"High pNic RX/TX Pause rate detected" on vmnic with little to no throughput/packets

Products

• VMware vSAN

Issue/Introduction

Symptoms:

• The alert "High pNic RX/TX Pause rate detected on vmnicX" is seen on vSAN hosts.

• Where vmnicX could be the standby vmnic or even the active vmnic which little to no throughput or packets passing through it, as seen from the vSphere Client > Select host > Monitor > vSAN Performance Graph > Physical Adapters.

Environment

• vSAN 7.x
• vSAN 8.x

Cause

This alarm triggers when the flow control pause frame rate exceeds hard-coded thresholds (typically >1%). There are two primary causes:

1. Low Traffic: The vmnic driver reduces receive queues during periods of low traffic. A sudden burst of IO fills the reduced queues, triggering pause frames. This is expected behavior.
2. High Traffic: The physical network switch port buffer is exhausted or the link has reached its maximum throughput/flow control limit. This indicates actual network saturation between the host and the upstream switch.

Example graph:

Resolution

1. If the affected vmnic is a standby adapter or shows minimal throughput (1-10 kb/s):

   1. No action is required.

   2. The vmnic driver will automatically increase queues as IO increases.

   3. The alert can be safely reset to green.

2. If the vmnic shows high throughput and the alarm persists:

   1. Engage Networking Team: Review the physical switch ports for buffer exhaustion or micro-bursts.

   2. Check Flow Control: Verify if 'Pause' is enabled on the physical switch ports and ensure it matches the ESXi configuration.