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Foundry NetIron M2404C and M2404F Metro Access Switches
Configuring DHCP Features (Rev. 03)
DHCP Relay
© 2008 Foundry Networks, Inc.
Page 46 of 63
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Sub-option Type (one byte) – the value of this field is 2. The format of the packet is as shown
in
Figure 15
.
•
Total Length (one byte) – the total length in bytes of all the fields that follow. In case of
Remote ID the value of the Total Length field is 8.
•
Remote ID type (one byte) – this field has a constant value that should be 0.
•
Length (one byte) – the length of the Remote ID, that is, the length in bytes of the MAC
Address (six bytes) – this is the MAC address of the relay agent device.
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
or additional criteria.
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 16
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 16: 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.