Cannot set RB1100 MTU above 1600 bytes

Hardware MikroTik hardware general
1

Not sure if this is a hardware, software or usererror.

I have tried on RouterOS 4.10, 5b3 and 5b4.

I have two RB1100 and I cannot set MTU >1600:

[admin@m2] > /interface print
Flags: D - dynamic, X - disabled, R - running, S - slave
 #     NAME                                         TYPE             MTU   L2MTU
 0  R  ether12                                      ether            1500  1600
 1  R  ether13                                      ether            1500  1600
 2     ether11                                      ether            1500  1600
 3     ether6                                       ether            1500  1598
 4     ether7                                       ether            1500  1598
 5     ether8                                       ether            1500  1598
 6     ether9                                       ether            1500  1598
 7     ether10                                      ether            1500  1598
 8  R  ether1                                       ether            1500  1598
 9     ether2                                       ether            1500  1598
10     ether3                                       ether            1500  1598
11     ether4                                       ether            1500  1598
12     ether5                                       ether            1500  1598
[admin@m2] > /interface set mtu=9192 num=1
could not set mtu
[admin@m2] > /interface ethernet set mtu=9192 num=1
could not set mtu
[admin@m2] > /interface ethernet set l2mtu=9192 num=1
failed to update l2mtu
[admin@m2] > /interface set mtu=1600 num=1
[admin@m2] > /interface print
Flags: D - dynamic, X - disabled, R - running, S - slave
 #     NAME                                         TYPE             MTU   L2MTU
 0  R  ether12                                      ether            1500  1600
 1  R  ether13                                      ether            1600  1600
 2     ether11                                      ether            1500  1600
 3     ether6                                       ether            1500  1598
 4     ether7                                       ether            1500  1598
 5     ether8                                       ether            1500  1598
 6     ether9                                       ether            1500  1598
 7     ether10                                      ether            1500  1598
 8  R  ether1                                       ether            1500  1598
 9     ether2                                       ether            1500  1598
10     ether3                                       ether            1500  1598
11     ether4                                       ether            1500  1598
12     ether5                                       ether            1500  1598

The L2MTU indicates the largest possible frame to be transmitted by the inteface, right?
http://wiki.mikrotik.com/wiki/Manual:Maximum_Transmission_Unit_on_RouterBoards says the RB1100 should support up to 9498.

2 
[admin@RB1100test] /interface ethernet> set ether2 l2mtu=9192
[admin@RB1100test] /interface ethernet> set ether2 mtu=9192

mtu can not be larger than l2mtu

3

Ahh, the syntax is different. This was confusing me.

[admin@m2] > /interface ethernet set numbers=1 l2mtu=9192
failed to update l2mtu
[admin@m2] > /interface ethernet set ether2 l2mtu=9192   
[admin@m2] >

Thank you for your fast response!

This is unintuitive though, because all other settings in RouterOS are done via the ‘numbers=#’ syntax.

4

I’m not sure why using ‘set numbers=’ didn’t work for you. I’m running 4.10 and the following syntax works.

/interface ethernet
set numbers=8 l2mtu=9192
set 8 l2mtu=9192
set ether1 l2mtu=9192
set l2mtu=9192 [find name="ether1"]

Notice there are multiple supported syntax for changing values in ROS, not just numbers=#.

5

Sorry, user error.

ether11-13 on RB1100 are limited to 9116 bytes. 9192 is larger than 9116.

Strange though, because ether1-10 which are behind the switchchip can do 9498.

6

max l2mtu on ether11-13 is 9116

look at the table:
http://wiki.mikrotik.com/wiki/Manual:Maximum_Transmission_Unit_on_RouterBoards#MAC.2FLayer-2.2FL2_MTU