Jumbo frames not fragmented on CRS with OS version 6.42.5

RouterOS General
1

The Goal is fragmentation of large IP packets and jumbo frames (up to 9000Bytes) to normal size IP packets (MTU <=1500Bytes).

A PC ) directly connected to Ether3 is sending ICMP packets to a PC directly connected to Ether4.
Routing is set on both ends
No firewalling rule or NAT applied

The issue at hand:
All is working fine when packet size is smaller than 1473 Bytes For ICMP packet bigger than 1472 Bytes the packet is seen sent over the cable but dropped by the CCR MTU for the ingress is set for 3000Bytes

In addition, the CCR does not send packets with MTU bigger than 1500Bytes on ether3 even though it is set to 1600



To summarize:
“jumbo” packets do not seem to work as well as fragmentation of jumbo packets to normal size packets

2

Did you change L2MTU as well?

https://wiki.mikrotik.com/wiki/Manual:Maximum_Transmission_Unit_on_RouterBoards#MAC.2FLayer-2.2FL2_MTU

3

Yes I did

actual MTU is set to 8000 L2 MTU is set to 9018 on eth 3 and 1500/1580 on eth4

4

Can you share more details about your setup?
RB1<->CCR<->RB2

#RB1
/interface ethernet
set [ find default-name=ether1 ] l2mtu=9000 mtu=9000
/ip address
add address=192.168.1.2/24 interface=ether1
/ip route
add gateway=192.168.1.1

#RB2
/interface ethernet
set [ find default-name=ether2 ] l2mtu=1580 mtu=1500
/ip address
add address=192.168.2.2/24 interface=ether2
/ip route
add gateway=192.168.2.1

#CCR
/interface ethernet
set [ find default-name=ether1 ] l2mtu=9000 mtu=1500
set [ find default-name=ether2 ] l2mtu=1580 mtu=1500
/ip address
add address=192.168.1.1/24 interface=ether1
add address=192.168.2.1/24 interface=ether2

This configuration seems to be working properly, even with 6.42.5
Also, there is a known issue with CCR series devices that L2MTU is not updated, if it is changed to anything lower than 1662 bytes.