Cisco Systems 2950 - page 420

20-4
Catalyst2950 and Catalyst2955 Switch Software Configuration Guide
78-11380-10
Chapter20 Configuring DHCP Features
Understanding DHCP Features
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, t he s witch adds the I P add ress in t he DHCP pack et.
•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 remoteID and possibly
the circuitID 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 previously described sequence of events occur s, the v alues in these f ields in Figure20-2do 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 circuit ID type
Figure 20-2 shows the packet formats for the remote ID suboption and the circuit ID suboption. The
switch uses the packet formats when DHCP snooping is globally enabled and when the ip dhcp
snooping information option global configuration command is entered.