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 Nokia 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 (toward the customer) the information-option is stripped (per the RFC).
Drop
The DHCP packet is dropped and a counter is incremented.
Keep
The existing information is kept on the packet and the router does not add any more information. On egress the information option is not stripped and is sent on to the downstream node.
In accordance with the RFC, the default behavior is to keep the existing information; except if the giaddr of the packet received is identical to a local IP address on the router, then the packet is dropped and an error incremented regardless of the configured action.
The maximum packet size for a DHCP relay packet is 1500 bytes. If adding the Option 82 information would cause the packet to exceed this size, the DHCP relay request is forwarded without the Option 82 information. This packet size limitation exists to ensure that there is no fragmentation on the end Ethernet segment where the DHCP server attaches.
In the downstream direction, the inserted Option 82 information should not be passed back toward the client (as per RFC 3046, DHCP Relay Agent Information Option). To enable downstream stripping of the option 82 field, DHCP snooping should be enabled on the SDP or SAP connected to the DHCP server.