Title: [Solved] Multi-WAN Failover Not Working - Root Cause: Same Gateway on Both Interfaces
Device: GL-BE3600 (Slate 7)
Firmware: 4.8.1
OpenWrt: 23.05-SNAPSHOT
Problem Description
Failover between Ethernet (Powerline) and Repeater (Wi-Fi) was not working.
When Powerline connection lost internet, router did NOT switch to Repeater automatically.
My Setup
- Primary WAN: Ethernet via Powerline adapter → ONT (ISP router)
- Backup WAN: Repeater connected to same ONT's Wi-Fi
- Goal: Automatic failover when Powerline fails
Symptoms
- Both interfaces had IP addresses and could ping internet
- Manual route deletion worked - traffic switched to Repeater
- But kmwan never triggered automatic failover
- In logs: kmwan only monitored "wwan" (Repeater), completely ignored "wan" (Ethernet)
Investigation
Spent hours checking:
- Firewall zones ✓ (wwan was in WAN zone)
- Interface Status Track ✓ (enabled with ping to 1.1.1.1, 8.8.8.8)
- kmwan config ✓ (track_mode='force' for both interfaces)
- Metrics ✓ (wan=1, wwan=2)
Test: Blocked ICMP on eth0 for 30 seconds - kmwan did NOT remove the route.
Root Cause Found
Both interfaces connected to SAME gateway:
- eth0 (Powerline): 192.168.5.54 → gateway 192.168.5.1
- sta0 (Repeater): 192.168.5.52 → gateway 192.168.5.1
This is architectural limitation of kmwan, not a bug.
kmwan detects duplicate gateways and monitors only ONE interface to avoid:
- Routing conflicts
- ARP flux issues
- Race conditions
When kmwan pings 8.8.8.8 to check "wan" health, the packet can go through
"wwan" (same gateway), so monitoring gives false positives.
Why This Happens
Common scenario: You want backup path to same ISP router via different
physical medium (Ethernet vs Wi-Fi). Seems logical, but creates
"Same Subnet / Same Gateway" situation that Multi-WAN cannot handle properly.
Solution
For real failover you need DIFFERENT internet sources:
- ISP router (primary)
- Mobile hotspot / USB modem (backup)
- Neighbor's Wi-Fi (backup)
- Different ISP (backup)
Different sources = different gateways = different subnets = kmwan works correctly.
What Does NOT Work
- Ethernet + Repeater to same router
- Two Ethernet ports to same switch/router
- Any combination where gateway IP is identical
Lesson Learned
Before configuring Multi-WAN failover, check: do your WAN interfaces have
DIFFERENT gateway IPs? If not, failover will not work regardless of settings.
Hope this saves someone hours of debugging!