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Cisco ME 3400 EthernetAccess Switch SoftwareConfiguration Guide
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Chapter18 Configuring DHCP Features and IP Source Guard
Understanding DHCP Features
Figure18-1 DHCP Relay Agent in a Metropolitan Ethernet Network
When you enable the DHCP snooping information option 82 on the switch, this sequence of
events occur s:
•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 is the switch MAC address (the remote ID suboption) and the port id entifier,
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 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 in spect ing the re moteID
and possibly the circuit ID fields. The switch removes the option-82 field and forwards th e packet
to the switch port that connects to the DHCP client that sent the DHCP request.
When the previously described sequence of events occurs, the values in these fields in Figure18- 2 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
–
Length of the circuit ID type
Subscribers
Cisco ME switch
(DHCP relay agent)
Host A
(DHCP client)
Access layer
DHCP
server
Host B
(DHCP client)
92999
VLAN 10