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35.4 DHCP Relay
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.
35.4.1 DHCP Relay Agent Information
The Switch can add information about the source of client DHCP requests that it relays to a DHCP
server by adding Relay Agent Information. 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 Relay Agent Information feature adds an Agent Information field (also known as the
Option 82 field) to DHCP requests. The Option 82 field is in the DHCP headers of client DHCP
request frames that the Switch relays to a DHCP server.
35.4.2 DHCP Relay Agent Information Format
A DHCP Relay Agent Information option has the following format.
Figure 195 DHCP Relay Agent Information Option Format
Code
(82)
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.
35.4.3 Sub-Option Format
There are two types of sub-option: "Agent Circuit ID Sub-option" and "Agent Remote ID Sub-
option". They have the following formats.
Figure 196 DHCP Relay Agent Circuit ID Sub-option Format
SubOpt
1
N
(1 byte)
(1 byte)
Figure 197 DHCP Relay Agent Remote ID Sub-option Format
SubOpt
2
N
(1 byte)
(1 byte)
VES1724-56 User's Guide
Length
i1
(N)
Length
String or System Name
Length
String or Port Name
i2
iN
...
Value
Value
Chapter 35 DHCP
313
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