Option-82 Data Insertion - Cisco 2950 - Catalyst Switch Configuration Manual

Chapter 20 Configuring DHCP Features
The switch drops a DHCP packet when one of these situations occurs:
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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 enabled globally and on the
Note
VLANs to which subscriber devices using this feature are assigned.
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 Relay Agent in a Metropolitan Ethernet Network
(DHCP client)
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A packet from a DHCP server, such as a DHCPOFFER, DHCPACK, DHCPNAK, or
DHCPLEASEQUERY packet, is received from outside the network or firewall.
A packet is received on an untrusted interface, and the source MAC address and the DHCP client
hardware address do not match.
The switch receives a DHCPRELEASE or DHCPDECLINE broadcast message that contains a MAC
address in the DHCP snooping binding table, but the interface information in the binding table does
not match the interface on which the message was received.
A DHCP relay agent forwards a DHCP packet that includes an relay-agent IP address that is not
0.0.0.0, or the relay agent forwards a packet that includes option-82 information to an untrusted port.
is an example of a metropolitan Ethernet network in which a centralized DHCP server
Catalyst switch
(DHCP relay agent)
Subscribers
Host A
DHCP
server
Access layer
VLAN 10
Host B
(DHCP client)
Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
Understanding DHCP Features
20-3