Switch User Manual

Basic Configuration
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Web – If the address assigned by DHCP is no longer functioning, you will not be
able to renew the IP settings via the web interface. You can only restart DHCP
service via the web interface if the current address is still available.
CLI – Enter the following command to restart DHCP service.
DHCP Relay and Option 82 Information
The switch supports 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 discard the Option 82
information, keep the existing information, or replace it with the switch’s relay
information.
Command Usage
You must specify the IP address of at least one DHCP server. Otherwise, the
switch’s DHCP relay agent will not operate and all DHCP request and reply packets
will be flooded to the entire VLAN.
Command Attributes
• DHCP Relay Option 82 – Enables the DHCP relay agant with Option 82 support.
• DHCP Relay Option 82 Policy – Sets the DHCP relay policy for DHCP client
packets that include Option 82 information. (Default: Drop)
• Replace – Overwrites the DHCP client packet information with the switch’s relay
information.
• Keep – Retains the client’s DHCP information.
Console#ip dhcp restart 4-246
Console#