3Com Switch 7750 Configuration Guide

626 CHAPTER 59: DHCP SNOOPING CONFIGURATION
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 495.
When receiving a DHCP client’s request without Option 82, the DHCP snooping
device will add the option field with the configured sub-option and then forward
the packet. For details, see Table 496.
n
The circuit ID and remote ID sub-options in Option 82, which can be configured
simultaneously or separately, are independent of each other in terms of
configuration sequence.
Upon receiving a packet returned by the DHCP server, the DHCP snooping device
checks the Option 82 field:
■ If it was added by the local device, the device strips off Option 82 and forwards
the packet to the DHCP client.
■ If it was not added by the local device, the device obtains the VLAN
information in Option 82 and broadcasts this packet within the VLAN.
Table 495 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.
Replace 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.
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.
Table 496 Ways of handling a DHCP packet without Option 82
Sub-option
configuration The DHCP-Snooping device will ...
Neither of the two
sub-options is configured.
Forward the packet after adding Option 82 with the default
contents.
The format of Option 82 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 adding Option 82 with the configured
circuit ID sub-option in ASCII format.
Remote ID sub-option is
configured.
Forward the packet after adding Option 82 with the configured
remote ID sub-option in ASCII format.