I'm trying to create a lab on PT to learn about duplex mismatch. I made one end of Fast Ethernet half duplex and other end is full duplex. I was hoping to see an increment in late collisions which I studied that should indicate duplex mismatch. But the late collisions is 0. Any reason why this is happening?


2 Answers 2


Packet Tracer is a simulator aimed at CCNA level courses.

It doesn't accurately simulate real devices but only a subset of their functions. (and still those simulations aren't perfect).

I would not rely on this tool to test whatever part is not explicitly teached in Cisco courses that use Packet Tracer.

TLDR you cannot trust PT to correctly simulate this.


A late collision is defined as any collision that occurs after 512 bits of the frame have been transmitted.

In order to test it in your lab, you can try sending a big size of data, several time on the link having the duplex mismatch, using an extented ping as shown below.

!Make sure you change the target and source IP address to your need

Router A#ping
Protocol [ip]:
Target IP address:
Repeat count [5]: 1000
Datagram size [100]: 18000
Timeout in seconds [2]:
Extended commands [n]: 
Source address or interface:

Hope it helps and it is not too late.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.