Now go build your topology, connect those serial cables, and watch show interface serial0 finally display the glorious words:
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.
A: Because IOU/IOL images are binary-level simulations that run natively on Linux. They do not use the Dynamips virtual device layer. They treat serial interfaces as internal software constructs, not hardware emulations. Conclusion: A Feature, Not a Bug The message "Virtual device serial0 will start disconnected" is the emulator's honest way of telling you, "I have no information about what this port should connect to, so I am leaving it in a safe, disconnected state." virtual device serial0 will start disconnected
config t interface serial0 no shutdown After this, show ip interface brief still shows Serial0 down/down .
A: In Dynamips/GNS3, no. The hardware state is decided at boot. You must shut down, connect the cable, and restart the router. In EVE-NG with newer QEMU images, you may hot-plug, but it is not recommended. Now go build your topology, connect those serial
In a real Cisco router, Serial interfaces use . If no cable is plugged in, the interface remains "down/down." However, emulators are not real circuits. If an emulator tried to auto-detect every possible connection at boot, it would slow down the entire lab startup process.
Always read the console boot messages. That "disconnected" line is not a warning; it is a direct status report. Frequently Asked Questions (FAQ) Q: Does this happen on real Cisco hardware? A: No. Real hardware will show "%LINK-3-UPDOWN: Interface Serial0/0, changed state to down" if no cable is plugged in, but it will never announce "virtual device." A: Because IOU/IOL images are binary-level simulations that
The no shutdown command fixes administratively down (interface disabled by software). It does fix hardware down (no cable, no carrier signal). The "virtual device will start disconnected" message is a hardware-level notification. You cannot fix it with IOS commands; you must fix it in the emulator GUI. 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.