Traffic Steering using BGP FlowSpec with SR Policy

2.1. Terminology

• Mode 1 (Transport Steering Only): Steering traffic into an SR Policy (SR-MPLS or SRv6) toward a target Endpoint solely for transport path selection.¶

• Mode 2 (Steering with Egress Service Action): Steering traffic into an SRv6 Policy toward a target Endpoint while executing an Egress Service SID function (e.g., decapsulation followed by routing table lookup via End.DT4/DT6/DT46, or L3 interface cross-connect via End.DX4/DX6 [RFC8986][RFC9252]) at the egress PE.¶

• Redirect-to-IP: The FlowSpec Redirect-to-IP Extended Community defined in [I-D.ietf-idr-flowspec-redirect-ip].¶

2.2. Acronyms

• BGP FS: BGP Flow Specification [RFC8955][RFC8956][RFC9117]¶

• EC: Extended Community [RFC4360]¶

• LPM: Longest Prefix Match¶

• NLRI: Network Layer Reachability Information [RFC4271]¶

• SID: Segment Identifier [RFC8402]¶

• SR-MPLS: Segment Routing over MPLS data plane [RFC8660]¶

• SRH: Segment Routing Header [RFC8754]¶

• SRv6: Segment Routing over IPv6 data plane [RFC8754][RFC8986]¶

• USD: Ultimate Segment Decapsulation (Flavor defined in [RFC8986])¶

3.1. Operational Steering Modes

Traffic steering specified in this document leverages BGP FlowSpec filtering rules to steer flows into SR Policies [RFC9256].¶

FlowSpec-based steering corresponds to Policy-Based Routing (PBR) steering per Section 8.7 of [RFC9256] and does not perform per- destination steering (Section 8.4 of [RFC9256]). Implementations MUST NOT apply per-destination route resolution, BGP multipath path selection, or On-Demand BSID recursion [RFC9256] to FlowSpec routes.¶

A BGP FlowSpec sender MUST construct the BGP UPDATE message using one of two operational modes: Mode 1 (Transport Steering Only) or Mode 2 (Steering with Egress Service Action).¶

3.2. Procedures for the Sending BGP Speaker / Controller

A BGP speaker or controller originating or re-advertising a FlowSpec route to steer traffic into an SR Policy MUST perform the following:¶

1. Construct the explicit (Color, Endpoint) tuple by attaching the Color Extended Community [RFC9012] and setting the target IP address in the Redirect-to-IP Extended Community [I-D.ietf-idr-flowspec-redirect-ip].¶

2. For Mode 2 steering (Section 3.1.2), attach the BGP Prefix-SID Attribute carrying the SRv6 Service SID constructed per Section 3.6. For Mode 1 steering (Section 3.1.1), the BGP Prefix-SID Attribute MUST NOT be attached.¶

A BGP speaker re-advertising a FlowSpec route MUST preserve these steering attributes unless local policy explicitly modifies them.¶

3.3. Procedures for the Receiving BGP Speaker (Headend / Ingress PE)

The procedures specified in this section apply exclusively to BGP FlowSpec receivers acting as SR Headends. Intermediate BGP speakers (e.g., Route Reflectors) that do not perform data-plane steering MUST transparently reflect and propagate FlowSpec routes and their attached attributes per [RFC8955] without attempting SR Policy binding.¶

Upon receiving a FlowSpec route, the headend performs evaluation and forwarding programming strictly per the sequential logic in Section 4.1:¶

1. Validation and Mode Determination: Syntactic validation follows [RFC7606] and [RFC8955]. Co-existing steering attributes dictate whether the route is Mode 1 or Mode 2 per Section 4.1.¶

2. SR Policy Binding & Synthesis: The headend uses the extracted (Color, Endpoint) tuple to resolve an active SR Policy Candidate Path per Section 8.8.1 of [RFC9256].¶

   • For Mode 1, traffic is encapsulated using the active Segment List.¶

   • For Mode 2, the headend MUST synthesize the effective Segment List with the Egress Service SID extracted per [RFC9252] and Section 3.6 by applying one of the following rules:¶

     • Replace: If the final segment of the resolved Segment List is an Endpoint SID (e.g., End SID) belonging to the target Egress PE that matches the Locator prefix (or longest matching Locator mask) of the Egress Service SID, the headend MUST replace that final segment with the Egress Service SID.¶

     • Append: Otherwise (or if explicit path preservation is required), the headend MUST append the Egress Service SID as the final segment (innermost SID) of the Segment List.¶

• Default Longest Prefix Match (LPM) redirection [I-D.ietf-idr-flowspec-redirect-ip] MUST NOT be applied when an SR Policy is successfully resolved. Non-steering FlowSpec actions in the same route remain applicable.¶

3.

