Interface Flapping Due to Duplex Mismatch

INTERFACE FLAPPING

I was working on a branch router that kept dropping its uplink every few minutes. Users were complaining about VPN sessions cutting out and internet being slow. At first glance, the config looked fine and the interface was up. But the logs told another story: the link was flapping.

🔍 What I Saw

• show interfaces GigabitEthernet0/0 → lots of line protocol down/up messages.

• show logging → repeated %LINK-3-UPDOWN events.

• show controllers → CRC errors and late collisions showing up.

That’s when I started thinking it might be a duplex mismatch between the router and the switch.

✅ What Fixed It

1. Checked the switch port → it was set to auto-negotiation.

2. On the router, I forced speed and duplex:

   Code

   interface GigabitEthernet0/0
   speed 100
   duplex full
   

3. Matched the same settings on the switch side (speed 100, duplex full).

4. Monitored again with:

   Code

   show interfaces GigabitEthernet0/0 | include line protocol
   

After that, no more flapping. CRC errors dropped to zero, and users said the connection was stable again.

📌 Lesson Learned

This one was sneaky—it wasn’t a hard failure, just constant small drops that added up to bad performance. Easy to miss if you only look at configs. Always worth checking both ends of a link when you see flapping or CRC errors.