Understanding Dhcp Option-82; Option 82 Relay Information Encapsulation - Cisco ASR 9000 Series Configuration Manual

Establishing Subscriber Sessions

Understanding DHCP Option-82

DHCP Option 82 allows the DHCP server to generate IP addresses based on the location of the client device.
This option defines these sub-options:
• Agent Circuit ID Sub-option—This sub-option is inserted by DSLAM and identifies the subscriber line
• Agent Remote ID Sub-option—This sub-option is inserted by DSLAM or BNG in an l2-connected
• VPN identifier sub-option—This sub-option is used by the relay agent to communicate the VPN for
• Subnet Selection Sub-option—This sub-option allows the separation of the subnet from the IP address
• Server Identifier Override Sub-option—This sub-option value is copied in the reply packet from the
The VPN Identifier, Subnet Selection, and Server Identifier Override sub-options are used by DHCP
Note
relay/proxy for supporting MPLS VPNs.

Option 82 Relay Information Encapsulation

When two relay agents are relaying messages between the DHCP client and DHCP server, the second relay
agent (closer to the server), by default, replaces the first option 82 information with its own option 82. The
remote ID and circuit ID information from the first relay agent is lost. In some deployment scenarios, it is
necessary to maintain the initial option 82 from the first relay agent, in addition to the option 82 from the
second relay agent.
The DHCP option 82 relay information encapsulation feature allows the second relay agent to encapsulate
option 82 information in a received message from the first relay agent, if it is also configured to add its own
option 82 information. This configuration allows the DHCP server to use option 82 information from both
the relay agents.
OL-28375-03
in the DSLAM.
topology. It is the client MAC address, but can be overridden. With the DHCP proxy or relay, the client
MAC address is lost by the time the packet gets to the DHCP server. This is a mechanism that preserves
the client MAC when the packet gets to the server.
every DHCP request that is sent to the DHCP server, and it is also used to forward any DHCP reply that
the DHCP server sends back to the relay agent.
and is used to communicate with the relay agent. In a DHCP processing, the gateway address specifies
both the subnet on which a DHCP client resides, and the IP address that the server uses to communicate
with the relay agent.
DHCP server, instead of the normal server ID address. This sub-option contains the incoming interface
IP address, which is the IP address on the relay agent that is accessible from the client. Using this
information, the DHCP client sends all renew and release packets to the relay agent, which in turn adds
all of the VPN sub-options and forwards the renew and release packets to the original DHCP server.
Cisco ASR 9000 Series Aggregation Services Router Broadband Network Gateway Configuration Guide, Release
Understanding DHCP Option-82
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