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19-4
Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
OL-10101-02
Chapter 19 Configuring DHCP Features
Understanding DHCP Features
Figure 19-1 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. The
option-82 information contains the switch MAC address (the remote-ID suboption) and the port
identifier, vlan-mod-port, from which the packet is received (the circuit-ID suboption).
• If the IP address of the relay agent is configured, the switch adds the 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. When the client and server are on the same subnet, the server broadcasts the reply. 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.
When the described sequence of events occurs, the values in these fields in Figure 19-2do 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
–
Length of the remote-ID type
Subscribers
Catalyst switch
(DHCP relay agent)
Host A
(DHCP client)
Access layer
DHCP
server
Host B
(DHCP client)
98813
VLAN 10