What is Port Mirroring?

Quick Definition: Port mirroring is a networking functionality that duplicates packets on specified ports or VLANs and forwards them to a specified port for further analysis. This functionality is easy to configure and can be helpful for troubleshooting and other scenarios.

In the field of networking, ports can be configured to do (or not do) many things, and they can be configured to allow or deny specific actions based on an individual rule or set(s) of rules. One of the many capabilities of ports is called port mirroring, which is what we will focus on in this article. 

Port mirroring is a capability of most modern network devices used to monitor traffic for performance and security issues. Simply put, port mirroring duplicates network traffic and sends one copy to the intended recipient and another to a monitoring port for analysis.

It’s important to note that port mirroring differs from flow mirroring because port mirroring duplicates and forwards all data to the monitoring port, whereas flow mirroring typically forwards data only when packets meet specific filters and criteria.

What is Port Mirroring?

Port mirroring creates copies of network traffic and passes those duplicate packets to a dedicated port for various purposes, including troubleshooting, compliance audits, and security analysis. We’ll elaborate on these in the next section, but first, let’s discuss the types of port mirroring that are available. 

Local Port Mirroring

Local port mirroring is the most common form of port mirroring that comes to mind when you hear the term. This form of port mirroring involves copying packets from ports on a switch to another dedicated port on the same switch. This option offers the lowest hop count since the traffic stays on the same switch. 

Remote Port Mirroring

Remote port mirroring differs from local port mirroring only in that the traffic is sent to a switch different from the point of origin. While this shouldn’t significantly impact performance, it does add to the total hop count for each packet. The further the designated port, the more hops and time packets will take to reach their destinations.

Encapsulated Port Mirroring

Encapsulated port mirroring, also known as encapsulated remote port mirroring (ERPM), is a form of port mirroring that involves sending duplicate packets across the network via layer 3. 

Multiple source ports can have their traffic sent to a single destination port. Encapsulated port mirroring is what most often comes to mind when we think of Network Operations Centers monitoring traffic for performance issues and other anomalies.

When Do You Use Port Mirroring?

Let’s discuss the use cases for port mirroring. We briefly mentioned troubleshooting, compliance audits, and security monitoring as use cases for port mirroring, but there are additional use cases as well. 

First, this technology can be used to troubleshoot and validate recent changes. Let’s say you’re a network engineer who recently implemented a change to your configuration that you believe will help optimize your network. 

You could wait for complaint tickets to roll in, or you could use port mirroring to validate whether your new network configuration is providing the desired outcome. 

Port mirroring can also be used as a security feature, mainly for detecting and preventing malicious behavior. Through port mirroring, you can monitor and analyze traffic as it is sent and received, which gives security analysts and network administrators insight into potential denial of service attacks, data exfiltration, and other anomalous behavior that might be indicative of an attack. 

Additionally, port mirroring can be used to detect a lack of encryption where plaintext is unacceptable. 

Port mirroring can also help during compliance audits. Depending on the specific audit, you may need to verify proper network segmentation, showing that specific data only traverses certain specified VLANs. Port mirroring can validate assertions that data is properly segmented. 

Finally, port mirroring can help with network performance and optimization. Maybe your network is running fine, and no one is complaining about latency or dropped packets, but what if it could be even better? 

Using port mirroring to examine how traffic actually flows through your network is a great way to detect opportunities for improvement. This can include identifying bottlenecks, decreasing hop count, and more. 

How to Implement Port Mirroring

If you want to configure port mirroring on your networking devices, you should first confirm that they are capable of doing so. You can usually confirm this in the documentation specific to your devices or by contacting your vendor for assistance. 

When modifying your existing network configurations, it is important to refer to the documentation specific to your equipment. Thankfully, configuring port mirroring is relatively simple across most network devices. 

Port mirroring can be configured using just a few commands. The first command specifies the source port or VLAN, and the second command specifies the destination port. You can choose to monitor traffic from an entire VLAN or just a single port, but you can only specify a single destination port. 

Configuring port mirroring on Cisco devices may look something like this:

You would then confirm that your changes have been saved by using the command monitor session 1 to display the current configuration settings.

Port Mirroring Limitations and Challenges

As with any technology, port mirroring has some potential drawbacks, but many can be avoided through due diligence prior to and during configuration. 

One of the first things you should consider is whether your network has the bandwidth to support port mirroring. You won’t be able to monitor traffic accurately if your packets are being dropped due to limited bandwidth and other performance issues. Moreover, port mirroring would likely cause more issues than it would solve. 

Another key consideration is attention to detail when modifying your configurations. As we saw in the configuration example in the previous section, networking involves a lot of numbers and letters that can mean entirely different things if they’re off, even by just a little. 

Double and triple-check that you are configuring the correct sources and destination before finalizing your configurations. It’s even better if you have your configuration changes in a script that goes through some form of change review process to get additional review of the port information prior to implementation. 

One final concern with port mirroring is that it is CPU intensive. You’re doubling the amount of data being sent between the source and destination, and the more sources you have, the more resources will be used. 

This may lead to increased latency to some extent. Whether your users will notice depends entirely on the total amount of data being copied, your network’s bandwidth, and your equipment’s processing capabilities. 

Conclusion

Port mirroring is a networking feature that allows one or more ports to send duplicate packets to a specified port for analysis. This analysis can be used for many reasons, such as helping troubleshoot ongoing network issues, monitoring for anomalous behavior and security risks, verifying compliance with the various frameworks, and enhancing network operations overall. 

While relatively easy to configure, port mirroring does have a few potential drawbacks, such as increased latency and being CPU-intensive. 

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