Multicast and Routing Guide K/KA/KB.15.15

For more information, see the documentation provided with the server application.
Option 82 server support
To apply DHCP Option 82, the routing switch must operate in conjunction with a server that supports
Option 82. (DHCP servers that do not support Option 82 typically ignore Option 82 fields.) Also,
the routing switch applies Option 82 functionality only to client request packets being routed to a
DHCP server. DHCP relay with Option 82 does not apply to switched (non-routed) client requests.
For information on configuring policies on a server running DHCP Option 82, see the documentation
provided for that application.
Figure 47 Example of a DHCP Option 82 application
Switch "A"
10.10.10.2
VLAN
10
10.10.10.1
Client
3
DHCP
Option
82
Server
Subnets 10 and 20 in relay agent "1"
form policy boundaries that can be
defined by the IP address of the subnet
on which the client request is received.
Relay Agent "1" (Routing Switch)
with DHCP Option 82 Enabled
Client
1
Client
2
VLAN
20
10.10.20.1
Switch "B"
10.10.20.3
Client
6
Client
4
Client
5
10.10.20.2
10.10.30.1
Relay Agent "2" (Routing
Switch) without DHCP
Option 82 Enabled
Policy Boundaries
General DHCP Option 82 requirements and operation
Requirements
DHCP Option 82 operation is configured at the global config level and requires the following:
• IP routing enabled on the switch
• DHCP-relay option 82 enabled (global command level)
• Routing switch access to an Option 82 DHCP server on a different subnet than the clients
requesting DHCP Option 82 support
• One IP helper address configured on each VLAN supporting DHCP clients
General DHCP-relay operation with Option 82
Typically, the first (primary) Option 82 relay agent to receive a client's DHCP request packet
appends an Option 82 field to the packet and forwards it toward the DHCP server identified by
the IP helper address configured on the VLAN in which the client packet was received. Other,
upstream relay agents used to forward the packet may append their own Option 82 fields, replace
the Option 82 fields they find in the packet, forward the packet without adding another field, or
drop the packet. (Intermediate next-hop routing switches without Option 82 capability can be used
to forward—route—client request packets with Option 82 fields.) Response packets from an Option
82 server are routed back to the primary relay agent (routing switch) and include an IP addressing
assignment for the requesting client and an exact copy of the Option 82 data the server received
with the client request. The relay agent strips off the Option 82 data and forwards the response
packet out the port indicated in the response as the Circuit ID (client access port.) Under certain
validation conditions described later in this section, a relay agent detecting invalid Option 82 data
in a response packet may drop the packet.
DHCP Option 82 239