Cisco IE 3000 Software Configuration Manual page 406

Understanding DHCP Features
Figure 20-1
assigns IP addresses to subscribers connected to the switch at the access layer. Because the DHCP clients
and their associated DHCP server do not reside on the same IP network or subnet, a DHCP relay agent
(the Catalyst switch) is configured with a helper address to enable broadcast forwarding and to transfer
DHCP messages between the clients and the server.
Figure 20-1
(DHCP client)
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
•
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.
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.
When the described sequence of events occurs, the values in these fields in
Circuit-ID suboption fields
•
–
–
–
–
Remote-ID suboption fields
•
–
Cisco IE 3000 Switch Software Configuration Guide
20-4
is an example of a metropolitan Ethernet network in which a centralized DHCP server
DHCP Relay Agent in a Metropolitan Ethernet Network
Catalyst switch
(DHCP relay agent)
Subscribers
Host A
Suboption type
Length of the suboption type
Circuit-ID type
Length of the circuit-ID type
Suboption type
DHCP
server
Access layer
VLAN 10
Host B
(DHCP client)
Chapter 20 Configuring DHCP Features
Figure 20-2 do not change:
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