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RFC 9871 - BGP Color-Aware Routing (CAR)

Document Information​

  • RFC Number: 9871
  • Title: BGP Color-Aware Routing (CAR)
  • Publication Date: November 2025
  • Status: EXPERIMENTAL
  • Authors:
    • D. Rao, Ed. (Cisco Systems)
    • S. Agrawal, Ed. (Cisco Systems)

Abstract​

This document describes a BGP-based routing solution to establish end-to-end intent-aware paths across a multi-domain transport network. The transport network can span multiple service provider and customer network domains. The BGP intent-aware paths can be used to steer traffic flows for service routes that need a specific intent. This solution is called BGP Color-Aware Routing (BGP CAR).

This document describes the routing framework and BGP extensions to enable intent-aware routing using the BGP CAR solution. The solution defines two new BGP SAFIs (BGP CAR SAFI and BGP VPN CAR SAFI) for IPv4 and IPv6. It also defines an extensible Network Layer Reachability Information (NLRI) model for both SAFIs that allows multiple NLRI types to be defined for different use cases. Each type of NLRI contains key and TLV-based non-key fields for efficient encoding of different per-prefix information. This specification defines two NLRI types: Color-Aware Route NLRI and IP Prefix NLRI. It defines non-key TLV types for the MPLS label stack, SR-MPLS label index, and Segment Routing over IPv6 (SRv6) Segment Identifiers (SIDs). This solution also defines a new Local Color Mapping (LCM) Extended Community.

What is Color-Aware Routing?​

Core Concepts​

Color (Intent):

  • A 32-bit integer value identifying a specific service level or traffic engineering policy
  • Example: Color 100 = "Low-Latency Path", Color 200 = "High-Bandwidth Path"
  • Unlike traditional "destination-only" routing, CAR considers both destination and color

Color-Aware Routing:

Traditional BGP Routing:
  Destination: 10.1.1.0/24 → Select best path

CAR Routing:
  Destination: 10.1.1.0/24 + Color: 100 → Select best path meeting Color 100 requirements
  Destination: 10.1.1.0/24 + Color: 200 → Select best path meeting Color 200 requirements

Use Cases​

Use Case 1: Service Differentiation​

Enterprise VPN Service:
- VIP customer traffic → Color 100 (Low-latency, High-reliability)
- Standard customer traffic → Color 200 (Best-effort)
- Backup traffic → Color 300 (Low-priority)

Use Case 2: Application Optimization​

Cloud Service Provider:
- Real-time video → Color 100 (Jitter-sensitive)
- File transfer → Color 200 (Bandwidth-priority)
- Bulk data → Color 300 (Cost-optimized)

Use Case 3: Multi-Cloud Interconnect​

Cross-cloud connectivity:
- Mission-critical apps → Color 100 (Dedicated circuit, Low-latency)
- General apps → Color 200 (Internet, Cost-optimized)

Technical Architecture​

BGP Extensions​

New BGP Attribute: Color Extended Community​

Type: Extended Community
Format: 
  Type: 0x0b (Color)
  Sub-Type: 0x0b
  Color Value: 32-bit integer

Examples:
  Color:100
  Color:200

BGP UPDATE Message​

Standard BGP UPDATE:
  NLRI: 10.1.1.0/24
  Next-Hop: 203.0.113.1
  AS-Path: 65001 65002
  
CAR BGP UPDATE:
  NLRI: 10.1.1.0/24
  Next-Hop: 203.0.113.1
  AS-Path: 65001 65002
  Extended Community: Color:100  ← New

Routing Process​

Step 1: Route Table Organization​

Traditional Route Table:
  Destination     Next-Hop
  10.1.1.0/24 → 203.0.113.1

CAR Route Table:
  Destination     Color   Next-Hop
  10.1.1.0/24 + None  → 203.0.113.1 (Default)
  10.1.1.0/24 + 100   → 203.0.113.2 (Low-latency)
  10.1.1.0/24 + 200   → 203.0.113.3 (High-bandwidth)

Step 2: Packet Classification​

Ingress Router:
1. Inspect packet (based on policy)
2. Assign color value
3. Lookup color-aware route table
4. Forward to appropriate next-hop

Step 3: Path Mapping​

Color mapping to physical paths:
Color 100 → SR Policy 1 → Via low-latency links
Color 200 → SR Policy 2 → Via high-bandwidth links
Color 300 → SR Policy 3 → Via cost-optimized path

Integration with Segment Routing (SR)​

SR Policy Binding​

Configuration example:
  sr-policy color 100
    candidate-path preference 100
      explicit segment-list path1
        segment 16001
        segment 16002
        segment 16003

End-to-End Flow​

1. BGP Advertisement: Prefix + Color:100
2. Controller/PCE: Compute SR Policy satisfying Color:100
3. Ingress PE: Map traffic to SR Policy
4. Data Plane: Forward along SR Policy
5. Egress PE: Decapsulate and deliver

