3Com Switch 4200G Configuration Guide

Option 82 Supporting Configuration 397
Configuring the dynamic user address entry updating function
When a DHCP client obtains an IP address from a DHCP server with the help of a
DHCP relay, the DHCP relay creates an entry (dynamic entry) in the user address table
to track the binding information about the IP address and MAC address of the DHCP
client. But as a DHCP relay does not process DHCP-RELEASE packets, which are sent
to DHCP servers by DHCP clients through unicast when the DHCP clients release IP
addresses, the user address entries maintained by the DHCP cannot be updated in
time. The dynamic user address entry updating function is developed to resolve this
problem.
The dynamic user address entry updating function works as follows: at regular
intervals, the DHCP relay sends a DHCP-REQUEST packet that carries the IP address
assigned to a DHCP client and its own MAC address to the corresponding DHCP
server. If the DHCP server answers with a DHCP-ACK packet, the IP address is
available (it can be assigned again) and the DHCP relay ages out the corresponding
entry in the user address table. If the DHCP server answers with a DHCP-NAK packet,
the IP address is still in use (the lease is not expired) and the DHCP relay remains the
corresponding user address entry unchanged.
Option 82 Supporting
Configuration
Prerequisites ■ Before configuring option 82 supporting on a DHCP relay, make sure that the
DHCP relay is configured and operates properly.
■ The DHCP server operates properly. Address allocation policy-related
configurations (such as address pools and the lease time) are performed.
■ The routes between the DHCP relay and the DHCP server are reachable.
Enabling Option 82
Supporting on a DHCP
Relay
The following operations are expected to be performed on a DHCP relay-enabled
network device.
Table 341 Configure the dynamic user address entry updating function
Operation Command Description
Enter system view system-view —
Set the interval to
update DHCP user
address entries
dhcp-security
tracker { interval |
auto }
Optional
By default, the update interval is automatically
determined by the number of DHCP user address
entries.
Table 342 Enable option 82 supporting on a DHCP relay
Operation Command Description
Enter system view system-view —
Enable option 82
supporting on the DHCP
relay
dhcp relay information
enable
Required
By default, this function is disabled.
Configure the strategy for
the DHCP relay to process
request packets
containing option 82
dhcp relay
information strategy
{ drop | keep |
replace }
Optional
By default, the replace policy is adopted,
that is, the DHCP relay replaces the original
option 82 carried in a request packet with
its own option 82.