VPN MTU

 The error you're encountering, "The VPN connection was terminated due to incorrect tunnel MTU," typically indicates an issue with the Maximum Transmission Unit (MTU) size for your VPN tunnel. The MTU determines the maximum size of data packets that can be transmitted through the network. If the MTU is set too high, packets may be too large to transmit through the VPN tunnel, leading to errors like the one you're experiencing.

Here's how you can resolve the issue:
1. Lower the MTU on the VPN Client (Cisco AnyConnect)
You can adjust the MTU on the client side to avoid the packet size issues.
Steps:
Open Cisco AnyConnect and disconnect from the VPN if you're currently connected.
Find and edit the configuration file (vpn_profile.xml or similar):
The configuration file is usually located in the directory: C:\ProgramData\Cisco\Cisco AnyConnect Secure Mobility Client\Profile on Windows or /opt/cisco/anyconnect/profile/ on Linux.
Look for the MTU setting in the configuration file. You might need to add or modify an entry for MTU:

<MTU>1400</MTU>
Lowering the MTU value to 1400 is a common practice to avoid fragmentation, but you may need to adjust this value based on your network configuration (try values like 1300 or 1500 if 1400 doesn't work).
Save the configuration file.
Reconnect to the VPN and test if the issue persists.
2. Adjust MTU on Windows (Network Adapter Settings)
If you're on a Windows machine, you can manually adjust the MTU for your network adapter.
Steps:
Open Command Prompt as an administrator.
Run the following command to find the name of your network interface:
netsh interface ipv4 show subinterfaces
Look for your VPN adapter (it will likely be labeled as "Ethernet" or "VPN" and will have a specific MTU size listed).
To change the MTU, run:
netsh interface ipv4 set subinterface "Interface Name" mtu=1400 store=persistent
Replace "Interface Name" with the exact name of your VPN network interface (e.g., Ethernet or VPN), and set the MTU to a lower value (e.g., 1400 or 1300).
After adjusting, disconnect and reconnect to the VPN.
3. Verify the MTU on the VPN Server Side (If You Have Control)
If you have access to the Cisco VPN server configuration, make sure the MTU is set appropriately. Misconfigured MTU settings on the VPN server can also cause issues on the client side.
4. Disable MTU Path Discovery (Optional)
If the issue persists, disabling MTU Path Discovery might help. This feature automatically detects the MTU of the network path and can cause issues in certain environments.
Open Command Prompt as an administrator.
Run the following command to disable Path MTU Discovery:

netsh interface ipv4 set global disablepathmtudiscovery=enabled
Reconnect to the VPN and check if it resolves the issue.
5. Check for Fragmentation
Ensure that there are no network devices (routers, firewalls, etc.) in your path that might be incorrectly handling fragmented packets. Sometimes, intermediaries on the network can drop or block fragmented packets.