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Bratislava
Roland
9.5.2026

This warning message, "virtual device serial0 will start disconnected", is one of the most common errors seen by users of VMware Workstation, VMware Player, and VMware Fusion.

It typically appears as a pop-up window immediately after you power on a virtual machine (VM). While it often looks like a critical error, it is usually just a configuration notification.

Here is your complete guide to understanding, fixing, and silencing this message.

Case Study: A Real-World Lab Failure

A student attempted to build a Frame Relay switch using a Cisco 3640 in GNS3. After configuring the Frame Relay map statements and inverse ARP, the PVCs remained inactive. The student spent six hours checking DLCI numbers and encapsulation types.

The root cause? The console log from boot clearly showed: "Virtual device serial0 will start disconnected" and "Virtual device serial1 will start disconnected". The student had never connected the cables from the router to the Frame Relay switch.

Moral of the story: Always read the console boot messages. That "disconnected" line is not a warning; it is a direct status report.

The Clock Rate Caveat (DCE vs DTE)

Even after you connect the virtual cable, you might still face a down/down or up/down status. After fixing the "start disconnected" message, ensure you have a clock rate set on the DCE side.

In a real lab, service providers provide the clock. In an emulator, you must provide it.

  1. Determine which router is DCE. When you created the link, the emulator designated one end as DCE.
  2. On that router’s serial interface, enter: 
    clock rate 64000
no shutdown
  3. On the DTE router (the other end), simply no shutdown is enough.

Within 30 seconds, both interfaces should transition to up/up. The original disconnected message is now a relic of the boot sequence.

Scenario D: You actually NEED the Serial Port

If you are an embedded developer or network engineer requiring the serial port for debugging:

  1. Go to VM Settings > Serial Port.
  2. Instead of "Use physical serial port," change the setting to "Use output file" or "Use named pipe".
  3. This prevents VMware from looking for a physical hardware port on your motherboard (which you likely don't have) and instead creates a virtual bridge.
    • Example for Named Pipe: \\.\pipe\com_1 (Server application).