Glossary

Loopback Detection

What is Loopback Detection

Loopback Detection is a Layer 2 network protection mechanism that finds and reacts to physical or logical loops on a switch before they can degrade the network. A switch with loop detection enabled periodically sends detection packets out an interface or VLAN and watches to see whether those same packets come back — if they do, a loop exists, and the switch responds automatically.

Loopback Detection exists because Layer 2 networks have no hop-limit field the way IP does: a broadcast, multicast, or unknown-unicast frame caught in a loop keeps circulating and gets duplicated at every switch it passes through. Left unchecked, this quickly saturates link bandwidth and switch CPU, and can bring an entire broadcast domain down — commonly the result of an accidental cable connecting two ports on the same switch or VLAN, or a misconfigured connection between access-layer devices. Because these loops are usually created by cabling mistakes rather than protocol failures, Loopback Detection is built to catch them directly, independent of Spanning Tree.

How Loopback Detection Works

Loopback Detection identifies loops by sending its own traffic and checking whether it comes back:

  1. On any interface or VLAN with loop detection enabled, the switch broadcasts detection packets at a configurable interval (10–300 seconds, 30 by default). Each packet carries a distinct Ethertype (0x9998) and a payload identifying the sending port or VLAN, so the switch can recognize its own packets on return.

  2. Detection runs at one of two granularities. Interface-based detection sends and listens on a single port — if that port receives back the packet it just sent, the loop is local to that link. VLAN-based detection sends from every member interface of a VLAN and watches all of them — if any interface in the VLAN receives a packet that was sent from another interface in the same VLAN, a loop exists somewhere within that VLAN, even if it spans multiple ports.

  3. Once a loop is confirmed, the switch immediately executes the configured action on the affected interface(s): shutdown (the default — the port is disabled and an alarm is logged) or logging (the event is recorded but the port stays up). Repeated alarm logs can be throttled with a separate log interval, which is disabled (0 seconds) by default.

  4. A shut-down interface stays in a controlled state until it recovers. If an automatic recovery time is configured (10–65,535 seconds), the switch retries the interface after that period and brings it back up only if the loop is gone; otherwise it stays down. Without a configured recovery time, the interface stays down until an operator intervenes manually with no shutdown on that interface or clear loopback-detection to recover all shut-down ports at once.

Why Loopback Detection is Beneficial

Loopback Detection gives operators a fast, direct safeguard against one of the most common causes of Layer 2 outages:

· Catches loops STP alone may miss: Spanning Tree prevents loops within its own topology, but misconfigurations, non-STP devices, or unmanaged switches in the path can still create loops that only a direct loopback check will catch.

· Contains the blast radius automatically: Shutting down or flagging just the affected port(s) — rather than letting a loop saturate the whole broadcast domain — keeps one bad cable or misconfigured device from taking down unrelated users and services.

· Matches detection to the problem's scope: Interface-based detection isolates a single bad link; VLAN-based detection catches loops that cross multiple access ports within the same VLAN, which is exactly the failure mode most access-layer loops actually take.

· Recovers without manual intervention: A configurable automatic recovery time lets transient loops (a cable temporarily miswired, then corrected) clear themselves, while persistent loops stay safely isolated until fixed.

· Tunable to the environment: Detection interval, log interval, per-interface action, and recovery time are all independently configurable, so operators can trade off detection sensitivity against system overhead based on how sensitive a given deployment needs to be.

The trade-off is the same one any active-probing mechanism faces: shorter detection intervals catch loops faster but generate more control-plane traffic, so the interval should be set based on how quickly a loop needs to be caught versus how much overhead the network can absorb.

At Asteraix

What We Can Do at Asteraix

AsterNOS implements Loopback Detection as a standard part of its Ethernet switching feature set, configurable at both the global and interface/VLAN level from the CLI.

· Both detection granularities, side by side: AsterNOS supports interface-based and VLAN-based loop detection simultaneously on the same switch — for example, strict per-port shutdown on uplinks while a shared access VLAN runs its own VLAN-wide check, as shown in AsterNOS's own configuration examples.

· Independently tunable timers: Detection interval (loopback-detection interval-time), alarm log interval (loopback-detection log-time), and per-interface automatic recovery time (loopback-detection recovery-time) are all configured separately, so sensitivity and overhead can be tuned per deployment rather than fixed.

· Configurable response per interface: Each interface can be set independently to shutdown (log and disable, the default) or logging (record only), so critical uplinks and best-effort access ports can be handled differently on the same switch.

· Full visibility for operations: show loopback-detection config, show loopback-detection status, and show interface errdown give operators a direct view of current settings, active loop events, and every port currently down because of a detected loop.

· One-command recovery: clear loopback-detection re-enables every loop-detection-triggered shutdown at once, alongside standard per-interface no shutdown recovery — useful after a bulk cabling fix across many access ports.

· Built for campus and access-layer scale: Loopback Detection ships across AsterNOS's campus and enterprise switch line, so the same loop-protection behavior is consistent from a wiring-closet access switch up through aggregation.