Configuration Examples​

Basic Configuration​

Advertising Routes with Color​

router bgp 65001
  neighbor 203.0.113.1 remote-as 65002
  
  address-family ipv4 unicast
    network 10.1.1.0/24
    route-policy ADD-COLOR out
    
route-policy ADD-COLOR
  set extcommunity color 100
end-policy

Receiving and Processing Color​

router bgp 65001
  address-family ipv4 unicast
    neighbor 203.0.113.2 remote-as 65002
    route-policy MATCH-COLOR in
    
route-policy MATCH-COLOR
  if extcommunity color is 100 then
    set local-preference 200
  elseif extcommunity color is 200 then
    set local-preference 100
  endif
end-policy

Advanced Applications​

Application-Based Color Mapping​

class-map match-any REALTIME
  match protocol http
  match dscp ef
  
policy-map COLOR-MAPPING
  class REALTIME
    set extcommunity color 100
  class BULK-DATA
    set extcommunity color 300
  class class-default
    set extcommunity color 200

Multi-Color Redundancy​

router bgp 65001
  address-family ipv4 unicast
    network 10.1.1.0/24
    route-policy MULTI-COLOR out

route-policy MULTI-COLOR
  set extcommunity color 100 additive
  set extcommunity color 200 additive
end-policy

# Result: Route carries both Color 100 and 200
# Allows receiver to choose appropriate color

Deployment Considerations​

Incremental Deployment​

Phase 1: Core Network​

1. Enable CAR on core routers
2. Define color policies
3. Configure SR Policies (if using SR)
4. Test color routing functionality

Phase 2: Edge Expansion​

5. Enable CAR on PE routers
6. Configure traffic classification
7. Map customer traffic to colors
8. Monitor and optimize

Phase 3: Network-Wide Coverage​

9. All BGP speakers support CAR
10. Refine color policies
11. Integrate with business systems
12. Automation and orchestration

Interoperability​

Non-CAR Routers:

  • Will ignore Color Extended Community
  • Process routes using traditional BGP
  • Does not affect basic connectivity

Mixed Environment:

CAR Domain 1 ← → Traditional BGP ← → CAR Domain 2
              Ignore color        Re-add color

Scalability​

Route Table Size:

Without CAR: N prefixes
With CAR: N prefixes × M colors

Example:
- 100K prefixes
- 10 colors
- Maximum: 1M routes (theoretical)
- Actual: Depends on color usage

Optimization Recommendations:

  • Limit number of colors (recommended < 100)
  • Use color aggregation
  • Apply colors only to prefixes requiring differentiation
  • Deploy Route Reflectors (RR)

Advantages and Limitations​

Advantages​

  1. Flexible Traffic Engineering

    • Path selection based on business requirements
    • Fine-grained QoS control

  2. Simplified Operations

    • Unified color semantics
    • Centralized policy management

  3. Multi-Tenancy Support

    • Different tenants use different colors
    • Isolation and differentiation

  4. Automation-Friendly

    • Easy integration with SDN controllers
    • Support for dynamic policy adjustment

Limitations​

  1. Requires Network-Wide Support

    • At least critical path devices need CAR support
    • Incremental deployment may be complex

  2. Additional Routing Overhead

    • More route entries
    • Larger BGP updates

  3. Configuration Complexity

    • Requires careful color scheme design
    • Policy management complexity

  4. Debugging Challenges

    • Multiple paths increase troubleshooting difficulty
    • Requires new monitoring tools

Monitoring and Troubleshooting​

Verification Commands​

# View routes with color
show bgp ipv4 unicast 10.1.1.0/24 detail
  Extended Community: Color:100

# View color statistics
show bgp color-aware-routing statistics
  Color 100: 150 routes
  Color 200: 200 routes

# Trace color path
traceroute 10.1.1.1 color 100

Common Issues​

Issue 1: Color Loss

Symptom: Color not preserved after route advertisement
Cause: Intermediate router doesn't support CAR or filters Extended Community
Solution: Check BGP neighbor config, enable Extended Community propagation

Issue 2: Suboptimal Path

Symptom: Traffic not using expected color path
Cause: BGP path selection or policy issues
Solution: Check local-preference, AS-Path and other BGP attributes

References​

Related RFCs:

  • [RFC4360] BGP Extended Communities
  • [RFC8664] Path Computation Element (PCE) Communication Protocol (PCEP) Extensions for Segment Routing
  • [RFC9256] Segment Routing Policy Architecture

Summary: BGP Color-Aware Routing provides powerful traffic engineering and service differentiation capabilities, enabling complex multi-path management through simple color identifiers. It is a key technology for modern SDN and 5G networks.

Official Document: ````https://www.rfc-editor.org/rfc/rfc9871.txt\````