MTU Troubleshooting on Cisco IOS

Hello Samir

Thanks for the details. This is an interesting one. I recreated the topology you suggest and I have been able to replicate your results. But it’s not immediately perceivable why we get this behavior. So let’s think this through.

When you ping using a size of 1400, what you capture using Wireshark is a frame size on the wire of 1414. So the actual Layer 3 PDU, including IP and ICMP headers is 1400, and the header of the Ethernet frame is 14 for a total of 1414 egressing the host.

Those 1414 bytes of frame reach the interface of the switch, which is set up to have an interface MTU of 1400. But it allows a frame of size up to 1424 on the wire to enter. So the Interface MTU must omit some of the headers of a total size of 24 bytes. Let’s examine what headers are added to an ICMP ping.

Looking at a Wireshark capture of a ping of size 1410, I see the following:

  • ICMP payload: 1382 bytes
  • ICMP header 8 bytes
  • IP header 20 bytes
  • Ethernet header 14 bytes

All of that equals 1424 bytes. So when H1 pings a size of 1410 bytes, it’s really 1382+8+20 = 1410. Plus the 14 bytes of the Ethernet makes it 1424 on the wire.

Now when the switch’s interface with an MTU of 1400 receives this, somehow, it does not consider 10 extra bytes as part of that MTU, and lets them through… To be honest, I don’t know. I’ll have to think about this some more, and share it with Rene to see if we can shed some light on it. Thanks for your patience.