Option-82 Data Insertion - Cisco Catalyst 2928 Software Configuration Manual

Understanding DHCP Snooping

Option-82 Data Insertion

In residential, metropolitan Ethernet-access environments, DHCP can centrally manage the IP address
assignments for a large number of subscribers. When the DHCP option-82 feature is enabled on the
switch, a subscriber device is identified by the switch port through which it connects to the network (in
addition to its MAC address). Multiple hosts on the subscriber LAN can be connected to the same port
on the access switch and are uniquely identified.
The DHCP option-82 feature is supported only when DHCP snooping is globally enabled and on the
Note
VLANs to which subscriber devices using this feature are assigned.
Figure 19-1
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 19-1
(DHCP client)
When you enable the DHCP snooping information option 82 on the switch, this sequence of
events occurs:
•
•
•
•
Catalyst 2928 Switch Software Configuration Guide
19-4
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
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.
Chapter 19 Configuring DHCP Features and IP Source Guard Features
DHCP
server
Access layer
VLAN 10
Host B
(DHCP client)
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