Chapter 44 DHCP
GS2220 Series User’s Guide
372
44.4 DHC Pv4 Re la y
Configure DHCP relay on the Switch if the DHCP clients and the DHCP server are not in the same
broadcast domain. During the initial IP address leasing, the Switch helps to relay network information
(such as the IP address and subnet mask) between a DHCP client and a DHCP server. Once the DHCP
client obtains an IP address and can connect to the network, network information renewal is done
between the DHCP client and the DHCP server without the help of the Switch.
The Switch can be configured as a global DHCP relay. This means that the Switch forwards all DHCP
requests from all domains to the same DHCP server. You can also configure the Switch to relay DHCP
information based on the VLAN membership of the DHCP clients.
44.4.1 DHC Pv4 Re la y Ag e nt Info rma tio n
The Switch can add information about the source of client DHCP requests that it relays to a DHCP server
by adding Re la y Ag e nt Info rma tion. This helps provide authentication about the source of the requests.
The DHCP server can then provide an IP address based on this information. Please refer to RFC 3046 for
more details.
The DHCP Re la y A g e nt Info rma tio n feature adds an Agent Information field (also known as the Option
82 field) to DHCP requests. The O ptio n 82 field is in the DHCP headers of client DHCP request frames that
the Switch relays to a DHCP server.
Re la y Ag e nt Info rm a tio n can include the Syste m Na me of the Switch if you select this option. You can
change the Syste m Na m e in Ba sic Se tting > Ge ne ra l Se tup.
The following describes the DHCP relay agent information that the Switch sends to the DHCP server:
44.4.1.1 DHCPv4 Re la y Ag e nt Info rm a tio n Fo rma t
A DHCP Relay Agent Information option has the following format.
i1, i2 and iN are DHCP relay agent sub-options, which contain additional information about the DHCP
client. You need to define at least one sub-option.
Table 190 Relay Agent Information
FIELD LABELS DESC RIPTIO N
Slot ID (1 byte) This value is always 0 for stand-alone switches.
Port ID (1 byte) This is the port that the DHCP client is connected to.
VLAN ID (2 bytes) This is the VLAN that the port belongs to.
Information (up to 64 bytes) This optional, read-only field is set according to system name set in Ba sic
Se tting > Ge ne ra l Se tup.
Table 191 DHCP Relay Agent Information Option Format
Code
(82)
Length
(N)
i1 i2
...
iN
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