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 primary 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 Operation on the Routing Switch
Syntax:
append: Configures the routing switch to append an Option 82 field to the client DHCP packet. If the client packet has any existing Option 82 field(s) assigned by another device, then 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 instead of the switch MAC address for the remote ID, use the ip option (below).
replace: Configures the routing switch to replace any existing Option 82 field(s) in an inbound client DHCP packet with one Option 82 field for the current routing switch.
The replacement 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 instead of the switch MAC address for the remote ID, use the ip option (below).