3Com Switch 7750 Configuration Guide Guide

DHCP Relay Configuration 581
■ When the flag field is set to 0, the DHCP relay agent unicasts the response
packets to the clients.
In actual networking, if clients have special requirements, the Switch 7750 Family
supports the following commands so as to force the DHCP relay agent to
broadcast the responses to the clients. After this function is enabled, even if the
flag field in the DHCP-DISCOVER packet is set to 0, the DHCP relay agent still
broadcasts responses to the clients.
Configuring DHCP Relay
Security
Configuring address checking
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 IP-MAC address binding information about the DHCP client. You
can also configure user address entries manually (static entries) to bind an IP
address and a MAC address statically.
The purpose of the address checking function on DHCP relay is to prevent
unauthorized users from statically configuring IP addresses to access external
networks. With this function enabled, a DHCP relay inhibits a user from accessing
external networks if the IP address configured on the user end and the MAC
address of the user end do not match any entries (including the entries
dynamically tracked by the DHCP relay and the manually configured static entries)
in the user address table on the DHCP relay.
Configuring dynamic entries
Through this configuration task, you can validate or invalidate the dynamic
IP-to-MAC mapping entries generated by the DHCP relay agent. Only valid entries
Table 452 Configure the DHCP relay agent to broadcast responses to clients
Operation Command Description
Enter system view system-view -
Configure the DHCP relay
agent to broadcast responses
to clients
dhcp relay reply broadcast
Required
Generally, the DHCP relay
determines to broadcast or
unicast responses to the
clients according to the flag
field in the DHCP-DISCOVER
packet.
Table 453 Configure address checking
Operation Command Description
Enter system view system-view -
Create a DHCP user address
entry manually
dhcp-security static
ip-address mac-address
Optional
By default, no DHCP user
address entry is configured
Enter interface view
interface interface-type
interface-number
-
Enable the address checking
function
address-check enable
Required
By default, the address
checking function is disabled