Link failures

Simulating link failures

Help index · Project overview

While the lab runs, click/tap a cable to open Link traffic. Right-click also opens the actions, including on a stopped lab; keyboard users can focus a cable and press Enter or Space. The controls work on the floating topology too. Endpoints are named explicitly, so A → B means frames sent by A toward B.

Action	Effect
Block A → B	Drop every frame sent from A; B → A continues.
Block B → A	Drop every frame sent from B; A → B continues.
Block both directions	Drop frames in both directions; keep interfaces up.
Clear frame loss	Clear directional frame-loss settings. It does not reconnect an unplugged endpoint.
Unplug a supported endpoint	Request guest link-down and immediately block traffic both ways on its cable. IOL detects loss after about ten seconds.
Reconnect that endpoint	Report link-up again. Any separately selected frame-loss settings remain active.

Directional loss is a silent unidirectional-fiber failure model for exercises such as UDLD and STP loop guard: the reverse direction remains intact and guest carrier is not lowered. All Ethernet frames are affected, including BPDUs, LLDP, OSPF multicast and VLAN-tagged traffic. Protocol availability and its response still depend on the selected image and configuration. This is not a simulation of optical power levels or every physical fiber/transceiver failure mode.

Unplug/Reconnect is available for running IOSv/IOSvL2 and the tested IOL profiles cisco_iol-17.18.02.bin / cisco_iol-l2-17.18.02.bin. Other IOL filenames/releases, EXOS, Arista and Alpine retain directional frame-loss controls only. No guest console commands, IOS shutdown or saved configuration changes are involved. Unplug/reconnect both supported endpoints if both guests should report cable loss; each endpoint's carrier is controlled separately.

IOSv uses QEMU's local QMP set_link on a named e1000 NIC. See QEMU set_link for notification limitations. The tested IOL profiles launch with -l and use a separate local L1 datagram socket. The controller sends periodic L1 messages independently of Ethernet forwarding. Unplug stops those messages to the selected port; the guest normally detects loss after about ten seconds. Reconnect resumes them. The UI shows requested IOL state and an estimated detection interval, not a guest acknowledgment. Allow ARP/protocol convergence after carrier returns. Unconnected ports receive no L1 messages. No instant explicit IOL down command is claimed, and other IOL image profiles require native validation before opt-in.

Green selection appears only with both endpoint nodes running, no configured frame loss, both requested carriers up, and no pending interval or signaling error. Active frame-loss choices and interrupted endpoint controls use amber; keyboard focus has a neutral outline. Clear frame loss does not reconnect a cable; Reconnect does not clear frame loss. While unplugged, no frame-loss choice is marked as the effective mode; the status text still lists any configured directional loss.

Interrupted cables are orange/dashed with an × marker. The tooltip and actions show the direction or unplugged endpoint, and the map footer counts interrupted links. The server shares these states across browser sessions. Closing the window or refreshing the page does not restore traffic. If an endpoint stops or exits, the dialog lists its state/error and keeps its carrier button visible but disabled when the image supports carrier control. The other endpoint's carrier remains independently controlled; a missing peer can therefore stop BPDUs without lowering the surviving interface. The server logs unexpected process exits with UTC time and available exit status.

Faults are temporary runtime state: they are not topology edits and are not included in JSON, configuration-text exports or saved-state ZIP. Stopping the whole lab or restarting the server clears them. Directional loss remains while one node restarts if other nodes keep the lab running. An endpoint's simulated carrier state resets when that endpoint stops; a restarted NIC starts connected. A failed/timed-out QMP operation or failed IOL L1 signal is shown as uncertain and blocks traffic until you successfully Reconnect the endpoint or stop it. Check the reported error before treating the cable as restored.

Scope

Packet capture/decoding, random packet loss, latency injection, payload corruption and moving a cable while devices run are not implemented. Stop the lab before recabling. See development and testing for the opt-in carrier and forwarding tests.