Investigating Bandwidth and Infrastructure Issues

A routine network performance test using iPerf has led to unexpected disruptions in multiple environments, raising concerns about infrastructure readiness and the true capacity of supposedly enterprise-grade networks. Some IT professionals have reported that running bandwidth tests with iPerf is causing network crashes, slowdowns, or even temporary outages, leading many to ask: Is the network really that slow — or is something else at play?

What Is iPerf, and Why Use It?

iPerf is an industry-standard tool used to measure network performance between two endpoints. It assesses various metrics such as bandwidth, latency, jitter, and packet loss, by sending streams of TCP or UDP data through the network. When used correctly, it provides accurate insights into available bandwidth and the health of the network.

But in some cases recently, especially in large enterprise networks or virtualized environments, iPerf has been doing more than just measuring performance — it's been pushing networks to the point of failure.

Problem Symptoms

System administrators and network engineers have reported:

Sudden loss of connectivity after initiating an iPerf test
High CPU utilization on routers or switches
Access point or firewall crashes
Network services going offline temporarily
Unusually low bandwidth readings just before the crash

These issues point to underlying constraints within the network rather than a flaw in iPerf itself.

Is Low Bandwidth Really the Issue?

The initial reaction to poor iPerf results might be to assume your network simply has inadequate bandwidth. However, that’s rarely the full story.

Here are some deeper reasons a network could "crash" during iPerf testing:

Common Causes of Network Failure During iPerf Tests

1. Overwhelming Legacy Infrastructure

Older switches, routers, or firewalls that are not optimized for high-throughput traffic may struggle when hit with data streams at full line speed. iPerf can easily saturate network links and overwhelm devices with limited processing capability.

Fix: Check hardware specifications and upgrade aging network devices where needed.

2. Improper QoS or Traffic Shaping Rules

iPerf can consume all available bandwidth on a network interface by design. If Quality of Service (QoS) or bandwidth throttling policies are not configured, the test can interfere with mission-critical services.

Fix: Implement QoS to properly prioritize essential services over test traffic.

3. Misconfigured iPerf Parameters

By default, iPerf attempts to use maximum throughput. Without limiting parameters, this can overload a network, especially on low-bandwidth or VPN links.

Fix: Use -b (bandwidth limit) or -u (UDP mode) flags to avoid overly aggressive testing.

4. Broadcast Storms or Multicast Issues

Improper deployment of iPerf in environments relying on multicast or VLAN trunking can result in broadcast storms, especially if loops exist or spanning tree isn't configured correctly.

Fix: Follow best networking practices including loop prevention (STP), VLAN isolation, and controlled test environments.

5. Virtualized Network Bottlenecks

When running iPerf from VMs, internal vSwitch bottlenecks or misconfigured virtual NICs can exaggerate performance issues or impact other hosted VMs.

Fix: Validate vSwitch performance limits and assign dedicated virtual NICs for testing.

Recommendations for Safe iPerf Testing

To safely test your network using iPerf without causing outages:

Run tests during off-peak hours
Limit bandwidth with flags:
iperf -c [server] -b 100M limits traffic to 100 Mbps.
Use multiple small tests instead of one large continuous stream.
Isolate test environment from production to prevent collateral damage.
Monitor equipment during tests for CPU and memory usage.