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4.9 Forward Error Correction (FEC)

Document Information

Field Value
Document ID SDWAN-POL-4.9
Version 1.0
Last Updated December 2025
Author Network Architecture Team
Classification Internal Use

1. Executive Summary

Forward Error Correction (FEC) and Packet Duplication are techniques used in Cisco Catalyst SD-WAN to mitigate packet loss on unreliable WAN links. This section defines Abhavtech's FEC strategy for protecting real-time and business-critical traffic.

1.1 FEC Architecture Overview

+------------------------------------------------------------------+
|                 FEC & PACKET DUPLICATION                          |
+------------------------------------------------------------------+
|                                                                   |
|  Forward Error Correction (FEC):                                  |
|  +----------------------------------------------------------+    |
|  | Original Packets:  [P1] [P2] [P3] [P4]                    |    |
|  | FEC Encoded:       [P1] [P2] [P3] [P4] [FEC1] [FEC2]      |    |
|  |                                                           |    |
|  | If P2 lost:        [P1] [X ] [P3] [P4] [FEC1] [FEC2]      |    |
|  | Recovered:         [P1] [P2] [P3] [P4] (reconstructed)    |    |
|  +----------------------------------------------------------+    |
|                                                                   |
|  Packet Duplication:                                              |
|  +----------------------------------------------------------+    |
|  | Original:    [P1]---> Path A --->[P1]                     |    |
|  |              [P1]---> Path B --->[P1]                     |    |
|  |                                                           |    |
|  | Receiver: First copy wins, duplicates discarded           |    |
|  +----------------------------------------------------------+    |
|                                                                   |
+------------------------------------------------------------------+

2. FEC Fundamentals

2.1 FEC vs Packet Duplication

Feature FEC Packet Duplication
Mechanism Add parity packets Send copies on multiple paths
Bandwidth Overhead 10-30% 100% (full duplication)
Loss Recovery Up to FEC threshold Any single path loss
Latency Impact Minimal Path difference latency
Use Case Moderate loss circuits Critical real-time traffic
Path Requirement Single path Multiple diverse paths

2.2 When to Use Each

Scenario Recommended Reason
Voice over Internet FEC Lower overhead
Video conferencing FEC Burst loss recovery
Executive calls Duplication Zero tolerance
Trading applications Duplication Mission-critical
General business FEC Cost-effective

3. FEC Configuration

3.1 FEC Policy Configuration

! App-Route Policy with FEC
policy
 app-route-policy _ABHAVTECH-FEC-POLICY_
  vpn-list VPN-10-EMPLOYEE
   sequence 10
    match
     app-list VOICE-APPS
    action
     sla-class REALTIME
     preferred-color mpls biz-internet
     loss-correction fec-always
    !
   !
   sequence 20
    match
     app-list VIDEO-APPS
    action
     sla-class REALTIME
     preferred-color mpls biz-internet
     loss-correction fec-adaptive
    !
   !
  !

3.2 FEC Modes

Mode Description Trigger Bandwidth
fec-always FEC always enabled Constant 10-30% overhead
fec-adaptive FEC on loss detection Loss > threshold Variable

3.3 FEC Parameters

! FEC algorithm selection
! Default: XOR-based, lightweight
! Advanced: Reed-Solomon for higher loss tolerance

! Configure FEC threshold for adaptive mode
policy
 app-route-policy _FEC-ADAPTIVE_
  vpn-list VPN-10-EMPLOYEE
   sequence 10
    match
     app-list VOICE-APPS
    action
     loss-correction fec-adaptive
     ! FEC activates when loss > 1%
    !
   !

4. Packet Duplication Configuration

4.1 Packet Duplication Policy

! App-Route Policy with Packet Duplication
policy
 app-route-policy _ABHAVTECH-DUPLICATION_
  vpn-list VPN-10-EMPLOYEE
   sequence 10
    match
     app-list EXECUTIVE-CALLS
    action
     sla-class REALTIME
     preferred-color mpls biz-internet
     loss-correction packet-duplication
    !
   !
  !

4.2 Duplication Requirements

+------------------------------------------------------------------+
|              PACKET DUPLICATION REQUIREMENTS                      |
+------------------------------------------------------------------+
|                                                                   |
|  Source WAN Edge                      Dest WAN Edge               |
|  +------------------+                +------------------+         |
|  |                  |    Path A      |                  |         |
|  | [P1] -----+------|----------------|---+              |         |
|  |           |      |    (MPLS)      |   +----> [P1]    |         |
|  |           +------|----------------|---+              |         |
|  |                  |    Path B      |   (First wins)   |         |
|  |                  |  (Internet)    |                  |         |
|  +------------------+                +------------------+         |
|                                                                   |
|  Requirements:                                                    |
|  - At least 2 diverse paths                                       |
|  - Both paths must meet basic connectivity                        |
|  - Receiver handles de-duplication                                |
+------------------------------------------------------------------+

5. Application-Specific FEC

5.1 Voice Traffic FEC

policy
 app-route-policy _VOICE-FEC_
  vpn-list VPN-10-EMPLOYEE
   sequence 10
    match
     app-list VOICE-APPS
    action
     sla-class REALTIME
     preferred-color mpls
     backup-sla-class REALTIME
     loss-correction fec-adaptive
    !
   !
  !
!
policy
 lists
  app-list VOICE-APPS
   app rtp audio
   app sip
   app cisco-phone
   app skinny
  !

