Home > Switches > Monitoring and Reporting > Packet Captures and Port Mirroring on the MS Switch

Packet Captures and Port Mirroring on the MS Switch

Workstations in promiscuous mode can sniff LAN packets within their broadcast domain. A workstation connected to Cisco Meraki switches can capture these packets through port mirroring. This article will cover how to capture traffic passed by an MS switch, using the following steps:

  1. Enable port mirroring on your switch
  2. Connect a workstation on your destination port
  3. Capturing packets in promiscuous mode

 

Enable port mirroring on your switch

The most effective way to capture traffic passed on a given switchport is to mirror that port to another available port, so all traffic passed by the source port will be sent out on the mirrored destination port. The following steps outline how to mirror one or more ports on an MS switch:

  1. In Dashboard, navigate to Switch > Monitor > Switch ports.
  2. Select one or more ports to be mirrored. Note that multiple source ports can be mirrored to a single destination port.
  3. Click Mirror:

     
  4. Specify the destination mirror port, which will be used to capture traffic on the source ports. Note that both ports must be on the same switch, or within the same switch stack.
  5. Click Create port mirror:
     

Note: The MS switch does not support port mirrors involving aggregated ports.

Connect a workstation to your destination port

Physically connect a workstation to your destination port. Make sure DHCP is enabled on the host, and check that it receives a 169.254.X.X IP address.

a8e71f97-cfa4-4762-b883-962c8d2b5678

Using Wireshark for Packet Captures

Wireshark is a utility that will display the packets seen by a device. Packets contain the data that is transmitted between computers. Viewing this information can often aid in the diagnosis of issues that may be occurring in a network. It is possible for a device to not see all packets transmitted on a network if a device is hardwired. In this instance, the device may only see broadcast packets and packets addressed to itself due to the functionality of modern networking equipment.

Installation

Please visit Wireshark's download page to download Wireshark. When downloading, simply follow the prompts. 

Taking Packet Captures

  1. Open Wireshark.
  2. Click Capture Options.

Screen Shot 2018-03-08 at 12.37.36 PM.png

  1. Uncheck "Enable promiscuous mode on all interfaces", check the "Promiscuous" option for your capture interface and select the interface.

Screen Shot 2018-03-08 at 1.05.12 PM copy.png

Note: Rolling captures can be configured if required. This option will allow packets to be captured continuously without filling up the storage on your device.

  1. Click start. This will take you to a new window that will show the packets that the device is picking up. 

Screen Shot 2018-03-08 at 1.05.12 PM.png

  1. When the desired packets have been obtained click stop. 

Screen Shot 2018-03-08 at 1.10.57 PM.png

  1. Save the capture from the "File menu" with a distinct name. 

Rolling Captures

A "Rolling Capture" is a capture which automatically saves the output to files at set intervals and can break up a large capture into multiple smaller files. This can be extremely useful when trying to run a long-term capture for troubleshooting intermittent troubles such as choppy audio on VOIP.

Best Way to Run Rolling Captures

For some issues, it may be necessary to perform port mirrors or span port captures which run for long periods of time until the issue occurs. The goal is to run a capture and once the issue surfaces stop the packet capture. If a packet capture is run for a long duration of time, 6 hours, for example, the .pcap file will be too large for your computer to open as captures larger than 100mb become too difficult to open on some computers. To mitigate this trouble, the capture can be set with multiple different options which makes this easier. 

What is the Ring Buffer

Ring Buffers can be set to ensure that you will not fill up all of the disk space on your device. It will start overwriting the oldest file based off how many files you specify. This does not have to be used, but it is useful to ensure you do not fill up your HDD.

Taking a Rolling Capture
  1. Open Wireshark.
  2. Click Capture Options.

Screen Shot 2018-03-08 at 12.37.36 PM.png

  1. Uncheck "Enable promiscuous mode on all interfaces", check the "Promiscuous" option for your capture interface and select the interface.

Screen Shot 2018-03-08 at 1.05.12 PM copy.png

  1. In the "Output" tab, click "Browse...".

Screen Shot 2018-03-08 at 12.56.40 PM.png

  1. Enter a filename in the "Save As:" field and select a folder to save captures to. Click Save.

Screen Shot 2018-03-08 at 12.54.29 PM.png

  1. Select "Create a new file automatically after..." and "Use a ring buffer with x files". This creates a maximum of number of files, with each file set to the size or timeframe configured. For example, creating a new file automatically after 32 megabytes, with a ring buffer of 128 files, will provide 4 gigabytes of rolling captures.

Screen Shot 2018-03-08 at 1.02.53 PM.png

  1. Click start. This will take you to a new window that will show the packets that the device is picking up. 

Filtering Packet Captures

In certain instances, it can be beneficial to filter a capture for a specific client's IP address or for a specific type of traffic. This filtering can be done prior to the capture as well as after the capture, however, the filters used in these instances differ. 

What is a Capture Filter

A capture filter is a type of filter which is used to limit the type of data which is captured and saved to the file and is not used as frequently because of this. The syntax for these differs from the Display Filters. Additional information regarding the different filters and syntax which can be used as a Capture Filter can be found in the Wireshark database

What is a Display Filter

Display filters are the more common type of filter as they do not reduce the type of traffic which is being captured. This eliminates the possibility of having an incorrect filter applied and missing the traffic required in order to troubleshoot. This type of filter is applied inside of Wireshark when viewing the completed capture and outlined below. Additional information regarding the different filters and syntax which can be used as a Display Filter can be found in the Wireshark database

Using a Display Filter
  1. Open your packet capture
  2. Select the filter box.

Screen Shot 2018-03-08 at 1.14.35 PM.png

  1. Input filter string as provided by support engineer. Click the "Apply" button.

Screen Shot 2018-03-08 at 1.15.36 PM.png

  1. To save the filtered data, go to File -> Export Specified Packets...

Screen Shot 2018-03-08 at 1.19.16 PM.png

  1. Make sure that the "Displayed" radio button is checked and that the file has a unique filename. Once this is complete, select "Save".

Screen Shot 2018-03-08 at 1.21.06 PM.png 

Port Mirror Egress Modes

On Meraki switches, there are two egress modes for port mirrors:

  • True egress
    • Native VLAN traffic is untagged
  • Tagged egress
    • Native VLAN is tagged

 

The following table lists support for the two modes:

True Egress MS22, MS42, MS220, MS320, MS350, MS410
Tagged Egress MS225, MS250, MS420, MS425
Last modified

Tags

Classifications

This page has no classifications.

Explore the Product

Click to Learn More

Article ID

ID: 1167

Explore Meraki

You can find out more about Cisco Meraki on our main site, including information on products, contacting sales and finding a vendor.

Explore Meraki

Contact Support

Most questions can be answered by reviewing our documentation, but if you need more help, Cisco Meraki Support is ready to work with you.

Open a Case

Ask the Community

In the Meraki Community, you can keep track of the latest announcements, find answers provided by fellow Meraki users and ask questions of your own.

Visit the Community