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19-4
Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
OL-9639-06
Chapter 19 Configuring DHCP Features and IP Source Guard
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. 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. Beginning with Cisco IOS
Release
12.2(25)SEG, you can configure the remote ID and circuit ID. For information on
configuring these suboptions, see the
“Enabling DHCP Snooping and Option 82” section on
page 19-11.
• 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 in
Figure 19-2 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
Subscribers
Cisco ME switch
(DHCP relay agent)
Host A
(DHCP client)
Access layer
DHCP
server
Host B
(DHCP client)
92999
VLAN 10