I am new to RouterOS. I am experiencing IP fragment on my LAN.
2 Windows PCs, I have set MTU 4KB on every NIC, and on Winbox I set the Interfaces MTU and L2MTU to 4074 (the max for my RB2011UiAS)
When pinging from 10.0.0.10 (PC1) to 10.0.0.20 (PC2) using “ping 10.0.0.20 -l 1500”, Wireshark displays: ip.fragment - fragment offset: 1480
When changing the command to “ping 10.0.0.20 -l 1480”, also fragment offset is 1480.
Only on “ping 10.0.0.20 -l 1472”, I don’t have IP fragmentation.
Bridge MTU is 1500, and I can’t change it.
Could you help me?
Thank you in advance,
Manos.
You have misunderstood the -l option of ping.
It does not specify the total length of the packet but only the ICMP payload length.
So when you specify 1472 and the 8-byte ICMP and 20-byte IP header is added, you get 1500.
Nothing is wrong.
Thank you for your answer. I understand it better now.
But, I can’t take advantage of jumbo frames.
When running “-l 1500”, the length is 1500+8+20=1528. But NICs are set to 4KB, so why I can’t send packets without being fragmented?
I suppose that the Bridge’s MTU must change, right?
Can you explain what you need them for in the first place?
Do you understand that you can generally use jumbo-frames for communication between peers in your own LAN only?
Don’t use a bridge, use the switch.
However, I am not sure it supports jumbo frames.
Jumbo frames are generally not useful except for really closed networks like SAN.
Yes, I use them only in my LAN, for file transfers between Gigabit devices. (I am begginer on networking, so if I have understood it wrong, just tell me.)
I know that Routerboard is an internet-side router, but I want to have the best I can.
So, I’ll set up the switch chip, right?
When you are a starter in networking, learn that you should stay away from jumbo frames!
These are not useful for what you are doing now. They are for specialized networks like SAN.