RUTX11 Iperf3 packetloss

Networking Solutions Question
1

I have 2 RUTX11 devices and I’m trying to run iperf3 tests.

I have a simple network:

  • RUTX11-1 connected to switch 1

  • RUTX11-2 connected to switch 2

  • Switch 1 and switch 2 connected to each other (empty switches)

I can ping between them.
I can start iperf.
But I have too much packet loss. The connection is almost local. There are no other devices in between.

I am doing the following:

Server side:
iperf3 -s -B (ip address) -p 5001 -i 5

Client side:
iperf3 -c (ip address) -u -p 5001 -w 100M

I have disabled the firewall and attack prevention on the RUTX devices, but I still have packet loss.

I have almost 80% packet loss.

What could be the cause?

3

Greetings, @Appo ,

Welcome to Teltonika Community!

Thank you for reaching out. Based on your setup, the high packet loss you’re experiencing is most likely caused by a missing bandwidth limit in your iPerf3 command. Without it, UDP traffic can flood the router’s buffer faster than it can process, resulting in dropped packets.

Please try updating your client command as follows:

iperf3 -c -u -p 5001 -b 50M -i 5

The -b flag sets the target bitrate. We recommend starting at 50M and adjusting up or down based on results. You can also remove the -w 100M flag for now, as the large window size may be contributing to buffer saturation.

Additionally, please verify the following:

  1. The server is bound to the correct LAN IP - Run “ip addr” to confirm, and ensure the -B flag references your LAN interface address, not a WAN or mobile interface.

  2. Run a quick TCP test to isolate the issue - Use “iperf3 -c -p 5001 -i 5” (no -u flag). If TCP works cleanly, the issue is specific to UDP handling.

In most cases, adding the -b flag resolves the issue immediately. Please give it a try and let us know how it goes.

Best regards,
V.

4

-w100m is a typo.

iperf3 -c (ip address) -u -p 5001 -b 100M correct command.

also wit 50M,20M or 1MB still packetless.
Only -b 55k no issues.

5

Greetings, @Appo ,

I hope you’re doing well,

Following the UDP iperf3 tests on your RUTX11 devices, please follow these steps to troubleshoot the packet loss issue:

  1. Allow UDP Traffic
  • Go to Network → Firewall → Traffic Rules and ensure UDP port 5001 is allowed between test endpoints.
  1. Check Attack Prevention
  • Navigate to Network → Firewall → Attack Prevention. Temporarily disable TCP flood protections (SYN flood) to rule out interference during tests.
  1. Reduce UDP Packet Size
  • Run iperf3 with smaller packets:

iperf3 -c -u -p 5001 -b 1M -l 512
4. Confirm Interface Binding

  • Verify the server binds to the correct LAN IP under Network → LAN using:

iperf3 -s -p 5001 -B
5. Direct Connection Test

  • Connect routers directly with a cable to rule out switches or intermediate devices.
  1. Firmware and Offloading
  • Ensure the latest RutOS firmware is installed (System → Firmware).
  • Toggle Software Flow Offloading if available (Network → Routing/NAT Offloading).
  1. Optional: Traffic Shaping
  • Use Network → Traffic Shaping to control UDP rates if needed for testing.

Note: The extreme packet loss at modest UDP rates indicates internal router limitations rather than network faults. These steps focus on confirming and mitigating those limits.

Best regards,
V.