Lab: ESXi Network Diagnostics & Path Failover

VCF 9.0Intermediatevcp-foundation⏱ 105 min

Objectives

Lab: ESXi Network Diagnostics & Path Failover

Prerequisites

VCF lab environment deployed and operational

Lab Environment

Standard VCF lab environment for vSphere Foundation 9.0 Support (Support Specialist)

Tasks

Task 1 Lab: ESXi Network Diagnostics & Path Failover

ESXi network diagnostics require understanding the packet path from VM through vNIC → port group → VDS → vmnic → physical switch. Each hop can introduce failures. Systematic diagnosis follows the packet path.

Step 1

SSH to ESXi host with ISCSI storage attached

Step 2

List active paths: esxcli storage nmp path list | grep active

Step 3

Disable one physical path (simulate cable fault): esxcli storage nmp path set -p vmnic2:0:0:0 -e false

Step 4

Verify failover: esxcli storage nmp path list (path should transition to dead)

Step 5

Test ISCSI LUN still accessible: iscsiadm -m session (should show target reachable via alternate path)

Step 6

Re-enable path: esxcli storage nmp path set -p vmnic2:0:0:0 -e true

Step 7

Run vmkping test: vmkping -I "Management Network" gateway_ip -c 4 (verify <2ms)

Validation Gate

Check: Diagnose a VM connectivity issue using: esxcli NIC stats, vmkping with jumbo frames, pktcap-uw for packet capture, and VDS port group VLAN verification

Expected: Root cause identified: VLAN mismatch, MTU issue, NIC speed negotiation, cable failure, or physical switch misconfiguration. Resolution applied and connectivity restored.

Common Errors

Using ping as the only diagnostic tool for network issues

Fix: Ping tests ICMP connectivity but doesn't diagnose: VLAN issues (use 'esxcli network vswitch dvs vmware list' to verify VLAN), MTU issues (use 'vmkping -d -s 8972' for jumbo frame test), NIC teaming issues (use 'esxcli network nic stats get -n vmnicX' for per-NIC traffic counters), or DNS issues (use 'nslookup' from ESXi shell).

Not checking physical NIC link status and speed negotiation

Fix: A NIC showing 'Up' in vCenter may be negotiated at 1Gbps instead of 25Gbps due to cable or switch port issue. Check: 'esxcli network nic get -n vmnic0' shows link status, speed, duplex. Mismatched speed causes performance issues that look like network congestion.

Forgetting to test path failover after NIC replacement

Fix: After replacing a failed NIC or cable, verify failover works correctly: (a) confirm new NIC is in the correct teaming group, (b) test failover by disconnecting the primary NIC and verifying traffic switches to standby, (c) restore primary and verify failback. Untested failover is not failover — it's hope.

Not verifying VLAN tagging consistency between VDS and physical switch

Fix: A VM on VLAN 100 port group with a physical switch port trunking only VLAN 200 results in silent packet drops — no errors, just no connectivity. Use packet capture (pktcap-uw) on the ESXi host to verify VLAN tags on packets leaving the vmnic match physical switch expectations.

Final Validation

Lab completed successfully

✓ All steps completed → No errors observed

Cleanup / Restore

• Revert to snapshot if needed

Design Reflection (VCDX)

Network troubleshooting methodology demonstrates operational maturity. VCDX designs should include diagnostic procedures for common network failure scenarios.

Requirements

Diagnose network connectivity issues systematically
Verify VLAN, MTU, NIC teaming, and physical connectivity
Test failover behavior after changes

Constraints

pktcap-uw captures can impact host performance at high rates
Some diagnostics require ESXi SSH access (may be disabled in lockdown mode)

Assumptions

SSH access is enabled for troubleshooting
Physical switch configuration is accessible for cross-reference

Risks

Silent packet drops from VLAN mismatch — no error messages
Performance degradation from NIC speed mismatch going unnoticed

⚠ Known Pitfalls (from Community KB)

Assuming ping failure = network down. Ping tests ICMP only — the issue might be VLAN, MTU, or application-layer.

Not verifying NIC speed negotiation after cable replacement — a 25GbE NIC negotiating at 1Gbps causes subtle performance issues.

Mark complete