5.2 Video Conferencing FEC

policy
 app-route-policy _VIDEO-FEC_
  vpn-list VPN-10-EMPLOYEE
   sequence 10
    match
     app-list VIDEO-APPS
    action
     sla-class REALTIME
     preferred-color mpls biz-internet
     loss-correction fec-always  ! Video needs constant FEC
    !
   !
  !
!
policy
 lists
  app-list VIDEO-APPS
   app webex-meeting
   app ms-teams
   app zoom
   app rtp video
  !

5.3 Critical Business Applications

policy
 app-route-policy _CRITICAL-APPS-FEC_
  vpn-list VPN-10-EMPLOYEE
   sequence 10
    match
     app-list TRADING-APPS
    action
     sla-class BUSINESS-CRITICAL
     preferred-color mpls
     loss-correction packet-duplication  ! Zero tolerance
    !
   !
   sequence 20
    match
     app-list DATABASE-REPLICATION
    action
     sla-class BUSINESS-CRITICAL
     preferred-color mpls
     loss-correction fec-always
    !
   !
  !

6. Per-VPN FEC Strategy

6.1 VPN 10 - Employee

Traffic Type FEC Mode Bandwidth Impact
Voice fec-adaptive ~15% when active
Video fec-always ~20% constant
Business Apps None 0%
General None 0%

6.2 VPN 40 - Voice

policy
 app-route-policy _VPN40-VOICE-FEC_
  vpn-list VPN-40-VOICE
   sequence 10
    match
     source-ip 0.0.0.0/0
    action
     sla-class REALTIME strict
     preferred-color mpls
     loss-correction fec-adaptive
    !
   !
  !

6.3 VPN 100 - PCI

! PCI uses packet duplication for critical transactions
policy
 app-route-policy _VPN100-PCI-FEC_
  vpn-list VPN-100-PCI
   sequence 10
    match
     destination-port 443 1433
    action
     sla-class BUSINESS-CRITICAL
     preferred-color mpls
     loss-correction packet-duplication
    !
   !
  !

7. FEC Bandwidth Planning

7.1 Bandwidth Overhead Calculation

FEC Mode Overhead Example (100 Mbps base)
None 0% 100 Mbps
fec-adaptive (inactive) 0% 100 Mbps
fec-adaptive (active) 15-25% 115-125 Mbps
fec-always 20-30% 120-130 Mbps
packet-duplication 100% 200 Mbps

7.2 Abhavtech Bandwidth Planning

Site Base BW Voice FEC Video FEC Total Needed
Mumbai Hub 1 Gbps 20 Mbps 100 Mbps 1.12 Gbps
Chennai Hub 500 Mbps 15 Mbps 75 Mbps 590 Mbps
Bangalore 200 Mbps 10 Mbps 30 Mbps 240 Mbps
Remote Sites 100 Mbps 5 Mbps 15 Mbps 120 Mbps

8. Monitoring and Verification

8.1 Verification Commands

! Check FEC status
show sdwan app-route fec

! View FEC statistics
show sdwan app-route statistics
show sdwan app-route fec statistics

! Check packet duplication status
show sdwan app-route packet-duplication

! View per-tunnel FEC counters
show sdwan bfd sessions
show sdwan tunnel statistics

! Sample output:
! TUNNEL            FEC-TX    FEC-RX    RECOVERED
! MPLS-Mumbai       12,345    10,234    1,234
! Internet-Mumbai   15,678    14,567    2,345

8.2 FEC Metrics

Metric Description Alert Threshold
FEC TX Packets Parity packets sent Info only
FEC RX Packets Parity packets received Info only
Recovered Packets Packets reconstructed >5% of total
Duplication TX Duplicate packets sent Info only
De-dup Discards Duplicates discarded Info only

9. Best Practices

9.1 FEC Guidelines

Guideline Recommendation
Use Adaptive For most scenarios
Use Always For constant loss circuits
Use Duplication Only for zero-tolerance apps
Monitor Overhead Track bandwidth consumption
Tune Thresholds Adjust based on baseline loss

9.2 Performance Considerations

Consideration Impact Mitigation
CPU Usage FEC encoding overhead Use hardware acceleration
Bandwidth 10-100% overhead Plan capacity accordingly
Latency Slight increase for FEC Minimal, acceptable for RT

10. Summary

Element Abhavtech Configuration
Voice FEC Adaptive mode
Video FEC Always mode
Critical Apps Packet duplication
VPN 40 (Voice) Adaptive FEC
VPN 100 (PCI) Packet duplication
Bandwidth Planning 20% headroom for FEC

Document Version: 1.0 Last Updated: December 2025 Next Review: March 2026