Web Management Guide-R04

Table Of Contents
Chapter 18
| IP Services
Dynamic Host Configuration Protocol
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request, including the VLAN ID, stack unit, and port. This allows DHCP client-server
exchange messages to be forwarded between the server and client without having
to flood them onto the entire VLAN.
The switch then forwards the packet to the DHCP server. When the server receives
the DHCP request, it allocates a free IP address for the DHCP client from its defined
scope for the DHCP client’s subnet, and sends a DHCP response back to the DHCP
relay agent (i.e., this switch). This switch then passes the DHCP response received
from the server to the client.
Figure 475: 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.
DHCP Snooping Information Option 82 (see page 368) 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 is “replace,” 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.
Provides IP address
compatible with switch
segment to which client
is attached
DHCP
Server