Control Plane Retention & Decoupling: If a matching active SR Policy cannot be resolved for a received FlowSpec route carrying an explicit steering intent, or if the resolved SR Policy subsequently transitions to Down state:¶

• Control Plane: The FlowSpec route MUST be retained in the BGP Loc-RIB as a valid route (unless rendered invalid by standard BGP path validation rules) and MUST continue to be advertised to downstream BGP peers according to standard BGP path selection and propagation rules. The headend MUST NOT implicitly invalidate or withdraw the BGP FlowSpec route solely due to underlay transport-layer resolution or SR Policy failures.¶

• Forwarding Plane: The associated SR steering forwarding state (e.g., Policy-Based Routing / PBR entry or ACL action) MUST NOT be activated in the forwarding plane (FIB/TCAM), or MUST be marked as Down/Inactive. Flow traffic matching the FlowSpec filtering rules MUST be handled according to the configured local failure policy (e.g., Drop as specified in Section 3.7 and Section 4.3), preventing silent fallback to default IP forwarding.¶

3.4. Interaction with Redirect-to-IP Extended Community

When a FlowSpec route carries both a Color Extended Community [RFC9012] and a Redirect-to-IP Extended Community [I-D.ietf-idr-flowspec-redirect-ip], the target IP address MUST be interpreted as the target SR Policy Endpoint to form the explicit (Color, Endpoint) tuple [RFC9256].¶

In accordance with Section 2.2.2 of [I-D.ietf-idr-flowspec-redirect-ip], SR Policy steering defined herein takes precedence over default LPM IP redirection. LPM lookup toward the target IP address MUST NOT be performed when an SR Policy is resolved. If multiple Color Extended Communities are present, color evaluation and fallback follow Section 8.4.1 of [RFC9256].¶

3.5. Scope Constraints

Steering procedures defined in this document strictly require an explicit (Color, Endpoint) tuple. Color-Only (CO) steering per Section 8.8.1 of [RFC9256] is outside the scope of this specification. If a received route contains non-zero CO bits in the Color Extended Community, the receiver MUST treat CO bits as zero and attempt explicit (Color, Endpoint) matching. If no matching SR Policy exists, steering failure procedures (Section 3.7) apply.¶

VPN FlowSpec address families (SAFI=134) and the Redirect-to-VRF Extended Community [RFC8955][RFC8956] are outside the scope of this document and MUST NOT be processed under these procedures.¶

3.6. BGP Prefix-SID Attribute Encoding Rules

The BGP Prefix-SID Attribute is used strictly for conveying SRv6 Service SIDs in SRv6 Mode 2 (Section 3.1.2) and MUST NOT be attached for SR-MPLS. Encoding rules per [RFC9252] and [RFC8986] apply:¶

• SRv6 Service TLVs: The SRv6 L3 Service TLV (Type 5) MUST be used, containing an SRv6 SID Information Sub-TLV (Type 1). The SRv6 SID Structure Sub-Sub-TLV (Type 1) MAY be included. SRv6 L2 Service TLVs (e.g., Type 6) MUST NOT be included for Layer 3 FlowSpec (SAFI 133) and MUST be ignored if received, treating the route as lacking an Egress Service SID.¶

• Endpoint Behaviors: Supported codepoints are restricted to Layer 3 behaviors (End.DT4, End.DT6, End.DT46, End.DX4, End.DX6) [RFC8986]. Layer 2 and transit behaviors MUST NOT be used.¶

• Disallowance of Transposition: Transposition Schemes [RFC9252] MUST NOT be used because FlowSpec NLRIs lack an MPLS Label field. The full SRv6 Service SID MUST be encoded entirely within the SRv6 SID Information Sub-TLV.¶

