Allied Telesis Rapier i Series User Manual

Page 17 | AlliedWare™ OS How To Note: DHCP Snooping on Rapier-style switches

In a layer 3 routing environment, the switch takes on a role of BOOTP Relay Agent, with 
support for DHCP Option 82. The relay agent inserts the information mentioned above 
when forwarding client-originated DHCP packets to a DHCP server. DHCP servers that are 
configured to recognise the relay agent information option may use the information to keep a 
log of switches and port numbers that IP addresses have been allocated to, and may also use 
this information for various address assignment policies. 

The DHCP server echoes the option back to the relay agent in server-to-client replies, and 
the relay agent strips the option before forwarding the reply to the client (RFC 3046). This 
process is shown in the following figure.

create vlan="Customers" vid=48

create vlan="Network" vid=50

Here the DHCP Server is on VLAN 50, while the DHCP clients are on VLAN 48.

add vlan="48" port=1-24

add vlan="50" port=25

X

Configure two VLANs for layer 3 access to the DHCP server: 

X A

dd ports to the VLANs: 

(1). DHCP client sends request.

(2). Relay agent appends

    Option 82 to client sourced 

    packets.

(4). Relay agent strips

    Option 82, and forwards

    to client.

(3). Option 82 enabled DHCP
    server allocates address

    

   and stores the 

    to the relay agent.

Option 82 information.

    Option 82 echoed 

    Server sends offer, with