Pseudowire Preferential Forwarding Status Bit
updated by
- rfc-7771 — Switching Provider Edge (S-PE) Protection for MPLS and MPLS Transport Profile (MPLS-TP) Static Multi-Segment Pseudowires
updates
- rfc-4447 — Pseudowire Setup and Maintenance Using the Label Distribution Protocol (LDP)
Extracted elements (25)
For Generalized PWid FEC tiebreaking in independent mode, the PE with the lowest system IP uses AGI::SAII::TAII while the PE with the highest system IP uses AGI::TAII::SAII, so that both PEs compare the same values (since SAII and TAII are mirror values at each end).
The independent mode is preferred over master/slave mode for applications requiring PE node protection, because master/slave mode has no mechanism to detect misconfiguration and does not specify backup master procedures. Independent mode handles PE node failures more robustly.
The Request Switchover bit was added because independent mode endpoints may fail to agree on a common active PW (e.g., due to misconfiguration or certain failure scenarios). Unlike master/slave mode, the request requires explicit peer acknowledgment before the requesting endpoint switches, preventing traffic loss.
This document updates RFC 4447 in two ways: (1) prescribing that any RFC 4447-compliant PE MUST ignore status bits it does not recognize, and (2) clarifying that a status bit may indicate non-fault status or an instruction to the peer PE, not just a fault condition.
When the Request Switchover bit is not supported by the receiving PE, it will be silently ignored, making the feature backward compatible. Coordination of PW switchover requires both sending and receiving PEs to support the Request Switchover bit.
A PE compliant with RFC 4447 that does not support the Preferential Forwarding or Request Switchover status bits MUST ignore these bits if set by a peer PE. This updates RFC 4447 to prescribe the same behavior for any status bit not originally defined in RFC 4447.
A PE implementation using these mechanisms MUST negotiate the use of PW Status TLV between its T-LDP peers as per RFC 4447. If PW Status TLV is not supported by either endpoint, the mechanisms in this document cannot be used.
All PWs in a redundant set MUST use the same FEC type. When using Generalized PWid FEC, each PW MUST have a unique identifier consisting of the triplet AGI::SAII::TAII.
At the requesting endpoint during switchover coordination, a timer SHOULD be configurable with a default value of 3 seconds while waiting for the remote endpoint to acknowledge the switchover request.
For MS-PWs, S-PEs MUST relay the PW status notification containing both the existing status bits and the new Preferential Forwarding status bits between ingress and egress PWs as per the procedures defined in RFC 6073.
In independent mode, if a management notification for failure to find an active PW was generated and an active PW is subsequently found, a management notification SHOULD be generated to clear the previous failure indication.
In independent mode, if more than one PW qualifies for the active state, each PW endpoint MUST implement a common default mechanism to choose the PW for forwarding, using lowest PWid value (for PWid FEC) or lowest AGI::SAII::TAII unsigned integer (for Generalized PWid FEC).
In master/slave mode, the slave node MUST stop forwarding over the previously active PW when the received status bit transitions from active to standby, and MUST start forwarding user traffic when the received status bit transitions from standby to active for a given PW.
In standby state, endpoints MUST NOT forward data traffic over the PW but MAY allow PW OAM packets (e.g., VCCV) to be sent and received in order to test the liveliness of standby PWs.
The master node MUST ignore any PW Preferential Forwarding status bits received from slave nodes; only the master selects and dictates which PW to activate.
When a PE enters the AC receive (or transmit) defect state, it MUST send a forward (reverse) defect indication to the remote peers over all PWs in the redundant set associated with this AC.
A new PW Preferential Forwarding status bit (0x00000020) is defined to signal the active or standby forwarding state of a PW. When set, it indicates PW forwarding standby; when cleared, it indicates PW forwarding active.
A second status bit, the Request Switchover status bit (0x00000040), is defined to allow peer PE nodes to coordinate a PW switchover operation. When set, it represents a request to switch over to this PW; when cleared, it represents no specific action.
In independent mode, active state requires PW labels exchanged, both UP/DOWN and Preferential Forwarding status active at both endpoints. Standby state requires labels exchanged but Preferential Forwarding status standby at one or both endpoints.
The Request Switchover coordination procedure requires the requesting endpoint to send a Status TLV with the Request Switchover bit set, start a timer, and not activate forwarding until it receives an explicit acknowledgment (Preferential Forwarding bit cleared on requested PW) from the peer.
Two modes of operation are defined: independent mode (each endpoint independently selects which PW to activate and advertises its preference; a PW is active only when both endpoints advertise active status) and master/slave mode (one designated master selects the active PW and dictates it to slave endpoints).
IANA has allocated two new entries in the 'Pseudowire Status Codes Registry': 0x00000020 for PW Preferential Forwarding Status (standby when set, active when cleared) and 0x00000040 for PW Request Switchover Status (request switchover when set).
LDP extensions and options that protect PWs must be implemented because the Preferential Forwarding and Request Switchover status bits have the same security considerations as the PW setup and maintenance protocol in RFC 4447. The security of a redundant PW set is only as good as the weakest security on any of its member PWs.
The PW state machine in master/slave mode defines two states (ACTIVE and STANDBY) with transitions: ACTIVE→STANDBY when master puts PW in standby (transmit Preferential Forwarding bit set) or slave receives bit set; STANDBY→ACTIVE when master activates PW (transmit bit clear) or slave receives bit clear. Unsupported bit reception causes no state change.
The Preferential Forwarding status bit (bit 0x00000020) and Request Switchover status bit (bit 0x00000040) are carried in the PW Status TLV defined in RFC 4447. Active state is signaled by clearing the Preferential Forwarding bit; standby state is signaled by setting it.