Foundry NetIron M2404C and M2404F Metro Access Switches
Configuring DHCP Features (Rev. 03)
DHCP Server
© 2008 Foundry Networks, Inc.
Page 7 of 63
Option 82 Policy
In the residential, metropolitan Ethernet-access environment, the DHCP can centrally manage the
IP address assignment for a large number of subscribers. When DHCP option 82 is not enabled, a
DHCP policy can be applied only per subnet scope and not per physical port. By enabling the
DCHP option 82 feature on the device, a subscriber is identified by the physical port through
which it connects to the network.
Figure 4
is an example of a metropolitan Ethernet network in which a centralized DHCP server
assigns IP addresses to subscribers connected to a device at the access layer. Because the DHCP
clients and their associated DHCP servers do not reside on the same IP network or subnet, a DHCP
relay agent is configured with a helper address to enable broadcast forwarding and to transfer
DHCP messages between the clients and the server.
Figure 4: Metropolitan Ethernet Network
With the DHCP option 82 feature enabled on the device, 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 as an IP unicast packet. During server-to-client exchanges, the DHCP server sends an
IP unicast reply that contains the option-82 field. By using this information to determine which
port is connected to the requesting client, the relay agent avoids forwarding the reply to the entire
VLAN.
The DHCP Servers can be configured with ranges of IP addresses assigned to a specific circuit-ID.
Addresses from this range will be given only to the client behind DHCP Relay agents relaying
packets with the same circuit-ID. If all the IP addresses in the range with a specific circuit-ID are
in use, the server will not assign an IP address to a DHCP client with the same circuit-ID.
There are two DHCP policies for treating Unknown circuit-IDs:
•
Permit policy
– the DHCP server assigns addresses from ranges that do not have assigned
Circuit-IDs (this is the default policy).
•
Deny policy
– the DHCP server does not assign IP addresses to DHCP clients that have
unknown Circuit-IDs.