Why Control Word Must Be Used in MPLS/VPLS

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ePMP will not fully support VPLS without Control Word

In modern networking—especially with performance-optimized platforms like ePMP radios that use hardware offloading to reduce OS workload—correct packet parsing is critical. Devices such as Network Soft Switches (NSS) must correctly interpret VPLS and MPLS pseudowire traffic. If the Control Word (CW) is not used, packet headers may be misinterpreted, leading to ambiguities, packet drops, or incorrect behavior. This is not just a theoretical concern: real-world deployments have suffered from issues when CW is omitted. Therefore, the CW should always be enabled when supported by both ends, aligning with updated standards like RFC 8469.

Understanding Control Word Usage in VPLS and MPLS Pseudowires

Virtual Private LAN Service (VPLS) and other Layer 2 VPNs over MPLS use pseudowires (PWs) to encapsulate Ethernet or other Layer 2 frames. The Control Word (CW) is a 4-byte field inserted between the MPLS label stack and the Layer 2 payload. Initially optional, it is now widely recommended—or even required—due to critical functional and interoperability reasons.

Early Standards and Optional CW Usage
RFC 4385 and RFC 4448 introduced the concept of the Control Word.

  • At the time, CW was defined as optional, used for features like frame sequencing and padding indication.
  • However, deployments soon uncovered operational risks in omitting the CW.

Technical Ambiguities Without CW
When the Control Word is not present, ambiguity arises because:

  • MPLS pseudowires expect a first nibble of 0x0 if a CW is used.
  • IP payloads in MPLS expect the first nibble to be 0x4 or 0x6, indicating IPv4 or IPv6.
  • If an Ethernet payload coincidentally starts with a nibble that mimics IP traffic, the receiving device may misinterpret the data, causing:
    • Faulty packet parsing
    • Incorrect forwarding decisions
    • Debugging difficulties and silent data loss

This issue is particularly acute in hardware-accelerated environments (like ePMP radios) where fast-path packet processing does not allow for deep protocol inspection.

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Evolution of the Standards

The industry responded with clearer and stricter guidance in later RFCs:

  • RFC 5085 – Pseudowire VCCV:
    Requires CW when using CW-based OAM (e.g., VCCV Type 1), for reliable control channel separation.

  • RFC 8469 – Pseudowire Congestion Considerations:
    States that CW MUST be used if supported by both ends, especially to eliminate parsing ambiguities.

These RFCs reflect lessons learned from early deployments that experienced interoperability issues due to lack of CW.

Practical Recommendations

To ensure reliable and standards-compliant VPLS/MPLS operation:

  • Always enable the Control Word when supported on both ends of the tunnel.
  • Be aware that hardware offloading platforms (like ePMP radios or NSS devices) require deterministic packet structures.
  • Consult RFCs 4385, 4448, 5085, and 8469 for full implementation guidelines.
  • Avoid legacy configurations or defaults that omit the CW, especially in multivendor or OAM-sensitive environments.
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