3.7. Steering Failure and Fallback Procedures

Steering failure scenarios are classified into two distinct operational cases: Attribute Incompleteness (Control Plane) and Policy Unavailability (Forwarding Plane).¶

3.8. Handling of the Redirect-to-IP C-bit

The Redirect-to-IP Extended Community [I-D.ietf-idr-flowspec-redirect-ip] includes a C-bit (Copy Bit) in its Local Administrator field:¶

• C-bit = 0 (Direct Redirection): Matching packets MUST be encapsulated and steered into the resolved SR Policy, bypassing normal IP routing.¶

• C-bit = 1 (Traffic Copying): The headend MUST duplicate matching packets. The original packet continues along normal IP routing without encapsulation, while the copy is encapsulated and steered into the resolved SR Policy per Section 3.1.¶

Setting C-bit = 1 does not alter SR Policy resolution logic; (Endpoint, Color) resolution and BGP Prefix-SID Attribute processing remain unchanged for the copied stream.¶

3.9. Load Balancing Considerations

When matching traffic is steered into an SR Policy with multiple active Segment Lists [RFC9256], the headend MUST distribute traffic across active Segment Lists according to their defined weights (ECMP/UCMP). FlowSpec filtering actions direct traffic into the policy as a single aggregate stream and MUST NOT alter Segment List selection.¶

For both SR-MPLS and SRv6, multipath hashing MUST use inner packet fields (e.g., 5-tuple). In SRv6 scenarios, the outer IPv6 Flow Label MUST NOT be overwritten by FlowSpec rules and SHOULD be computed from inner packet fields per Section 5.5 of [RFC8754] to prevent hash polarization. If the headend lacks line-rate capabilities to compute the IPv6 Flow Label from inner packet fields, the headend MAY set the Flow Label to zero or a fixed value, provided that entropy-based load balancing is handled by downstream SRv6 transit nodes or alternative path hashing mechanisms.¶

3.10. Backward Compatibility and Coexistence

A non-supporting BGP speaker that implements base FlowSpec [RFC8955][RFC8956] and Redirect-to-IP, but not this specification, processes updates under standard protocol rules:¶

• Attribute Pass-Through: As an optional transitive attribute, an unrecognized BGP Prefix-SID Attribute is propagated unmodified to peers per [RFC4271] and [RFC9252].¶

• Forwarding Fallback on non-supporting headends: A non-supporting headend ignores unrecognized BGP Prefix-SID Attributes [RFC4271] and falls back to default Redirect-to-IP behavior [I-D.ietf-idr-flowspec-redirect-ip].¶

To prevent unintended forwarding paths due to fallback, implementations MUST support administrative controls to enable or disable advertisement of SRv6 service steering attributes per neighbor and per service [RFC9252]. Operators SHOULD apply attribute filtering at domain boundaries per Section 3.2.1 of [RFC9252]. An exception to this SHOULD requirement occurs when multiple BGP domains (e.g., multiple ASes) reside within a single administrative control domain where end-to-end SRv6 policy propagation across domain boundaries is explicitly intended. In such controlled deployments, boundary filtering MAY be relaxed, provided that strict ingress filtering remains enforced at the untrusted edge of the single administrative domain.¶

4.1. Sequential Resolution Logic

The headend evaluates received attribute containers step-by-step:¶

Step 1: Redirect-to-IP Extended Community¶

• Absent: Procedures in this document do not apply. Process route per standard BGP FlowSpec [RFC8955][RFC8956].¶

• Present: Extract target IP as SR Policy Endpoint, process C-bit (Section 3.8), and proceed to Step 2.¶

Step 2: Color Extended Community¶

• Absent: Check for other explicit SR steering attributes.¶

  • If the route contains a BGP Prefix-SID Attribute, or contains an unparseable/malformed Color Extended Community, the steering intent for SR Policy selection is considered incomplete. The headend MUST NOT steer traffic into an SR Policy, MUST ignore the BGP Prefix-SID Attribute, and MUST process the matching traffic according to Section 4.3 (i.e., dropped or handled per local failure policy, rather than falling back to standard Redirect-to-IP forwarding).¶

  • If the route contains solely a valid Redirect-to-IP Extended Community and no SR-specific attributes (e.g., no BGP Prefix-SID Attribute), the route represents standard IP redirection. The headend MUST process the route per [I-D.ietf-idr-flowspec-redirect-ip] (out of scope for SR Policy steering defined in this document).¶

