Cisco Catalyst 2960 Series, Configuration Manual

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switch) is configured with a helper address to enable broadcast forwarding and to transfer DHCP messages
between the clients and the server.
Figure 86: DHCP Relay Agent in a Metropolitan Ethernet Network
When you enable the DHCP snooping information option 82 on the switch, the following 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.You can configure the remote ID and circuit ID.
• 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. Then the DHCP server echoes 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 do not change (see the illustration,
Suboption Packet Formats ):
• Circuit-ID suboption fields
◦ Suboption type
◦ Length of the suboption type
◦ Circuit-ID type
◦ Length of the circuit-ID type
Consolidated Platform Configuration Guide, Cisco IOS Release 15.2(4)E (Catalyst 2960-X Switches)
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Information About DHCP