Troubleshooting Steps for SD-WAN Best Path Verification

1. Objective

This document provides a structured procedure to verify SD-WAN Best Path behavior. The goal is to confirm that the SD-WAN engine detects WAN degradation, recalculates WAN scores, selects the correct WAN interface, and updates the forwarding path dynamically based on SLA evaluation.

2. Verification Flow

• First verify the active route from the client side.
• Check the SD-WAN device Best Path logs in real time.
• Confirm which WAN interface is selected as the current winner.
• Run traceroute to validate the actual forwarding path.
• Compare log output and traceroute result to confirm Best Path switching behavior.

3. Client-Side Route Verification

Before analyzing SD-WAN Best Path behavior, confirm that the client is forwarding traffic through the expected gateway or WAN path.

• 3.1 Windows Client

Use the following command to verify the active route:

         route print

Verify the default route, gateway IP address, interface metric, and active interface used for internet traffic.

• 3.2 Linux Client

Use the following command to check routing information:

           ip r

Verify that the default route and gateway are pointing toward the expected SD-WAN path.

 

4. SD-WAN Device Log Verification

Before monitoring the logs, first enable the required Detailed Best Path Logs option on the SD-WAN device.

Reference Image:

Ray Device – Best Path Selection Rules

This section explains how the Ray device compares WAN link quality and decides when traffic should be switched from the current WAN path to a better available path.

1. Minimum Improvement to Switch

Current Value: 0.1

This value defines how much better the challenger WAN link must be compared to the current active WAN link before the Ray device performs a path switch.

The WAN score range is generally from -1 to +1. In this range, a value of 0.1 is already considered a strong margin.

This prevents unnecessary switching when two WAN links have almost similar quality.

2. Reaction Speed

Current Value: 1

Reaction Speed controls how quickly the Ray device reacts to WAN quality changes.

A higher value makes the device react faster when link quality changes. A lower value makes the system calmer and more stable.

With the value set to 1, the system will react normally and avoid overly aggressive switching.

3. Jitter Priority

Current Value: 1

Jitter Priority defines how much influence jitter has during WAN score calculation.

Jitter means variation in packet delay. It is important for real-time traffic such as voice, video calls, and online meetings.

A value of 0 disables jitter from scoring. A higher value gives jitter more importance in the best path decision.

4. Detailed Best Path Logs

Current Value: Enabled

This option enables detailed logs for best path decision-making.

When enabled, the Ray device records detailed information about WAN score calculation, path comparison, and switching decisions.

This is useful for troubleshooting, analysis, and understanding why the device selected or changed a WAN path.

5. Success Cycles

Current Value: 3

Success Cycles defines how many consecutive cycles the challenger WAN link must remain better before the Ray device switches traffic to it.

With the value set to 3, the challenger WAN link must stay better for 3 continuous cycles before traffic is moved.

This helps avoid unnecessary switching caused by temporary link improvement.

6. Switch Cooldown

Current Value: 3

Switch Cooldown defines how many cycles the Ray device should wait after performing a switch before considering another voluntary switch.

With the value set to 3, the device waits for 3 cycles after switching before making another normal path change.

This prevents frequent path flapping. However, forced failover can still bypass this cooldown if the active link fails.

7. Latency Priority

Current Value: 1

Latency Priority defines how much influence latency has in WAN score calculation.

Latency means network delay. Lower latency is better for real-time and business-critical traffic.

A value of 0 disables latency from scoring. A higher value gives latency more importance in selecting the best WAN path.

8. Packet Loss Priority

Current Value: 100

Packet Loss Priority defines how much influence packet loss has in WAN score calculation.

The current value is 100, which means packet loss has very high importance in best path selection.

This is useful because packet loss directly affects application performance, voice/video quality, and tunnel stability. If a WAN link has packet loss, the Ray device will strongly prefer another better path.

How It Works

The Ray device continuously monitors all available WAN links and calculates a quality score for each link based on configured SLA parameters such as latency, jitter, packet loss, and other priorities.

After calculating the scores, the device compares the current active WAN link with the challenger WAN link. If the challenger link is better by at least the configured Minimum Improvement to Switch value, and it remains better for the configured Success Cycles, the Ray device selects it as the new best path.

After switching, the Switch Cooldown timer prevents frequent path changes. This keeps traffic stable and avoids unnecessary WAN flapping.

After enabling the option and confirming the client-side routing path, monitor the Best Path logs on the SD-WAN device using the following command:

logread -f -e best path

This command filters Best Path-related events and displays interface score, winner selection, and path change decisions.

 

5. Observed Best Path Log Evidence

The following captured logs show SD-WAN Best Path evaluation between eth0 and eth3. The logs include SLA values, WAN scores, winner selection, and no-change decision logic.