Troubleshooting Mindset Series #008: When a Critical HCI Alert Leads to the Physical Layer

Recently, our Sangfor HCI generated a Critical Alert:

"Node Disconnected from Gateway"

The affected node was our Cluster Controller, and the gateway reported in the alert was the Firewall interface.

At first, the alert looked like it could be related to the firewall, routing, or network configuration.

Instead of immediately changing any configuration, I decided to investigate layer by layer.

1. Verify HCI Cluster Health

First, I checked the HCI cluster.

✅ The affected node remained online.

✅ Other nodes remained operational.

✅ Virtual machines continued running normally.

✅ There was no indication of a cluster-wide failure.

This suggested that the problem might be an intermittent connectivity issue rather than a complete node failure.

2. Check the Firewall

The HCI alert identified the gateway as:

10.x.x.x

The affected HCI node was:

10.x.x.x

I reviewed the Firewall logs around the time of the incident.

I specifically looked for:

❌ Interface down/up events

❌ HA failover

❌ Firewall reboot

❌ Routing changes

❌ Abnormal resource utilization

No clear evidence indicated that the Firewall itself had failed during the incident.

So I moved to the next layer.

3. Investigate the Physical Layer

I physically inspected the network connection of the affected HCI host.

During the inspection, I noticed that the network cable was not seated as firmly as expected.

The connector appeared slightly loose.

I reseated the cable properly and ensured that the connection was secure.

4. Validation

This was the most important part of the investigation.

After reseating the network cable, I monitored the HCI environment for more than 48 hours.

The result:

✅ No further "Node Disconnected from Gateway" alerts were observed.

✅ The HCI cluster remained stable.

✅ The affected node remained online.

✅ No further similar connectivity symptoms were observed.

Based on the evidence and the fact that the alert did not recur after the physical connection was corrected, the loose network connection was identified as the most likely root cause of the intermittent connectivity issue.

Key Lesson

This incident reminded me that troubleshooting should not always start with the most complicated component.

When we see a Critical Alert, it is easy to immediately suspect:

Firewall

Routing

Configuration

Virtual machines

Software

But sometimes the actual problem is much simpler.

A loose network connection can cause intermittent connectivity problems without necessarily producing an obvious firewall event.

My Troubleshooting Flow

HCI Alert

→ Verify node health

→ Verify VM status

→ Identify the reported gateway

→ Check firewall logs

→ Eliminate firewall-side issues

→ Inspect physical connectivity

→ Reseat network cable

→ Monitor for recurrence

→ Validate the result

Final Takeaway

Follow the evidence, not the assumption.

A Critical Alert does not always mean that the most complex component is the problem.

Before changing firewall policies, routing, or HCI configuration, don't forget to check the basics — including the physical network connection.

Sometimes, the root cause is sitting right in front of us.

Have you ever spent hours troubleshooting a network or infrastructure issue, only to discover that the actual problem was a loose cable?

Thank you for sharing!

Thanks for sharing!

Thanks to share!

Thanks for sharing!

yes, loose cables and bad SFPs are probably the most common "boring" root cause behind alerts that look scary on paper. It's a good reminder for anyone reading this to check Layer 1 early, not last.