The DHCP Information option (Option
82) is commonly used in metro or large enterprise deployments to provide
additional information on “physical attachment” of the client.
Option 82 is
supposed to be used in distributed DHCP server/relay environment, where relays
insert additional information to identify the client’s point of attachment.
The DHCP relay agent information
option (DHCP option 82) enables a DHCP relay agent to include information about
itself when forwarding client-originated DHCP packets to a DHCP server. The
DHCP server uses this information to implement IP address or other parameter-assignment
policies.
In a metropolitan Ethernet-access
environment, the DHCP server can centrally manage IP address assignments for a
large number of subscribers. If DHCP option 82 is disabled, a DHCP policy can
only be applied per subnet, rather than per physical port. When DHCP option 82
is enabled, a subscriber is identified by the physical port through which it
connects to the network.
Specifically the option works by setting two sub-options: Circuit ID and Remote ID.
The Circuit ID sub-option is supposed to include information specific to which circuit the request came in on. It's an identifier that is specific to the relay agent, so what kind of circuit is described will vary depending on the relay agent. In an Ethernet-based network this is probably a port on a switch. In other networks, this might be a wireless modem or an ATM virtual circuit.
The Remote ID sub-option was designed to carry information relating to the remote host end of the circuit. In practice, this sub-option usually contains information that identifies the relay agent. In the case of an Ethernet network, this is the MAC address of the relay agent. In a wireless network, this would likely be a unique identifier of the wireless access point. It's worth noting that the Remote ID sub-option was designed to be globally unique. In other words, Remote ID "7" should not have different meanings depending on some other context. In it's default configuration, the DHCP Relay Agent Information Option passes along port and agent information to a central DHCP server. It is useful in statistical analysis, as well as, indicating where an assigned IP address physically connects to the network. It may also be used to make DHCP decisions based on where the request is coming from or even which user is making the request.
Below Figure :DHCP Relay Agent and DHCP Server Processing of Option 82 Suboptions
2. The DHCP relay agent
intercepts the broadcast DHCP request packet and inserts a server ID override
suboption and link selection suboption to its relay agent information option in
the DHCP packet. The server ID override and link selection suboptions contain
the incoming interface IP address, which is the IP address on the relay agent
that is accessible from the client (10.1.1.1 in this case).
3. The relay agent sets
the giaddr to the IP address of an interface that is reachable by the DHCP
server (typically the server-facing interface that will be used to transmit the
message, 10.3.1.2 in this case).
4.If the source interface
is explicitly configured on a loopback interface (ip dhcp-relay
source-interface command), the relay agent will use that address as the
source IP address (giaddr)
for messages relayed to the DHCP server (10.2.1.1 in this case).
The following processing occurs on the DHCP
server after receiving the forwarded packets from the relay agent:
5. The DHCP server uses
the link selection suboption to locate the correct address pools for the DHCP
client.
6.The DHCP server sets
the server ID option to the value specified by the server ID override suboption
of the DHCP packet.
The DHCP client will see the relay agent
address as the server ID and use that address when uni casting RENEW messages.
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