• Present: Extract Color value C to form the (Endpoint, Color) tuple, and proceed to Step 3.¶

Step 3: BGP Prefix-SID Attribute (PSID)¶

• Note: This step applies when steering into an SRv6 Policy (regardless of whether the Endpoint is encoded as an IPv4 or IPv6 address [RFC9256]).¶

• Absent (Mode 1): Resolve and steer traffic into the SR Policy corresponding to (Endpoint, Color) per Section 3.3.¶

• Present (Mode 2):¶

  • For SRv6 Steering: Extract the Egress Service SID from the PSID, resolve the SR Policy via (Endpoint, Color), and synthesize the active segment list per Section 3.3 and Section 3.6.¶

  • For SR-MPLS Steering: In accordance with Section 3.6, the BGP Prefix-SID Attribute MUST NOT be used for SR-MPLS steering. If the resolved (Endpoint, Color) tuple maps to an SR-MPLS Policy, the headend MUST ignore the BGP Prefix-SID Attribute, process the route as Mode 1 (Section 3.3), and SHOULD generate a diagnostic log entry.¶

4.2. Valid Attribute Combinations Summary

Table 1 summarizes all possible attribute combinations and their corresponding resolution modes after applying the sequential logic above.¶

Table 1: FlowSpec to SR Policy Steering Resolution

+-------------+----------+-----------+------------------------------+
| Redirect-   | Color EC | Prefix-SID| Steering Resolution Mode     |
| to-IP       |          |           |                              |
+-------------+----------+-----------+------------------------------+
| Present     | Present  | Absent    | Mode 1: Transport Steering   |
|             |          |           | (SR-MPLS / SRv6 USD)         |
+-------------+----------+-----------+------------------------------+
| Present     | Present  | Present   | Mode 2: Transport Steering + |
|             |          |           | Egress Service Action (SRv6) |
+-------------+----------+-----------+------------------------------+
| Present     | Invalid/ | Any*      | Incomplete Steering Intent:  |
|             | Missing* |           | Drop / Local Failure Policy  |
|             |          |           | (Section 3.7.1 / 4.3)        |
+-------------+----------+-----------+------------------------------+
| Present     | Absent   | Absent    | Out of Scope for SR Policy   |
|             |          |           | (Standard Redirect-to-IP)    |
+-------------+----------+-----------+------------------------------+
| Absent      | Ignored  | Ignored   | Out of Scope for SR Policy   |
|             |          |           | (Standard FlowSpec)          |
+-------------+----------+-----------+------------------------------+

* Note: "Invalid/Missing" refers to scenarios where explicit SR
  steering intent is present (e.g., carries a BGP Prefix-SID
  Attribute or a malformed/unparseable Color EC) but lacks a valid
  Color Extended Community. In such cases, the Prefix-SID state is
  marked as "Any" because steering intent is considered incomplete
  regardless of whether the BGP Prefix-SID Attribute is Present or
  Absent.

4.3. Protection Against Silent SLA Violations

To prevent unintentional redirection and SLA violations, implementations MUST strictly enforce the following rules:¶

1. Handling Invalid Steering Intent and Unreachable SIDs: If a route expresses SR steering intent (e.g., contains a Prefix-SID Attribute or a Color Extended Community) but required steering attributes (such as a valid Color EC) are invalid or incomplete, OR if the associated Egress Service SID (or its covering SRv6 Locator prefix) is unreachable in the local routing table, the receiver MUST NOT fall back to standard IP forwarding or Redirect-to-IP forwarding toward the target IP address. Matching traffic MUST be dropped or processed per local failure policy.¶

2. Enforcement of Non-Steering Actions: In all failure cases, other valid non-steering actions present in the FlowSpec route (e.g., Traffic-Rate/Drop [RFC8955], DSCP remarking) MUST continue to be enforced in the forwarding plane.¶

3. Multiple Redirect-to-IP Communities: When multiple valid Redirect-to-IP Extended Communities are attached alongside a Color Extended Community, load-sharing across the resulting target SR Policies MUST follow EC-level load-sharing defined in Section 2.2 of [I-D.ietf-idr-flowspec-redirect-ip].¶

