Web Management Guide-R05

Table Of Contents
Chapter 15
| IP Services
Dynamic Host Configuration Protocol
– 558
Figure 357: Layer 2 DHCP Relay Service
Command Usage
You must specify the IP address for at least one active DHCP server. Otherwise,
the switchs DHCP relay agent will not be able to forward client requests to a
DHCP server. Up to five DHCP servers can be specified in order of preference.
If any of the specified DHCP server addresses are not located in the same
network segment with this switch, specify the default router through which
this switch can reach other IP subnetworks (seeConfiguring the IPv4 Default
Gateway” or “Configuring the IPv6 Default Gateway”).
DHCP Snooping Information Option 82 (see page 366) and DHCP Relay
Information Option 82 cannot both be enabled at the same time.
DHCP request packets received by the switch are handled as follows:
If a DHCP relay server has been set on the switch, when the switch receives
a DHCP request packet without option 82 information from the
management VLAN or a non-management VLAN, it will add option 82 relay
information and the relay agent’s address to the DHCP request packet, and
then unicast it to the DHCP server.
If a DHCP relay server has been set on the switch, when the switch receives
a DHCP request packet with option 82 information from the management
VLAN or a non-management VLAN, it will process it according to the
configured relay information option policy:
If the policy isreplace,” the DHCP request packet’s option 82 content
(the RID and CID sub-option) is replaced with information provided by
the switch. The relay agent address is inserted into the DHCP request
packet, and the switch then unicasts this packet to the DHCP server.
If the policy is “keep,” the DHCP request packet's option 82 content will
be retained. The relay agent address is inserted into the DHCP request
packet, and the switch then unicasts this packet to the DHCP server.
If the policy is “drop,” the original DHCP request packet is
flooded onto the VLAN which received the packet but is not
relayed.
Provides IP address
compatible with switch
segment to which client
is attached
DHCP
Server