Hello Samir
After further investigation, there are a couple of interesting things involved with this scenario. The lab you created, and the one I created as well, were both on CML, so we created the exact same scenario. The first interesting thing is that on physical hardware, it’s not possible to reduce the L2 MTU per interface to a value of less than 1500 bytes. Indeed on most IOS platforms, you can only apply the system MTU globally, and with a minimum of 1500. Even on some physical platforms where you can configure MTU per interface, it’s still a minimum of 1500.
Secondly, Rene tested this out on real hardware (WS-C3850-48T), and with an MTU size of 1500, was able to ping up to a size of 1504 but not 1505:
Success rate is 100 percent (1/1), round-trip min/avg/max = 1/1/1 ms
R1#ping 192.168.12.2 size 1504 repeat 1
Type escape sequence to abort.
Sending 1, 1504-byte ICMP Echos to 192.168.12.2, timeout is 2 seconds:
!
Success rate is 100 percent (1/1), round-trip min/avg/max = 1/1/1 ms
R1#ping 192.168.12.2 size 1505 repeat 1
Type escape sequence to abort.
Sending 1, 1505-byte ICMP Echos to 192.168.12.2, timeout is 2 seconds:
.
On the wire, the frame length is 1518 bytes. This is expected behavior because it allows for an additional 14 bytes for the Ethernet header and another 4 bytes for a possible VLAN tag. He tried it again with an MTU of 1600, and a ping of 1604 went through but not 1605.
So it seems to be an issue with the emulator. I’m curious to find out what would happen in a similar topology in GNS3 or EVE-NG.
I hope this has been helpful!
Laz