MTU mismatch between two directly connected devices

[morabusa]

Hello, I am having a lot of issues to understand the MTU mismatch behavior on two network devices (two Nexus 9K connected through routed interfaces). I have been trying to find what happen if the device A (MTU 9000 bytes) connects to the device B (MTU 1500 bytes), and what would happen if a 8500 bytes frame is received in device B interface (1500 bytes MTU) from the device A interface (9000 bytes MTU). As far I have read, "MTU is the maximum packet size that can be sent through an interface and MTU does not limit the maximum packet size that can be received", but when I have tested it in my lab environment, I can see how the 8500 bytes packet is received in the Device B interface (with 1500 bytes MTU), but then the device A does not get answer from the device B (it looks like the packet is getting discarded at some point).

Why is this happening if "MTU doesn't limit the maximum packet size that can be received"? The behavior I was expecting to see is the following:

1- Device A sends an 8500 bytes ping to Device B IP (for example, ping 192.168.1.2 source 192.168.1.1 size 8500)
2- Device B receives the 8500 bytes ICMP packet (plus headers)
3- Device B then fragments the 8500 packet into smaller packets because of 1500 bytes MTU

I would like to understand why is this not happening and the packet is getting discarded? Thank you very much.

Best Regards.

[@confignetworks]

Hello,

My opinion is that device B is receiving a packet with MTU higher that its supported MTU. So it dropped it without any notification. 

if you send a ping without changing its packet size, you should receive a reply, if also you change the MTU of the device B as device A you should also receive a reply. 

Anyway, it is recommended to have the same MTU between 2 Devices. 

Thanks

[Joseph W. Doherty]

First, keep in mind that fragmentation only happens at L3 interface jumps. I.e if router x receives a packet on interface 1 which is 2000 bytes, when is routed to interface 2, which only supports 1000 bytes, it will be fragmented, assuming DF bit not set.

My understanding is, on Cisco switches, a L2 interface will accept a MTU frame larger than it's configured for, if the interface NIC is capable of physically accepting it, but the frame will not be transmitted out a L2 interface larger than the interface's configured MTU. Further, a too large L2 frame, when discarded, is done silently.