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.
Figure 50: Example of DHCP Option 82 Operation in a Network with a Non-Compliant Relay Agent
Option 82 field content
The remote ID and circuit ID subfields comprise the Option 82 field a relay agent appends to client requests. A
DHCP server configured to apply a different IP addressing policy to different areas of a network uses the values in
these subfields to determine which DHCP policy to apply to a given client request.
Remote ID
This configurable subfield identifies a policy area that comprises either the routing switch as a whole (by using
the routing switch MAC address) or an individual VLAN configured on the routing switch (by using the IP
address of the VLAN receiving the client request.)
• Use the IP address option if the server will apply different IP addressing policies to DHCP client requests
from ports in different VLANs on the same routing switch.
• Use the management VLAN option if a management VLAN is configured and you want all DHCP clients on
the routing switch to use the same IP address. (This is useful if you are applying the same IP addressing
policy to DHCP client requests from ports in different VLANs on the same routing switch.) Configuring this
option means the management VLAN's IP address appears in the remote ID subfield of all DHCP requests
originating with clients connected to the routing switch, regardless of the VLAN on which the requests
originate.
• Use the MAC address option if, on a given routing switch, it does not matter to the DHCP server which
VLAN is the source of a client request (that is, use the MAC address option if the IP addressing policies
supported by the target DHCP server do not distinguish between client requests from ports in different
VLANs in the same routing switch.)
Circuit ID
This nonconfigurable subfield identifies the port number of the physical port through which the routing switch
received a given DHCP client request and is necessary to identify if you want to configure an Option 82 DHCP
server to use the Circuit ID to select a DHCP policy to assign to clients connected to the port. This number is
the identity of the inbound port. On fixed-port switches, the port number used for the circuit ID is always the
same as the physical port number shown on the front of the switch. On chassis switches, where a dedicated,
sequential block of internal port numbers are reserved for each slot, regardless of whether a slot is occupied,
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Switch 16.08