4. Logging and Diagnostics: Implementations SHOULD generate diagnostic log entries whenever an SR Policy steering route contains corrupted or invalid steering attributes. An exception to this SHOULD requirement occurs during high-volume error events or flood conditions (e.g., malformed route injection attacks), where strict per-route logging could exhaust control plane CPU or storage resources. In such scenarios, implementations MAY rate-limit, aggregate, or temporarily suppress individual diagnostic log entries, provided that summary metrics or counter-based telemetry remain available to operators.¶

6.1. Cross-Table/VRF Traffic Injection and Service Hijacking

The BGP Prefix-SID Attribute in a FlowSpec route conveys an SRv6 Service SID (e.g., End.DT4/DT6/DT46, End.DX4/DX6 [RFC8986][RFC9252]) that determines the egress routing table (e.g., a specific VRF or the global table) or specific egress interface at the egress PE. A malicious or compromised speaker could steer traffic into an arbitrary remote VRF, the global routing context, or an unauthorized interface, causing unauthorized traffic injection, cross-table/VRF data leakage, or traffic interception.¶

To mitigate this, receiving PEs MUST validate that any SRv6 Service SID belongs to an authorized Locator range allocated for the target VRF, global table, or service context. BGP sessions with FlowSpec controllers MUST be secured using TCP-AO [RFC5925] and restricted to trusted entities.¶

6.2. Scope and Validation Limits of BGP FlowSpec Attributes

Standard BGP FlowSpec validation [RFC8955][RFC8956][RFC9117] validates the NLRI and Redirect-to-IP Extended Community against the origin of the destination prefix, but does not define validation for the Color Extended Community or BGP Prefix-SID Attribute. These unvalidated attributes dictate path selection and service contexts.¶

An attacker could alter or strip these attributes. Injecting an unauthorized Color EC could hijack traffic onto unintended SR paths. Conversely, stripping the Color EC from an SR-steering FlowSpec route causes an incomplete intent condition (Section 3.7.1), triggering drop behavior and resulting in a Denial of Service (DoS) for legitimate redirected traffic.¶

Headends MUST apply local policy checks to verify that the (Color, Endpoint) tuple resolves to an authorized SR Policy before binding traffic. Implementations MUST filter, strip, or reject unknown or unauthorized Color Extended Communities and BGP Prefix-SID Attributes at administrative boundaries. Implementations SHOULD generate log messages or management alarms when unexpected attribute stripping is detected. An exception to this SHOULD requirement occurs during high-volume route thrashing, rapid update sequences, or flood conditions, where logging rate-limiting, aggregation, or suppression MAY be applied to protect control plane stability.¶

6.3. SR Domain Leakage and Boundary Security

SRv6 SIDs and BGP Prefix-SID Attributes are intended strictly for use within a trusted SR domain [RFC8402][RFC8754][RFC8986][RFC9252]. Propagation across untrusted EBGP boundaries leaks internal SRv6 Locators and topology, exposing the domain to external steering attacks.¶

Autonomous System Border Routers (ASBRs) MUST filter or strip the BGP Prefix-SID Attribute and SRv6-related Extended Communities from BGP FlowSpec routes traversing administrative boundaries. An exception occurs when multiple BGP Autonomous Systems reside within a single administrative control domain with explicit inter-domain trust agreements, as specified in Section 3.10; in such controlled deployments, boundary filtering MAY be relaxed provided strict ingress filtering remains enforced at the untrusted domain edge.¶

6.4. Traffic Amplification and Path Pinning

A single FlowSpec route can match high-volume distributed flows across multiple Ingress PEs and redirect them to a single destination [RFC9117]. Binding redirected flows to an SR Policy allows an attacker to pin amplified traffic to a specific engineered path, potentially overloading transit nodes or egress interfaces.¶

Ingress PEs SHOULD combine FlowSpec redirect actions with Traffic Rate Extended Communities [RFC8955] to bound redirected bandwidth. An exception to this SHOULD requirement occurs when a FlowSpec route is deployed for legitimate traffic steering and rate limiting would cause unwanted packet drops, where adequate capacity planning guarantees sufficient bandwidth along the candidate path.¶

Implementations SHOULD monitor path utilization on SR Policies used for FlowSpec steering to detect and mitigate link congestion dynamically. An exception to this SHOULD requirement occurs in centralized control plane architectures where automated Controller-based TE or PCE monitoring dynamically adjusts policy pathing and prevents link congestion automatically.¶

