3-3
Figure 3-3 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, S4500 Series Ethernet Switches support
Option 82 in the standard format. Refer to
Figure 3-4 and Figure 3-5 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 3-4 Standard format of the circuit ID sub-option
Figure 3-5 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 3-1.
Table 3-1 Ways of handling a DHCP packet with Option 82
Handling
policy
Sub-option
configuration
The DHCP Snooping device will…
Drop — Drop the packet.
Keep — Forward the packet without changing Option 82.
Neither of the two
sub-options is
configured
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.
Circuit ID sub-option
is configured
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.
Replace
Remote ID
sub-option is
configured
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.
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