Product guide

IP Routing Features
Configuring DHCP Relay
Multinetted VLANs
On a multinetted VLAN, each interface can form an Option 82 policy boundary
within that VLAN if the routing switch is configured to use IP for the remote
ID suboption. That is, if the routing switch is configured with IP as the remote
ID option and a DHCP client request packet is received on a multinetted VLAN,
the IP address used in the Option 82 field will identify the subnet on which the
packet was received instead of the IP address for the VLAN. This enables an
Option 82 DHCP server to support more narrowly defined DHCP policy
boundaries instead of defining the boundaries at the VLAN or whole routing
switch levels. If the MAC address option (the default) is configured instead,
then the routing switch MAC address will be used regardless of which subnet
was the source of the client request. (The MAC address is the same for all
VLANs configured on the routing switch.)
Note that all request packets from DHCP clients in the different subnets in the
VLAN must be able to reach any DHCP server identified by the IP Helper
Address(es) configured on that VLAN.
Configuring Option 82
To configure DHCP Option 82 on a routing switch, enter the dhcp-relay option
82 command.
Syntax: dhcp-relay option 82 < append [validate] | replace [validate] | drop [validate]
| keep > [ip | mac | mgmt-vlan]
append: Configures the switch to append an Option 82
field to the client DHCP packet. If the client packet
has existing Option 82 field(s) assigned by another
device, the new field is appended to the existing
field(s).
The appended Option 82 field includes the switch
Circuit ID (inbound port number*) associated with
the client DHCP packet, and the switch Remote ID.
The default switch remote ID is the MAC address of
the switch on which the packet was received from the
client. To use the incoming VLAN’s IP address or the
Management VLAN IP address (if configured) for the
remote ID instead of the switch MAC address, use the
ip or mgmt-vlan option (below).
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