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| Configuring the Switch
Configuring Security
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Figure 32: DHCP Snooping Configuration
CONFIGURING DHCP
RELAY AND OPTION 82
INFORMATION
Use the DHCP Relay Configuration page to configure DHCP relay service for
attached host devices. If a subnet does not include a DHCP server, you can
relay DHCP client requests to a DHCP server on another subnet.
When DHCP relay is enabled and the switch sees a DHCP request
broadcast, it inserts its own IP address into the request (so that the DHCP
server knows the subnet of the client), 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 switch. The switch
then broadcasts the DHCP response to the client.
DHCP also provides a mechanism for sending information about the switch
and its DHCP clients to the DHCP server. Known as DHCP Option 82, it
allows compatible DHCP servers to use the information when assigning IP
addresses, or to set other services or policies for clients.
Using DHCP Relay Option 82, clients can be identified by the VLAN and
switch port to which they are connected rather than just their MAC
address. DHCP client-server exchange messages are then forwarded
directly between the server and client without having to flood them to the
entire VLAN.
In some cases, the switch may receive DHCP packets from a client that
already includes DHCP Option 82 information. The switch can be configured
to set the action policy for these packets. Either the switch can drop
packets that already contain Option 82 information, keep the existing
information, or replace it with the switch's relay information.
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