7.1. Interop-test Status

The traffic steering mechanism using BGP FlowSpec with SR-MPLS and SRv6 Policies described in this specification has been implemented by multiple major networking vendors across hardware platforms, router operating systems, and SDN controllers.¶

These implementations successfully participated in multi-vendor joint interoperability testing events organized by China Mobile between July 2021 and October 2021. The following hardware devices, software versions, and controllers passed the interoperability suite (listed in alphabetical order by vendor):¶

Routers:
+---------+---------------+--------------------------------+
| Vendor  | Device Model  | Version                        |
+---------+---------------+--------------------------------+
| Huawei  | NE40E-X8A     | NE40E V800R021C00SPC091T       |
+---------+---------------+--------------------------------+
| New H3C | CR16010H-FA   | Version 7.1.075, ESS 8305      |
+---------+---------------+--------------------------------+
| Ruijie  | RG-N8010-R    | N8000-R_RGOS 12.8(1)B08T1      |
+---------+---------------+--------------------------------+
| ZTE     | M6000-8S Plus | V5.00.10(5.60.5)               |
+---------+---------------+--------------------------------+

Controllers:
+----------------+---------------+-------------------------+
| Vendor         | System Name   | Version                 |
+----------------+---------------+-------------------------+
| China Unitechs | I-T-E SC      | V1.3.6P3                |
+----------------+---------------+-------------------------+
| Huawei         | NCE-IP        | V100R021C00             |
+----------------+---------------+-------------------------+
| Ruijie         | RG-ONC-AIO-H  | RG-ION-WAN-CLOUD_2.00T1 |
+----------------+---------------+-------------------------+
| ZTE            | ZENIC ONE     | R22V16.21.20            |
+----------------+---------------+-------------------------+

7.2. Deployment Status

The mechanisms specified in this document have been natively deployed and in active production use on large-scale IP backbone networks, including China Mobile's production network, since August 2022.¶

In production deployments featuring pure SRv6 architectures, traffic steering driven by FlowSpec routes with Color Extended Communities and IPv6 Redirect addresses operates seamlessly over SRv6 Policies. In hybrid or multi-AS network environments where SR-MPLS and SRv6 coexist, policy selection for FlowSpec traffic steering adheres strictly to the steering framework defined in [RFC9256].¶

11.1. Normative References

[I-D.ietf-idr-flowspec-redirect-ip]

Haas, J., Henderickx, W., and A. Simpson, "BGP Flow-Spec Redirect-to-IP Action", Work in Progress, Internet-Draft, draft-ietf-idr-flowspec-redirect-ip-16, 21 May 2026, <https://datatracker.ietf.org/doc/html/draft-ietf-idr-flowspec-redirect-ip-16>.

[RFC2119]

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>.

[RFC4271]

Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A Border Gateway Protocol 4 (BGP-4)", RFC 4271, DOI 10.17487/RFC4271, January 2006, <https://www.rfc-editor.org/info/rfc4271>.

[RFC4360]

Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended Communities Attribute", RFC 4360, DOI 10.17487/RFC4360, February 2006, <https://www.rfc-editor.org/info/rfc4360>.

[RFC4760]

Bates, T., Chandra, R., Katz, D., and Y. Rekhter, "Multiprotocol Extensions for BGP-4", RFC 4760, DOI 10.17487/RFC4760, January 2007, <https://www.rfc-editor.org/info/rfc4760>.

[RFC7606]

Chen, E., Ed., Scudder, J., Ed., Mohapatra, P., and K. Patel, "Revised Error Handling for BGP UPDATE Messages", RFC 7606, DOI 10.17487/RFC7606, August 2015, <https://www.rfc-editor.org/info/rfc7606>.

[RFC8174]

Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC8402]

Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L., Decraene, B., Litkowski, S., and R. Shakir, "Segment Routing Architecture", RFC 8402, DOI 10.17487/RFC8402, July 2018, <https://www.rfc-editor.org/info/rfc8402>.

[RFC8660]

Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S., Decraene, B., Litkowski, S., and R. Shakir, "Segment Routing with the MPLS Data Plane", RFC 8660, DOI 10.17487/RFC8660, December 2019, <https://www.rfc-editor.org/info/rfc8660>.

