RB750GL and packets over 1500bytes

Hardware MikroTik hardware general
1

We have a couple of RB750GL’s running RouterOS 5.5 connected via ether1.

They will not pass packets greater than 1500bytes between them, even with an MTU of 1598 on eth1 they will only pass up to 1500byte packets :frowning: Oddly, RouterOS thinks the packet was sent successfully as there are no fragmentation errors, the packet just never leaves the port!


I have logged a ticket with Mikrotik, so hopefully someone gets on to it before anyone else encounters this problem.

2

Are you adjusting the l2-mtu parameter, or just mtu?

3

I adjusted both MTU and L2-MTU

I can straight swap with RB750G units and they work perfectly (albeit with 1524byte MTU limit)

I factory defaulted them, then only put IP addresses on eth1 and set the MTU and L2-MTU and even then, could only send up to 1500byte packets.

4

FYI this is now confirmed by Mikrotik support, and will be fixed in the next release.


Thanks Mikrotik.

5

Anyone at MikroTik have an idea when a new release will hit the download servers to address this? (5.6?) I’ve got several 750GLs installed at a particular site that I cannot use until this is fixed.

Wondering,
Aaron out.

6

email support to get the pre-release version, so you can verify the issue is fixed

7

Thank you. I will follow your suggestion.

Aaron out.

8

The 5.6 prerelease DOES fix the 1500 byte problem on the 750GL boxes I’m using.

I can run packets cleanly as large as 4074 bytes.

Thanks!

I do wonder why the l2mtu by default is 1998 instead of 4074 on the GL…

Aaron out.

9

Great, I cant wait to get the pile of 750GL’s off my desk and in to production.