Pseudowire Redundancy on the Switching Provider Edge (S-PE)
Extracted elements (19)
Preferential Forwarding status signaling from RFC 6870 and RFC 6478 is reused without modification, and no changes are required to T-PE implementations, allowing S-PE-level redundancy to be deployed incrementally without upgrading T-PE nodes.
PW redundancy on S-PE reduces the burden on access T-PE nodes that may not support PW redundancy, halves the number of PW segments required between T-PE1 and S-PE1 compared to end-to-end MS-PW redundancy, and enables faster protection switching than end-to-end MS-PW protection.
The document explicitly excludes the scenario where an S-PE has two multi-homed sides from scope, keeping the specification focused on the single-homed/multi-homed asymmetric case where the Master/Slave/Independent mode assignments are well-defined.
The S-PE Bypass mode (RFC 6478) is prohibited when S-PE provides PW redundancy because the bypass would prevent the S-PE from receiving PW status messages from T-PEs, which are essential for the S-PE to determine correct Preferential Forwarding status and perform switchovers.
VCCV CC type 1 'PW ACH' can be used with the S-PE redundancy mechanism. VCCV CC type 2 'Router Alert Label' is not supported for MS-PW as specified in RFC 6073.
For VCCV using CC type 3 'TTL Expiry' with S-PE redundancy, the PW label TTL MUST be set to the appropriate value to reach the target PE. The hop count can be obtained via SP-PE TLVs, MS-PW path trace, or management-plane information.
On the multi-homed side, where both S-PE and T-PEs work in Independent mode, the PW segment that has both local and remote Up/Down status as Up AND both local and remote Preferential Forwarding status as Active MUST be selected for traffic forwarding.
The S-PE MUST advertise Standby Preferential Forwarding status to the single-homed side if it receives Standby status from ALL PW segments on the multi-homed side, and MUST advertise Active status to the single-homed side if it receives Active status from ANY PW segment on the multi-homed side.
The S-PE MUST pass the Preferential Forwarding status received from the single-homed side unchanged to all PW segments on the multi-homed side.
The S-PE that provides PW redundancy MUST work in Slave mode for the single-homed side and MUST work in Independent mode for the multi-homed side.
The T-PE on the single-homed side MUST work in Master mode, and T-PEs on the multi-homed side MUST work in Independent mode, consistent with the S-PE's Slave/Independent mode assignments.
When a switchover happens on the S-PE, if the S-PE supports SP-PE TLV processing as defined in RFC 6073, it SHOULD advertise the updated SP-PE TLVs by sending a Label Mapping message to the T-PEs.
When PW redundancy for MS-PW is provided on an S-PE, the optional S-PE Bypass mode defined in RFC 6478 MUST NOT be used; otherwise the S-PE will not receive PW status messages originated by T-PEs.
In MS-PW redundancy on S-PE, the S-PE has a single-homed side (one PW segment toward T-PE1) and a multi-homed side (multiple PW segments toward redundant T-PEs). The primary PW segment on the multi-homed side carries traffic while the secondary provides resiliency for T-PE or AC failure.
In Scenario 1 normal operation: S-PE1 receives Active Preferential Forwarding from T-PE1 (single-homed side) and propagates it to PW-Seg2 and PW-Seg3. T-PE2 responds Active and T-PE3 responds Standby (reflecting AC states), causing PW-Seg2 to be selected for forwarding.
In Scenario 2, T-PE1 operates in Master mode and advertises Active to S-PE1 and Standby to S-PE2 per configuration, causing S-PE1 to forward on PW1-Seg2 toward T-PE2 and S-PE2 to remain in standby. On S-PE1 failure, T-PE1 detects failure and switches to PW2 via S-PE2.
In the S-PE protection scenario, two MS-PWs are switched at S-PE1 and S-PE2 respectively. PW1 (primary) is switched at S-PE1 with PW1-Seg2 as primary segment, and PW2 (secondary) is switched at S-PE2, providing both S-PE node protection and multi-homed T-PE resiliency.
Since PW redundancy is provided on S-PE nodes of MS-PWs, the security mechanisms defined in RFC 4447, RFC 6073, and RFC 6478 MUST be implemented to ensure S-PE nodes and the messages sent and received by them are not compromised.
S-PE protection switching is driven by matching local and remote Up/Down status and Preferential Forwarding status on the multi-homed side. On AC failure, the affected T-PE sends a PW status Notification (Down); the peer T-PE advertises Active; the S-PE selects the new active PW segment without changing status toward the single-homed side as long as any Active segment remains.