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Configuring DHCP
Information About Configuring DHCP
Figure 60 DHCP Relay Agent in a Metropolitan Ethernet Network
When you enable the DHCP snooping information option-82 on the switch, this sequence of events occurs:
The host (DHCP client) generates a DHCP request and broadcasts it on the network.
When the switch receives the DHCP request, it adds the option-82 information in the packet. By default, the
remote-ID suboption is the switch MAC address, and the circuit-ID suboption is the port identifier, vlan-mod-port,
from which the packet is received.
If the IP address of the relay agent is configured, the switch adds this IP address in the DHCP packet.
The switch forwards the DHCP request that includes the option-82 field to the DHCP server.
The DHCP server receives the packet. If the server is option-82-capable, it can use the remote ID, the circuit ID, or
both to assign IP addresses and implement policies, such as restricting the number of IP addresses that can be
assigned to a single remote ID or circuit ID. The DHCP server then repeats the option-82 field in the DHCP reply.
The DHCP server unicasts the reply to the switch if the request was relayed to the server by the switch. The switch
verifies that it originally inserted the option-82 data by inspecting the remote ID and possibly the circuit ID fields. The
switch removes the option-82 field and forwards the packet to the switch port that connects to the DHCP client that
sent the DHCP request.
In the default suboption configuration, when the described sequence of events occurs, the values in these fields in
Figure 61 on page 392 do not change:
Circuit-ID suboption fields
— Suboption type
— Length of the suboption type
— Circuit-ID type
— Length of the circuit-ID type
Remote-ID suboption fields
— Suboption type
— Length of the suboption type
— Remote-ID type
Subscribers
Catalyst switch
(DHCP relay agent)
Host A
(DHCP client)
Access layer
DHCP
server
Host B
(DHCP client)
98813
VLAN 10
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