3Com Switch 7750 Configuration Guide Guide

DHCP Relay Configuration 583
■ If the DHCP server returns the DHCP-NAK packet, it indicates that the lease of
the IP address is not expired. The DHCP relay does not age the corresponding
entry.
After the DHCP relay handshake function is disabled, the DHCP relay does not
send the handshake packet (the DHCP-REQUEST packet) periodically to the DHCP
server.
■ When the DHCP client releases this IP address, the client unicasts the
DHCP-RELEASE packet to the DHCP server.
■ The DHCP relay does not process this packet, so the user address entries of the
DHCP relay cannot be updated in real time.
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 bridge 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 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.
Configuring Option 82
Supporting
Prerequisites
Before configuring option 82 supporting on a DHCP relay, you need to:
Table 456 Enable/disable DHCP relay handshake
Operation Command Description
Enter system view system-view -
Enable DHCP relay handshake dhcp relay hand enable
By default, the DHCP relay
handshake function is
enabled.
Disable DHCP relay
handshake
dhcp relay hand disable
Table 457 Configure the dynamic user address entry updating function
Operation Command Description
Enter system view system-view -
Enable DHCP relay handshake dhcp relay hand enable Required
Set the interval at which the
DHCP relay dynamically
updates the user address
entries
dhcp-security tracker {
interval | auto }
Optional
By default, the update interval
is determined automatically.