3Com Switch 7750 Configuration Guide

Configuring DHCP Relay Agent 615
n
■ You can configure up to eight external DHCP IP addresses in a DHCP server
group.
■ You can map multiple VLAN interfaces to one DHCP server group. But one
VLAN interface can be mapped to only one DHCP server group. If you execute
the dhcp-server groupNo command repeatedly, the new configuration
overwrites the previous one.
■ You need to configure the group number specified in the dhcp-server
groupNo command in VLAN interface view by using the command
dhcp-server groupNo ip ipaddress-address&<1-8> in advance.
Configuring a DHCP
Relay Agent to
Broadcast Responses to
Clients
Generally, the DHCP relay agent determines to broadcast or unicast responses
(including DHCP-OFFER, DHCP-ACK, or DHCP-NAK) from the DHCP server to the
clients according to the flag field in the DHCP-DISCOVER packet.
■ When the first bit of the flag field is set to 1, the DHCP relay agent broadcasts
the response packets to the clients.
■ 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, Switch 7750 Ethernet
switches support 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.
Specifying Gateways for
DHCP Clients
To implement this feature on the DHCP relay agent, you need to bind ports in a
VLAN to either the VLAN interface’s primary IP address or one of its secondary IP
addresses (a gateway address). The binding entries will be stored in a binding
table.
Upon receiving a DHCP request message with the giaddr field being 0 from a port
in this VLAN, the DHCP relay agent will search the binding table and then insert
the bound gateway IP address into the giaddr field of the message. If the DHCP
relay agent cannot find a match in the binding table, it inserts the primary IP
Map an interface to a DHCP
server group
interface interface-type
interface-number
Required
By default, a VLAN interface is
not mapped to any DHCP
server group
dhcp-server groupNo
Table 483 Configure an interface to operate in DHCP relay agent mode
Operation Command Description
Table 484 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 agent determines
to broadcast or unicast responses to the
clients according to the flag field in the
DHCP-DISCOVER packet.