Strange effect with routing MTU/Packet Sniffer

RouterOS Forwarding Protocols
1

I am tracking down a Problem where I could send only Packets of size 1486 to a router. Using another IP of this same router I could send 1500.

I tried to sniff at the router one hop before to see what is happening. From this router I can send 1500 byte packets to both IPs. As soon as I enable the Packet Sniffer (regardless which interface) the problem disappears. So problem seems to be the router before.

What does Packet sniffer change to the system which could solve this problem? Am I chasing a misconfig or a ROS-Bug? Version is 6.37.5.

2

Hi.
Running the sniffer will disable fast path and fasttrack. If you have fp/ft enabled, then is a thing to look.

3

In IP Setting fastpath/fasttrack counters are “0”.

4

Updated to 6.40.9. Problem stays.