Bit-Error-Triggered Switching for PW

Background

When PW redundancy is configured for L2VPN services, bit-error-triggered switching can be configured. With this function, if bit errors occur, services can switch between the primary and secondary PWs.

Principles

Trigger-LSP bit error detection must be enabled on each node's interfaces. PW redundancy can be configured in either a single segment or multi-segment scenario.

• Single-segment PW redundancy scenario

  In Figure 1, PE1 establishes a primary PW to PE2 and a secondary PW to PE3, which implements PW redundancy. If PE2 detects bit errors, the processing is as follows:

  • PE2 switches traffic destined for PE1 to the path bypass PW -> PE3 -> secondary PW -> PE1 and sends a BFD packet to notify PE1 of the bit errors.

  • Upon receipt of the BFD packet, PE1 switches traffic destined for PE2 to the path secondary PW-> PE3 -> bypass PW -> PE2.

  Traffic between PE1 and PE2 can travel along bit-error-free links.

  Figure 1 Bit-error-triggered switching for single-segment PW
  

• Multi-segment PW redundancy scenario

  In Figure 2, multi-segment PW redundancy is configured. PE1 is dual-homed to two SPEs. If PE2 detects bit errors, the processing is as follows:

  • PE2 switches traffic destined for PE1 to the path bypass PW -> PE3 -> PW2 -> SPE2 -> secondary PW -> PE1 and sends a BFD packet to notify SPE1 of the bit errors.

  • Upon receipt of the BFD packet, SPE1 sends an LDP Notification message to notify PE1 of the bit errors.

  • Upon receipt of the notification, PE1 switches traffic destined for PE2 to the path secondary PW -> SPE2 -> PW2 -> PE3 -> bypass PW-> PE2.

  Traffic between PE1 and PE2 can travel along bit-error-free links. If bit errors occur on a link between PE1 and SPE1, the processing is the same as that in the single-segment PW redundancy scenario.

  Figure 2 Bit-error-triggered switching for multi-segment PW
  

After traffic switches to the secondary PW, and bit errors are removed from the primary PW, traffic switches back to the primary PW based on a configured switchback policy.

If an RSVP-TE tunnel is established for PWs, and bit-error-triggered RSVP-TE tunnel switching is configured, a switchover is preferentially performed between the primary and hot-standby CR-LSPs in the RSVP-TE tunnel. A primary/secondary PW switchover can be triggered only if the primary/hot-standby CR-LSP switchover fails to remove bit errors in either of the following situations:

• The hot standby function is not configured.
• Bit errors occur on both the primary and hot-standby CR-LSPs.

Usage Scenario

If L2VPN is used to carry user services and PW redundancy is deployed to ensure reliability, deploy bit-error-triggered switching for PW to minimize the impact of bit errors on user services and improve service quality.