• SonicOS 7.1
• About SonicOS
  • Working with SonicOS
  • SonicOS Workflow
  • How to Use the SonicOS Administration Guides
  • Guide Conventions
• Introduction to Diagnostics for Classic Mode
• Terminal
• Tech Support Report
• Check Network Settings
• DNS Name Lookup
• Network Path
• Ping
• Trace Route
• Real-Time Blacklist
• Reverse Name Lookup
• Connection TopX
• Geo and Botnet
• MX and Banner
• GRID Check
• URL Rating Request
• PMTU Discovery
• Switch Diagnostics
• SonicWall Support
  • About This Document

PMTU Discovery

PMTU Discovery is a diagnostic tool that uses a standardized technique for determining the maximum transmission unit (MTU) size on the network path between two Internet Protocol (IP) hosts, usually with the goal of avoiding IP fragmentation. PMTU Discovery works with both IPv4 and IPv6 protocols.

To troubleshoot with PMTU Discovery:

1. Navigate to Device > Diagnostics > PMTU Discovery.
2. Type the IP address or domain name of the destination host in the Path MTU Discovery to this host or IP address field.
3. In the Interface drop‐down menu, select which WAN‐specific interface you want to test the trace route from. Selecting ANY, the default, allows the firewall to choose among all interfaces—including those not listed in the drop‐down menu.

4. Click GO.

   Depending on the route, this may take a few minutes. A pop-up table displays with each hop to the destination host. By following the route, you can diagnose where the connection fails between the firewall and the destination.