CDP data shows multiple ESXi host uplinks (vmnics) as being connected to the same upstream neighbor

Products

• VMware vSphere ESXi

Issue/Introduction

On VMware ESXi hosts, multiple physical NICs (vmnics) may report as being connected to the same CDP (Cisco Discovery Protocol) neighbor when viewed through vSphere Client or from the output of the "esxcli network nic get" command.

 

Example command run on an ESXi host.  Be sure to replace with the proper vmnic #'s:

#esxcli network nic get -n vmnic# 

Environment

VMware vCenter Server 
VMware vSphere ESXi

Cause

Having the two vmnics report as being connected to the same CDP neighbor may, or may not be a problem.  It depends on the topology within the network underlay as well as the switch configurations.

Resolution

When two ESXi vmnics unexpectedly see the same CDP neighbor, you're likely facing a physical loop, an incorrect port/VLAN configuration, or a VDS/pSwitch issue, so check physical cabling (looped cables!), verify VLANs/port groups on the vSwitch, use esxtop for real-time mapping, and ensure your physical switch config matches the host's intended uplinks.

1. Physical Layer & Cabling (Most Common)

• Check for Loops: The most frequent cause is a physical cable loop.
• Inspect Physical Ports: Visually confirm which physical switch ports your vmnics are connected to.

 

2. ESXi vSwitch Configuration

• Verify Port Groups: In vCenter, check the vSwitch topology.
• Check vmnic Assignment: Use esxtop (press n) in the ESXi shell to see exactly which vmnic is handling traffic for which VMkernel adapter (vmk0, vmk1, etc.).

 

3. Physical Switch Configuration

• Verify CDP/LLDP: On the physical switch, use show cdp neighbors detail or show lldp neighbors to see what the switch thinks is connected to its ports.
• Check VLANs: Ensure the switch port(s) connected to your ESXi host uplinks are configured as trunk ports (if using VLANs) or as access ports for the correct native VLAN, and that the VLANs match what's in ESXi.

Additional Information

ESXi will therefore report the same Device-ID for uplinks connected to different members of the same stack.