[RFC8669]

Previdi, S., Filsfils, C., Lindem, A., Ed., Sreekantiah, A., and H. Gredler, "Segment Routing Prefix Segment Identifier Extensions for BGP", RFC 8669, DOI 10.17487/RFC8669, December 2019, <https://www.rfc-editor.org/info/rfc8669>.

[RFC8754]

Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J., Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header (SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020, <https://www.rfc-editor.org/info/rfc8754>.

[RFC8955]

Loibl, C., Hares, S., Raszuk, R., McPherson, D., and M. Bacher, "Dissemination of Flow Specification Rules", RFC 8955, DOI 10.17487/RFC8955, December 2020, <https://www.rfc-editor.org/info/rfc8955>.

[RFC8956]

Loibl, C., Ed., Raszuk, R., Ed., and S. Hares, Ed., "Dissemination of Flow Specification Rules for IPv6", RFC 8956, DOI 10.17487/RFC8956, December 2020, <https://www.rfc-editor.org/info/rfc8956>.

[RFC8986]

Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer, D., Matsushima, S., and Z. Li, "Segment Routing over IPv6 (SRv6) Network Programming", RFC 8986, DOI 10.17487/RFC8986, February 2021, <https://www.rfc-editor.org/info/rfc8986>.

[RFC9012]

Patel, K., Van de Velde, G., Sangli, S., and J. Scudder, "The BGP Tunnel Encapsulation Attribute", RFC 9012, DOI 10.17487/RFC9012, April 2021, <https://www.rfc-editor.org/info/rfc9012>.

[RFC9117]

Uttaro, J., Alcaide, J., Filsfils, C., Smith, D., and P. Mohapatra, "Revised Validation Procedure for BGP Flow Specifications", RFC 9117, DOI 10.17487/RFC9117, August 2021, <https://www.rfc-editor.org/info/rfc9117>.

[RFC9252]

Dawra, G., Ed., Talaulikar, K., Ed., Raszuk, R., Decraene, B., Zhuang, S., and J. Rabadan, "BGP Overlay Services Based on Segment Routing over IPv6 (SRv6)", RFC 9252, DOI 10.17487/RFC9252, July 2022, <https://www.rfc-editor.org/info/rfc9252>.

[RFC9256]

Filsfils, C., Talaulikar, K., Ed., Voyer, D., Bogdanov, A., and P. Mattes, "Segment Routing Policy Architecture", RFC 9256, DOI 10.17487/RFC9256, July 2022, <https://www.rfc-editor.org/info/rfc9256>.

11.2. Informative References

[RFC3411]

Harrington, D., Presuhn, R., and B. Wijnen, "An Architecture for Describing Simple Network Management Protocol (SNMP) Management Frameworks", STD 62, RFC 3411, DOI 10.17487/RFC3411, December 2002, <https://www.rfc-editor.org/info/rfc3411>.

[RFC5424]

Gerhards, R., "The Syslog Protocol", RFC 5424, DOI 10.17487/RFC5424, March 2009, <https://www.rfc-editor.org/info/rfc5424>.

[RFC5925]

Touch, J., Mankin, A., and R. Bonica, "The TCP Authentication Option", RFC 5925, DOI 10.17487/RFC5925, June 2010, <https://www.rfc-editor.org/info/rfc5925>.

[RFC7942]

Sheffer, Y. and A. Farrel, "Improving Awareness of Running Code: The Implementation Status Section", BCP 205, RFC 7942, DOI 10.17487/RFC7942, July 2016, <https://www.rfc-editor.org/info/rfc7942>.

[RFC8639]

Voit, E., Clemm, A., Gonzalez Prieto, A., Nilsen-Nygaard, E., and A. Tripathy, "Subscription to YANG Notifications", RFC 8639, DOI 10.17487/RFC8639, September 2019, <https://www.rfc-editor.org/info/rfc8639>.

[RFC9800]

Cheng, W., Ed., Filsfils, C., Li, Z., Decraene, B., and F. Clad, Ed., "Compressed SRv6 Segment List Encoding", RFC 9800, DOI 10.17487/RFC9800, June 2025, <https://www.rfc-editor.org/info/rfc9800>.

