3Com Switch 7750 Configuration Guide Guide

582 CHAPTER 55: DHCP RELAY CONFIGURATION
can pass DHCP security check; otherwise you cannot access the network even if
you have obtained a valid IP address. If you invalidate the dynamic IP-to-MAC
mapping entries generated by the DHCP relay agent, this means that you specify
the clients as freely-connected hosts.
This configuration will take effect only after the address checking function of the
DHCP relay on the VLAN interface is enabled.
Configuring whether to allow freely-connected clients to pass DHCP
security check
A freely-connected client refers to the client whose IP address and MAC address
are not in the DHCP security table. When the freely-connected client is not
allowed to pass DHCP security check, you cannot access the network on this client
even if the freely-connected client has a valid IP address.
This configuration will take effect only after the address checking function of the
DHCP relay on the VLAN interface is enabled.
Configuring DHCP relay handshake
When the DHCP client obtains an IP address from the DHCP server through the
DHCP relay, the DHCP relay records the binding relationship of the IP address and
the MAC address. After the DHCP relay handshake function is enabled, the DHCP
relay sends the handshake packet (the DHCP-REQUEST packet) which carries
includes the IP address recorded in the binding and its own bridge MAC address to
the DHCP server periodically.
■ If the DHCP server returns the DHCP-ACK packet, it indicates that the IP
address can be assigned. The DHCP relay ages the corresponding entry in the
user address table.
Tabl e 454 Configure dynamic entries generated by DHCP relays
Operation Command Description
Enter system view system-view -
Enter VLAN interface view
interface interface-type
interface-number
-
Validate the dynamic entries
generated by the DHCP relay
address-check dhcp-relay
enable
Optional
By default, the dynamic
IP-to-MAC mapping entries
generated by the DHCP relay
are valid
Tabl e 455 Configure whether to allow freely-connected clients to pass DHCP security
check
Operation Command Description
Enter system view system-view -
Enter VLAN interface view
interface interface-type
interface-number
-
Forbid freely-connected
clients to pass DHCP security
check
address-check no-matched
enable
Optional
Freely-connected clients are
not allowed to pass DHCP
security check