This has to be done at the source, not just at the router/firewall. I’ve read that it’s best if they do match however. For instance, if you set the MTU to 1300 on the ethernet port, set the MTU to 1300 on the firewall. If you set the MTU at the firewall to 1300 and not the source, packet loss will still occur.

Changing MTU size . - Cisco Community Hi, I have VLAN X with MTU 9216 on Switch-A. I also have VLAN Y with MTU 9216 on Switch-B. But not their SVIs are MTU 9216.They are not communicating to each other on packet size of above 1500. This is because SVI VLAN X and SVI VLAN Y connect to each other through SVI VLAN Z (Point-to-point Layer Where did my post go regarding PPTP MTU? Jan 06, 2010 Check & Set MTU Size? - TechGenix

How to configure MTU (maximum transmission unit) settings

Ping Test to determine Optimal MTU Size on Router | Answer Nov 28, 2016 Changing MTU size . - Cisco Community Hi, I have VLAN X with MTU 9216 on Switch-A. I also have VLAN Y with MTU 9216 on Switch-B. But not their SVIs are MTU 9216.They are not communicating to each other on packet size of above 1500. This is because SVI VLAN X and SVI VLAN Y connect to each other through SVI VLAN Z (Point-to-point Layer

How to configure MTU (maximum transmission unit) settings

VPN, Connection issues when connecting over IPv6 Aug 28, 2018 How do I set a custom MTU size for a virtual machine using