[RFC9830]

Previdi, S., Filsfils, C., Talaulikar, K., Ed., Mattes, P., and D. Jain, "Advertising Segment Routing Policies in BGP", RFC 9830, DOI 10.17487/RFC9830, September 2025, <https://www.rfc-editor.org/info/rfc9830>.

[RFC9862]

Koldychev, M., Sivabalan, S., Sidor, S., Barth, C., Peng, S., and H. Bidgoli, "Path Computation Element Communication Protocol (PCEP) Extensions for Segment Routing (SR) Policy Candidate Paths", RFC 9862, DOI 10.17487/RFC9862, October 2025, <https://www.rfc-editor.org/info/rfc9862>.

A.1. SR-MPLS Steering Example (Mode 1)

Figure 1 illustrates steering FlowSpec traffic into an SR-MPLS Policy without egress service actions (Section 3.1.1).¶

   +------------+
   |  BGP FS    |
   | Controller |
   +------------+
      | FlowSpec Route:
      |   NLRI: Filter Rules
      |   Redirect-to-IP: Tailend Address
      |   Color EC: C0
      |   (Prefix-SID: Absent)
      |          .-----.
      |         (       )
      V     .--(         )--.
+-------+  (                 )  +-------+
|       |_(  SR-MPLS Network  )_|       |
|Headend| ( ================> ) |Tailend|
+-------+  (SR List<S1,S2,S3>)  +-------+
            '--(         )--'
                (       )
                 '-----'

Control Plane: The BGP FS Controller advertises a FlowSpec route with matching rules, Redirect-to-IP (Tailend Address), and Color C0. The BGP Prefix-SID Attribute is omitted.¶

Headend Resolution: The headend resolves the SR-MPLS Policy matching the tuple (Endpoint: Tailend, Color: C0).¶

Encapsulation: Matching traffic is encapsulated into the policy's MPLS label stack:¶

+---------------------------------------------+
| Outer IP / Ethernet Header                  |
+---------------------------------------------+
| MPLS Label S1 (Top)                         |
| MPLS Label S2                               |
| MPLS Label S3 (Bottom with S=1)             |
+---------------------------------------------+
| Inner Payload (Original IPv4/IPv6 Packet)   |
+---------------------------------------------+

A.2. SRv6 Steering Examples

In SRv6 networks, FlowSpec traffic steering operates under either Mode 2 (Transport Steering with Egress Service Action) or Mode 1 (Transport Steering Only).¶

   +------------+
   |  BGP FS    |
   | Controller |
   +------------+
      | FlowSpec Route:
      |   NLRI: Filter Rules
      |   Redirect-to-IPv6: Tailend Address
      |   Color EC: C1
      |   Prefix-SID: Egress_Service_SID
      |          .-----.
      |         (       )
      V     .--(         )--.
+-------+  (                 )  +-------+
|       |_( SRv6 Core Network )_|       |
|Headend| ( ================> ) |Tailend|
+-------+  (SR List<S1,S2,S3>)  +-------+
            '--(         )--'   SRv6 SID: Egress_Service_SID
                (       )       (e.g., End.DT4, End.DT6, etc.)
                 '-----'

+-----------------------------------------------------+
| Outer IPv6 Header (DA = S1)                         |
+-----------------------------------------------------+
| SRH (Segments Left = 2 for Scenario A,              |
|      Segments Left = 3 for Scenario B)              |
|   Segment List: <S1, S2, Egress_Service_SID> (A)    |
|             OR: <S1, S2, S3, Egress_Service_SID> (B)|
+-----------------------------------------------------+
| Inner Payload (Original IPv4/IPv6 Packet)           |
+-----------------------------------------------------+

   +------------+
   |  BGP FS    |
   | Controller |
   +------------+
      | FlowSpec Route:
      |   NLRI: Filter Rules
      |   Redirect-to-IPv6: Tailend Address
      |   Color EC: C2
      |   (Prefix-SID: Absent)
      |          .-----.
      |         (       )
      V     .--(         )--.
+-------+  (                 )  +-------+
|       |_( SRv6 Core Network )_|       |
|Headend| ( ================> ) |Tailend|
+-------+  (SR List<S1,S2,S3>)  +-------+
            '--(         )--'
                (       )
                 '-----'
Note: S3 is an SRv6 SID with USD flavor on the tailend.