Network Connectivity Loss on a Specific VM Adapters (vNIC)

Products

• VMware vSphere ESXi

Issue/Introduction

A specific Virtual Machine (VM) may lose network connectivity on a secondary or specific network adapter (e.g., eth1), even while other adapters remain functional. In this scenario, the VM cannot reach its default gateway, and it becomes unreachable via ICMP (ping) from both local and remote subnets.

Symptoms include:

• Inability to ping the gateway from the affected VM.
• The VM is unreachable from other hosts within the same VLAN.
• The VM is unreachable from different VLANs/subnets.

Crucially: ESXi host logs and VM event logs show no errors (no NIC flaps, link-down events, or resource contention), making the cause difficult to identify through standard log review.

Environment

VMware ESX

Resolution

Immediate Workaround:

If an immediate restoration of service is required, the following steps have proven effective in resetting the virtual port state:

• Disable the impacted network adapter within the Guest OS.
• Add a new network adapter to the VM configuration.
• Assign the original IP address configuration to the new adapter.
• Once connectivity is verified, you may re-enable the original adapter or remove it.

Diagnostic Requirements for Root Cause Analysis:

• Because ESXi does not natively log granular VM-to-VM or VM-to-Gateway communication (it only logs physical link or port failures), standard logs are insufficient for root cause identification.
• If the issue recurs, you must perform a live packet capture while the state is failing to determine where the traffic is being dropped:
• Utilize the pktcap-uw tool: Capture traffic at the vNIC, the Virtual Switch (vSwitch), and the physical uplink (vmnic).
• Analyze the flow: Determine if packets are entering the vSwitch but failing to exit the physical NIC, or if they are being dropped before reaching the virtual switch.
• Verify the physical switch port the vmnic that the vNIC is using is configured correctly to allow the appropriate network traffic, including vlan tagged. 

Additional Information

Reference: For detailed instructions on packet capture methodology, refer to Broadcom Article Network Connectivity Loss on a Specific VM Adapters (vNIC)