1. Purpose
To prevent frequent automatic switching between Primary Wired WAN and LTE backup in dual-uplink routers by stabilizing failover detection and reducing unnecessary link transitions.
The objective is to ensure LTE activates only during genuine wired outages, not during minor fluctuations.
2. Scope
Applies to branch routers and gateways configured with:
Primary ISP via Ethernet (Wired WAN)
Secondary ISP via LTE (4G/5G SIM-based)
Automatic failover enabled
3. Background
Dual-uplink routers continuously monitor the primary connection using:
Connectivity tests (ping or gateway reachability)
Packet loss detection
Link state monitoring
If the router is configured to immediately revert back to primary WAN once it responds again, the device may continuously switch between uplinks — creating instability.
This behavior is commonly referred to as failover oscillation.
4. Problem Description
The router alternates between:
Primary Wired WAN
LTE Backup
Even though the primary circuit is not fully down.
Switching may occur every few seconds or minutes.
5. Business Impact
Frequent uplink switching can cause:
Interrupted POS transactions
Web session logouts
Payment gateway resets
Cloud application re-authentication
These issues are especially disruptive during peak hours.
6. Common Root Causes
Failover Detection Too Aggressive
Short detection interval
Very low failure count threshold
No stabilization delay before failback
Borderline LTE Signal
Weak or fluctuating signal
LTE tower congestion
Router installed in enclosed rack
Immediate Failback Enabled
Router switches back to primary WAN instantly once it detects minimal recovery.
7. Detailed Troubleshooting Procedure
Step 1: Review Event Logs
Check system event logs:
WAN down / WAN up entries
LTE Uplink UP / LTE uplink Down messages
Switchover Events between Wan and LTE uplinks
If events are occurring repeatedly within short intervals, oscillation is confirmed.
Step 2: Observe Timing Pattern
Identify:
How often switching occurs
Whether switching correlates with business peak hours
Whether it aligns with ISP modem restarts
Frequent transitions within 1–2 minutes usually indicate threshold misconfiguration.
Step 3: Validate Primary ISP Stability
Ask site to:
Confirm modem power stability
Replace WAN patch cable
Ensure no loose Ethernet connections
If primary link briefly drops physical connectivity, failover will trigger correctly — but repeatedly.
Step 4: Check LTE Signal Quality
From router status page, review:
Signal strength indicator (Check the Uplink info to get Signal quality dbm)
LTE network type (LTE/4G)
Connection uptime
If signal is weak or fluctuating significantly, LTE may briefly disconnect, causing reverse switching.
8. Resolution Scenarios
Scenario A: Improve LTE Stability
Reposition router for better signal
Install external antenna
Avoid metal cabinets
Ensure SIM is active and stable
Scenario B: Stabilize Primary ISP
Replace ISP modem if unstable
Confirm no power fluctuation
Change Cable in case faulty o lose
Escalate to ISP if latency spikes exceed SLA
Scenario C: Change Failover and Failback Time
Change Failover Time (Increase failover check to monitor longer before performing switchover to Secondary Uplink)
Change Failback Time (Increase failback check to monitor longer before performing switchover to Primary Uplink)
9. Validation After Resolution
Confirm:
Router remains on primary WAN under normal conditions
LTE activates only during actual cable disconnect or WAN uplink down
No repeated switching in logs
Stable application sessions during peak usage
Monitor for at least one full business day.
10. Preventive Measures
Use conservative failover detection values
Always configure failback stabilization timer
Perform quarterly LTE signal review
Periodically test failover manually
Maintain ISP modem health checks
11. Escalation Guidelines
Escalate to ISP if:
Frequent micro-outages observed
Latency and packet loss exceed agreed SLA
Modem logs show recurring drops
Escalate to Network Support if:
Router firmware suspected unstable
LTE registration frequently drops
Provide:
Event log screenshots
Timing of failover occurrences
Current failover configuration
LTE signal readings