Web Management Guide-R05

Table Of Contents
Chapter 15
| IP Services
Dynamic Host Configuration Protocol
– 557
Figure 356: Specifying A DHCP Client Identifier
Configuring
DHCP Relay Service
Use the IP Service > DHCP > Relay page to configure DHCP relay service for
attached host devices, including DHCP option 82 information. DHCP provides an
option for sending information about its DHCP clients to the DHCP server
(specifically, the interface on the relay server through which the DHCP client
request was received). Also known as DHCP Relay Option 82, it allows compatible
DHCP servers to use this information when assigning IP addresses, or to set other
services or policies for clients.
Option 82 information contains information which can identify both the relay
agent and the interface through which the DHCP request was received:
The DHCP Relay Information Option Remote ID (RID) is the access node
identifier – a string used to identify the switch to the DHCP server.
The DHCP Relay Information Option Fields are the Option 82 circuit
identification fields (CID – including VLAN ID, stack unit, and port). These fields
identify the requesting device by indicating the interface through which the
relay agent received the request.
If DHCP relay is enabled, and this switch sees a DHCP client request, it inserts its
own IP address into the request so that the DHCP server will know the subnet
where the client is located. Depending on the selected frame format set for the
remote-id, this information may specify the MAC address, IP address, or an arbitrary
string for the requesting device (that is, the relay agent in this context).
By default, the relay agent also fills in the Option 82 circuit-id field with information
indicating the local interface over which the switch received the DHCP client
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 clients 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.