Troubleshooting Network Performance on a Sonicwall Firewall

Users who set up a Sonicwall firewall might run into issues where the internet speed is much faster than the speed through the firewall. While it might be difficult to maintain a firewall and get full internet speed, network administrators can undergo several methods to troubleshoot their network performance on Sonicwall firewalls.

Here are some basic troubleshooting steps to follow.

  1. Check the status of the WAN interface of the Sonicwall. Make sure that it the connection is full duplex, and at the correct speed. While interfaces will auto-negotiate their speed and duplex status, this might not set the correct mode. Users can set the interface to its proper status in settings.
  2. Do the same thing for the LAN interface. Users will need to consult the documentation for their switch to set the speed and proper duplex status.
  3. Set the Interface MTU to its correct value on the WAN interface. Use this ping command:

ping -f -l 1500

The -f modifier prevents the fragmentation of the ping command’s ICMP Echo Requests. -l sets the size of the request in bytes, which sends a 1500 byte request to, which accepts ping requests for testing purposes.

If the ping command doesn’t go through, reduce the -l modifier by 8 bytes on further requests until it works. Once there is an MTU value that works, add 28 to this number, and use that as the MTU size.

If the internet speed is still slower compared to the speed ahead of the firewall, there are a few things to check. In the Sonicwall security service settings, consider lowering from maximum security to performance optimized. If speeds now match internet speed, this is the source of the bottleneck.

If internet speeds are not up to par, try disabling these security features, and test after each one:

  • Content Filtering
  • Gateway Antivirus
  • Anti-Spyware
  • Intrusion Prevention
  • App Control

Be sure to turn these back on after testing. If disabling these features alleviates the bottleneck, it means the Sonicwall firewall is underpowered. It might be time to upgrade to a more powerful firewall for the network’s throughput and security needs.

Have more firewall troubleshooting questions? Contact our team of Sikich experts to help.

This publication contains general information only and Sikich is not, by means of this publication, rendering accounting, business, financial, investment, legal, tax, or any other professional advice or services. This publication is not a substitute for such professional advice or services, nor should you use it as a basis for any decision, action or omission that may affect you or your business. Before making any decision, taking any action or omitting an action that may affect you or your business, you should consult a qualified professional advisor. In addition, this publication may contain certain content generated by an artificial intelligence (AI) language model. You acknowledge that Sikich shall not be responsible for any loss sustained by you or any person who relies on this publication.

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