Executive summary

The message "virtual device serial0 will start disconnected" is a benign informational log from virtualization software (commonly QEMU/KVM, VirtualBox, or libvirt) indicating that a virtual serial device (named serial0) is configured but will not be connected to a host endpoint at VM start. It does not by itself indicate an error but explains why a guest serial port is inactive; troubleshooting depends on whether this was intentional.

Step 4: Verify via .vmx File

You can also edit the VMX file directly (with VM powered off):

Look for lines like:

serial0.present = "TRUE"
serial0.startConnected = "FALSE"

Change to:

serial0.startConnected = "TRUE"
serial0.fileType = "file"
serial0.fileName = "nul"

Or to remove completely:

serial0.present = "FALSE"

Decoding "Virtual Device Serial0 Will Start Disconnected": Causes, Fixes, and Best Practices

If you have ever fired up a Cisco router in Dynamips, GNS3, EVE-NG, or Cisco Packet Tracer, you have likely seen the console output slowly crawl by until it halts at a seemingly concerning line:

"Virtual device serial0 will start disconnected"

For many students, this message triggers an instinct to panic. Is the router broken? Is the image corrupted? Will Serial0/0 ever come up?

The short answer is: No, your router is not broken. Everything is functioning as designed.

However, understanding why this message appears, what it actually means for your virtual lab, and how to resolve the underlying connectivity issue is crucial for building functional network topologies. This article provides a deep dive into the causes, step-by-step troubleshooting, and permanent solutions for the "Virtual device serial0 will start disconnected" message.

Solution 1: GNS3 (Dynamips Backend)

1. Shut down the router. Do not attempt to connect cables while the router is powered on. Right-click the router and select Stop.
2. Open the topology. Look for a dotted line or a link tool.
3. Connect the interface. Use the "Add a link" tool (the cable icon).
   • Click on Router A.
   • Select Serial0 from the list of interfaces.
   • Click on Router B.
   • Select Serial0 from its list.
   • Choose the cable type: Usually "Serial DTE" or "Serial DCE" . If you select DCE, you must also set the clock rate later in the config.
4. Restart the router. Power it back on. The message "Virtual device serial0 will start disconnected" should be replaced by initialization handshakes.

Common platforms — what “start disconnected” means

• QEMU: serial device defined with "-serial" target may be configured to start disconnected (e.g., using chardev with “-chardev socket,id=ch1,server,nowait” vs. an explicit connect). If disconnected, QEMU still exposes the emulated UART but no chardev backend is attached until connected.
• VirtualBox: A virtual COM port can be configured but left disconnected; guest sees the COM port but host-side pipe/socket/file isn’t created.
• Containerized device passthrough (e.g., socat to a pseudo-tty): The container exposes /dev/ttyS0 but host-side endpoint isn’t present until socat creates it.
• Embedded/Simulators: Device models in simulators may instantiate serial0 but leave it disconnected to mimic hardware not wired to a console.

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.