Figure 159 and Figure 160 for the extended format
of the sub-options (with the default padding contents). That is, the circuit ID or
remote ID sub-option defines the type and length of a circuit ID or remote ID.
The remote ID type field and circuit ID type field are determined by the option
storage format. They are both set to 0 in the case of HEX format and to 1 in the
case of ASCII format.
Figure 159 Extended format of the circuit ID sub-option
Figure 160 Extended format of the remote ID sub-option
In practice, some network devices do not support the type and length identifiers
of the Circuit ID and Remote ID sub-options. To interwork with these devices, the
Switch 7750 supports Option 82 in the standard format. Refer to Figure 161 and
Figure 162 for the standard format of the sub-options (with the default padding
contents). In the standard format, the Circuit ID or Remote ID sub-option does not
contain the two-byte type and length fields of the circuit ID or remote ID.
Figure 161 Standard format of the circuit ID sub-option
Figure 162 Standard format of the remote ID sub-option
Suboption type
0715
Length Circuit ID type Length
23 31
VLAN ID Port Index
Suboption type
0715
Length Reomte ID type Length
23 31
Bridge MAC Address
Suboption type
0715
Length
23 31
VLAN ID
Port Index
Suboption type
0715
Length
23 31
Bridge MAC Address