DHCP Principles
Page 936 7210 SAS-X OS Services Guide
DHCP Features
• Using Option 82 Field on page 936
• Trusted and Untrusted on page 937
• DHCP Snooping on page 937
Using Option 82 Field
Option 82, or the relay information option is specified in RFC 3046, DHCP Relay Agent
Information Option, allows the router to append some information to the DHCP request that
identifies where the original DHCP request arrives from.
There are two sub-options under Option 82:
• Agent Circuit ID Sub-option (RFC 3046, section 3.1): This sub-option specifies data
which must be unique to the box that is relaying the circuit.
• Remote ID Sub-option (RFC 3046 section 3.2): This sub-option identifies the host at the
other end of the circuit. This value must be globally unique.
Both sub-options are supported by the Alcatel-Lucent 7210 SAS and can be used separately or
together.
Inserting Option 82 information is supported independently of DHCP relay.
When the circuit id sub-option field is inserted by the 7210 SAS, it can take following values:
• sap-id: The SAP index (only under a IES or VPRN service)
• ifindex: The index of the IP interface (only under a IES or VPRN service)
• ascii-tuple: An ASCII-encoded concatenated tuple, consisting of [system-name|serviceid|
interface-name] (for VPRN or IES) or [system-name|service-id|sap-id] (for VPLS).
• vlan-ascii-tuple: An ASCII-encoded concatenated tuple, consisting of the ascii-tuple
followed by Dot1p bits and Dot1q tags.
Note that for VPRN the ifindex is unique only within a VRF. The DHCP relay function
automatically prepends the VRF ID to the ifindex before relaying a DHCP Request.
When a DHCP packet is received with Option 82 information already present, the system can do
one of three things. The available actions are:
• Replace — On ingress the existing information-option is replaced with the information-
option parameter configured on the 7210 SAS. On egress (towards the customer) the
information-option is stripped (per the RFC).
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