In this blog, we will walk through how to diagnose and fix network issues on ESXi hosts, focusing especially on CRC errors and packet drops. We’ll cover using tools like esxtop, esxcli, and log files to get to the root of the problem — all with practical steps you can follow today.
Using esxtop to Identify Network Issues
Let’s start with what most seasoned admins reach for first: esxtop.
- SSH into the ESXi host
- Launch
esxtop:esxtop - Press
nto jump into network view
| Column | What It Tells You |
|---|---|
PKTTX/s, MbTX/s, PSZTX | Packets sent, bandwidth, avg packet size |
PKTRX/s, MbRX/s, PSZRX | Same, but for incoming traffic |
DRPTX, DRPRX | Dropped packets – zero is the goal |
Hit f in esxtop and enable:
f: TEAM-PNICj: USED-BY (who’s using the interface?)k: PORT-IDn: VSWITCHo: VLAN-ID
You’ll now be able to correlate exactly which VM or kernel port is responsible for traffic spikes or anomalies.
Detecting Saturation or Imbalance
If you’re hitting high throughput but things still feel sluggish, you might be saturating your link or not distributing load properly.
Saturation: Watch MbTX/s and MbRX/s. If you’re nearing 940 Mbps on a 1 Gbps NIC, you’re at the ceiling.
Imbalance: Using NIC teaming? Make sure the TEAM-PNIC column isn’t showing one uplink doing all the work while others sit idle. Load-based teaming helps here.
Deep Dive: NIC-Level Stats with esxcli
Sometimes the issue lies deeper than what esxtop reveals. Try:
esxcli network nic stats get -n vmnic7Check for:
- Transmit errors
- Receive drops
- CRC or framing errors (indicates cabling or physical port issues)
Logs are your silent witnesses. Here’s where to look:
| Log File | Path | What to Look For |
vmkernel.log | /var/log/vmkernel.log | Packet drops, link state changes |
vobd.log | /var/log/vobd.log | NIC up/down events |
hostd.log | /var/log/hostd.log | Switch config changes, VM attach events |
syslog.log | /var/log/syslog.log | General networking/system events |
Use grep to pull highlights:
grep -i 'drop\|error\|vmnic\|link' /var/log/vmkernel.log
grep -i 'link.*up\|link.*down' /var/log/vobd.log
for crc errors, bad packets, usually physical or config related.
grep -i 'crc\|error\|link' /var/log/vmkernel.log
- Confirming errors on NIC and switch
- Matching duplex/speed mismatches
- Replacing cables
- Avoiding interference
- Updating firmware
- Checking logs for additional clues
Tail logs in real time while recreating the issue:
tail -f /var/log/vmkernel.logFind VM-ID on ESXi to locate the vm causing issues
esxcli vm process list
or
vim-cmd vmsvc/getallvms | grep <vmid>