3Com 5500-EI PWR Install Manual page 625

Figure 4-3 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, S5500-EI Series Ethernet Switches support
Option 82 in the standard format. Refer to
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 4-4 Standard format of the circuit ID sub-option
Figure 4-5 Standard format of the remote ID sub-option
Mechanism of DHCP-snooping Option 82
With DHCP snooping and DHCP-snooping Option 82 support enabled, when the DHCP snooping
device receives a DHCP client's request containing Option 82, it will handle the packet according to the
handling policy and the configured contents in sub-options. For details, see
Table 4-1 Ways of handling a DHCP packet with Option 82
Handling
policy configuration
Drop —
Keep —
Neither of the two
sub-options is
configured
Replace
Circuit ID sub-option
is configured
Remote ID
sub-option is
configured
Figure 4-4
Sub-option
Drop the packet.
Forward the packet without changing Option 82.
Forward the packet after replacing the original Option 82
with the default content.
The storage format of Option 82 content is the one specified
with the dhcp-snooping information format command or
the default HEX format if this command is not executed.
Forward the packet after replacing the circuit ID sub-option
of the original Option 82 with the configured circuit ID
sub-option in ASCII format.
Forward the packet after replacing the remote ID sub-option
of the original Option 82 with the configured remote ID
sub-option in ASCII format.
and Figure 4-5
The DHCP Snooping device will...
4-3
for the standard format of the
Table 4-1.