Knowledge Base Article | Audience: Customer network / site operations teams | Scope: Basic isolation of a physical Ethernet link-down condition between a Versa VOS interface and a third-party ISP router or handoff.

Purpose

Use this procedure when a physical interface (for example, vni-0/1) on a Versa VOS appliance reports link down while connected to a third-party ISP router, modem, switch, or Ethernet handoff. The goal is to identify whether the issue is most likely associated with the original Versa port, the cable/transceiver, the remote device, or the link negotiation between the two endpoints.

This is an isolation procedure, not a definitive hardware diagnostic. A link-state result by itself does not prove a hardware failure; repeat tests with known-good components and collect evidence before concluding that a port or device requires replacement.

Before you begin

• Confirm the affected interface name and current administrative/operational state (example: vni-0/1 by checking "show interfaces brief" on VOS cli).
• Check that the remote ISP router port is enabled and that the provider confirms the handoff should be active.
• Record interface LEDs, alarms, recent changes, and the time the issue was observed.
• Use a known-good spare VOS port and a known-good Ethernet cable wherever possible.
• If the connection uses an SFP/SFP+ or other pluggable optic, record its vendor, part number, type, wavelength, and supported speed. Use a Versa-supported/certified module for the platform and port.
• Schedule the test with the customer/ISP if moving the cable may interrupt service. Do not move production links without authorization.

Decision flow

Step 1 — Move the ISP cable to a different VOS port

If vni-0/1 is down, move the same ISP-facing cable to a known-good, unused interface such as vni-0/3. Before connecting the cable, make sure vni-0/3 is administratively enabled (admin-up) and configured for a compatible physical mode. Avoid changing unrelated configuration.

Observe the operational link state after the cable is moved:

• If vni-0/3 comes up while vni-0/1 remains down under the same conditions, the evidence points toward a problem specific to vni-0/1 or its port-specific configuration. Continue to Step 3 to test the original port with a laptop before classifying it as a hardware fault.
• If vni-0/3 also remains down, do not conclude that the Versa appliance is faulty yet. Continue to Step 2 and test the ISP handoff independently.

Step 2 — Test the ISP handoff with a laptop

Disconnect the ISP-facing cable from the VOS appliance and connect it to a laptop with a known-good Ethernet NIC. Ensure the laptop Ethernet adapter is enabled. If required, use a compatible adapter; avoid introducing a USB/Ethernet dongle or cable of unknown condition if a known-good alternative is available.

Check the physical Ethernet link indication on the laptop and, where available, the NIC status in the operating system. IP addressing, DHCP, ping, or Internet access are not required to establish physical link; the key observation for this step is whether Ethernet carrier/link comes up.

• Laptop link remains down: the issue is likely outside the VOS port. Check the cable, patch panel, ISP handoff, remote router interface, transceiver/media compatibility, and provider-side port status. A laptop link-down result is not conclusive if the laptop NIC or adapter does not support the handoff speed/media.
• Laptop link comes up: the ISP handoff and cable can establish physical link with the laptop. Continue to Step 3 to test the original VOS port directly.

Step 3 — Connect the laptop directly to the original VOS port

With authorization, disconnect the ISP handoff from vni-0/1 and connect the laptop directly to vni-0/1 using the known-good cable. Confirm vni-0/1 is administratively enabled and that the laptop NIC is enabled. Observe whether the physical link comes up.

• Laptop link stays down on vni-0/1, but the same laptop/cable links successfully to another compatible VOS port: this strongly suggests a port-specific issue or port-specific configuration on vni-0/1. Check administrative state, supported speed/media, and interface configuration; collect diagnostics and escalate for hardware evaluation if the result is repeatable.
• Laptop link comes up on vni-0/1, and the laptop also links to the ISP handoff: a hard physical failure of vni-0/1 is less likely. Investigate interoperability between the VOS interface and the third-party router, including transceiver compatibility, supported speeds, autonegotiation, duplex, and remote-port configuration.

Step 4 — Check media compatibility and link negotiation

If the laptop links to both endpoints but the VOS-to-ISP connection does not, review the physical-layer settings at both ends:

• SFP/SFP+ or other transceiver: verify the module is supported for the Versa platform and the remote device, and that both sides use compatible media, wavelength, fiber type, and speed. For copper RJ45 links, verify cable category and length.
• Speed and duplex: first confirm the speed/duplex capabilities and the vendor-recommended settings for both devices. Autonegotiation mismatches can prevent link establishment or cause errors. If supported by both endpoints, test a matching static speed/duplex configuration as a controlled diagnostic; use settings supported by the exact interface and platform.
• Autonegotiation: for copper Ethernet, autonegotiation is generally preferred when supported at both ends. Do not force settings on only one side. Any static setting should match on both ends and be reverted if it does not resolve the issue.
• Remote interface: verify it is enabled, configured for the correct media/speed, and not reporting a fault. Ask the ISP to check its port counters, alarms, and physical-layer diagnostics.

How to interpret the results

Observation	Most likely direction	Next action
Link comes up on vni-0/3, not vni-0/1	Original port or port-specific configuration	Test laptop directly on vni-0/1; compare configuration and supported mode.
Link stays down on VOS ports and laptop	Cable, ISP handoff, remote port, or media compatibility	Validate with known-good cable/NIC and engage ISP.
Laptop links to ISP and vni-0/1	VOS-to-ISP interoperability or negotiation	Check SFP/media, speed/duplex, autonegotiation, and remote-port settings.
Laptop links to ISP but not to vni-0/1; another VOS port works	Port-specific VOS-side issue likely	Verify admin state/configuration; collect diagnostics and escalate for hardware evaluation.

Evidence to collect before escalation

• Appliance model, VOS version, affected interface, and interface configuration (redact sensitive details before sharing publicly).
• Operational and administrative state of the original and test interfaces before and after each test.
• Photos or notes of interface LEDs, cable path, SFP label, and remote router port indicators where practical.
• Exact test matrix: vni-0/1 to ISP, vni-0/3 to ISP, laptop to ISP, and laptop to vni-0/1; include cable/module used and link outcome.
• Supported speed/media information for both endpoints and whether autonegotiation or static settings were used.
• Relevant VOS logs, alarms, interface counters, and timestamps, collected using commands documented for the installed VOS release.

Important cautions

• A link-up result proves only that physical carrier was established; it does not prove IP connectivity, routing, or service health.
• A link-down result with a laptop does not automatically prove an ISP fault. The laptop NIC, adapter, cable, speed support, and media type must also be known-good and compatible.
• Do not declare a hardware failure based on a single cable swap. Repeat the comparison with known-good components and check administrative state and configuration.
• Do not change speed/duplex or transceiver settings on a production link without approval and a rollback plan.
• This article covers basic physical-link isolation. If carrier is up but traffic fails, troubleshoot VLANs, IP addressing, routing, policy, and counters separately.

Quick customer checklist

• ☐ Confirm vni-0/1 is admin-up and record its link state.
• ☐ Move the cable to a known-good free port that is admin-up; record the result.
• ☐ Test the ISP handoff with a known-good laptop/NIC and cable.
• ☐ If the ISP handoff works with the laptop, test the laptop directly on vni-0/1.
• ☐ Verify supported SFP/media and matching speed/duplex/autonegotiation on both ends.
• ☐ Share the test results and diagnostics with Versa TAC and/or the ISP as indicated.

 

Suggested public KB disclaimer: This procedure is intended for initial troubleshooting and fault isolation. You can reach out to Versa TAC by providing these observations, while opening a ticket, for a faster resolution