Juniper JUNOS OS 10.3 - SOFTWARE, Manual

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Overview and Topology
If DHCP option 82 is enabled on the switch, then when a network device—a DHCP
client—that is connected to the switch on an untrusted interface sends a DHCP request,
the switch inserts information about the client's network location into the packet header
of that request. The switch then sends the request to the DHCP server. The DHCP server
reads the option 82 information in the packet header and uses it to implement the IP
address or other parameter for the client.
DHCP option 82 is enabled on an individual VLAN or on all VLANs on the switch.
When option 82 is enabled on the switch, then this sequence of events occurs when a
DHCP client sends a DHCP request:
1.
The switch receives the request and inserts the option 82 information in the packet
header.
2.
The switch forwards the request to the DHCP server.
3.
The server uses the DHCP option 82 information to formulate its reply and sends a
response back to the switch. It does not alter the option 82 information.
4.
The switch strips the option 82 information from the response packet.
5.
The switch forwards the response packet to the client.
Figure 75 on page 2899 illustrates the topology for this example.
Figure 75: Network Topology for Configuring DHCP Option 82 on a Switch
That Is on the Same VLAN as the DHCP Clients and the DHCP Server
2899 Copyright © 2010, Juniper Networks, Inc.
Chapter 94: Examples: Port Security Configuration