EtherHaul Operation, Administration and Maintenance Manual
Page 136
14.6
DHCP Relay (Option 82)
Since DHCP packets cannot travel across a router, a relay agent is necessary in order to
have a single DHCP server handle all leases.
Relay agents receive broadcast DHCP packets and forward them as unicast packets to a
DHCP server.
With the DHCP option-82 feature enabled, port-to-port DHCP broadcast isolation is
achieved when the client ports are within a single VLAN. During client-to-server
exchanges, broadcast requests from clients connected to VLAN access ports are
intercepted by the relay agent and are not flooded to other clients on the same VLAN.
The relay agent forwards the request to the DHCP server. During server-to-client
exchanges, the DHCP server sends a broadcast reply that contains the option-82 field.
The relay agent uses this information to identify which port connects to the requesting
client and avoids forwarding the reply to the entire VLAN.
When enabling DHCP relay agent option 82:
The host (DHCP client) generates a DHCP request and broadcasts it on the
network.
The system (DHCP relay agent) intercepts the broadcast DHCP request packet
and inserts the relay agent information option (option 82) in the packet. The
relay information option contains the system’s MAC address (the remote ID sub-
option) and the port SNMP ifindex from which the packet is received (circuit ID
sub-option).
The system forwards the DHCP request that includes the option-82 field to the
DHCP server.
The DHCP server receives the packet. If the server is option-82 capable, it might
use the remote ID, the circuit ID, or both to assign IP addresses and implement
policies, such as restricting the number of IP addresses that can be assigned to a
single remote ID or circuit ID. Then the DHCP server echoes the option-82 field
in the DHCP reply.
If the server does not support option 82, it ignores the option and does not echo
it in the reply.
The DHCP server unicasts the reply to the relay agent. The relay agent makes
sure that the packet is destined for it by checking the IP destination address in
the packet. The relay agent removes the option-82 field and forwards the packet
to the system port that connects to the DHCP client, which sent the DHCP
request.
Note:
Configuring DHCP Relay supported in the CLI only.
