CCR2004 – Short downtime when changing L2MTU on unused interfaces

RouterOS
1

Hello everyone,

I’ve observed an unexpected behavior on multiple CCR2004 devices running RouterOS v7.

During test, I changed the L2MTU on a few ports. What surprised me is that even when the affected ports were:

  • not in a running state,
  • not passing traffic, and
  • not part of any bridge, bonding, or the active routing path,

the router still experienced a short downtime of about 20-25 seconds.

This happened consistently on several different routers. I expected the change to only affect the modified interface, not the whole device.

My questions are:

  • Why does changing the L2MTU on unused or non-critical interfaces cause the device to briefly go down?
  • Is this expected behavior (e.g., because of internal driver re-initialization or hardware reset)?
  • Has anyone else experienced this in RouterOS v7.x, especially on CCR2004?

Thanks

2

When changing L2MTU, the internal memory table must be recalculated, hence the interface reset.

3

It's even in Help pages...

L2MTU configuration changes evoke all interface reloads (link down/link up) due to necessary internal processes.
It is recommended to configure L2MTU with caution by keeping in mind that it can cause short interruption with connected devices.