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VPN, Connection issues when connecting over IPv6

IPv6 addresses and headers take up more space in the data packet than IPv4 addresses and headers do. Because of this some users are not able to connect to the VPN at all, and others can connect but can't download files, read email, or do other things that use large data payloads in their data packets. Adjusting the packet MTU to a lower value will make sure that there is enough space in the packet for the larger IPv6 headers.

This only affects customers that connect over IPv6.  Cisco's AnyConnect (or Cisco Secure Client) software will always use IPv4 if it is available, so this will mostly affect customers using openconnect, or customers that only have IPv6 (which is rare).  The default MTU for wireless and Ethernet is 1500 bytes.  When using IPv6, especially if it is being tunneled, you'll need to set it down to between 1380 and 1450 depending on the your setup.

How can you tell if this is the problem?
  1. If you can't connect at all and your client just times out trying to connect (and is using IPv6 to get to the VPN*), then first check to see if you can ping6 the vpn (unix/Mac OS command is "ping6"). If that doesn't work, this is not the problem.
  2. If ping6 worked, then see if you can load the website over IPv6. (or any of the VPNs).  If it loads, this probably isn't the problem.  If it loads, and the VPN connects, but then some things don't work, it might be the problem.
  3. If ping6 worked, but loading the website did not work, then there is a good chance this is your problem. Please try changing your MTU setting and see if that fixes the issue.  
If you think this is the problem you are having, then you need to set your MTU size down to at lest 1450, possibly as low as 1380 to make everything work.  

Mac OS: 
  1. Go to Network settings 
  2. lick on the interface being used for their network connection, if it is not already selected.
  3. Click on the "Advanced" button. 
  4. Click on the "Hardware" tab. 
  5. Change the "Configure:" drop down to Manually.
  6. Change the "MTU:" drop down to Custom.
    1. Mac OS MTU window 
  7. Type in 1380, click Okay, and then click Apply.

Windows 7, 8, and 10:

Follow the instructions from this website: but replace "ipv4" in all the commands with "ipv6". 

Open a command prompt

  1. Click the Windows button on the task bar.
  2. Click All Programs.
  3. Click Accessories.
  4. Right-click on Command Prompt and click Run as administrator
  5. If prompted click the Allow button.

Set the MTU size:

  1. Once the Command Prompt window is open follow the steps below to change the MTU size:

    1. Type netsh interface ipv6 show subinterface
    2. Press Enter.
    3. You will see a list of network interfaces.
    4. Type netsh interface ipv6 set subinterface “Local Area Connection” mtu=1450 store=persistent
      You should replace Local Area Connection with the name that appeared in the “Interface” column from steps 1-3.
    5. Press Enter.
    6. Restart you computer and then test again.

    If you still have problems after modifying the MTU repeat the above steps - replacing the numbers 1458 with 1430, or 1380 – restart the computer and test again. 



  1. In Linux there are multiple ways to do it. Here are two possibilities:
    1. If you are using openconnect, use the "-m "option to specify the MTU like this
      1. openconnect -m 1380 -v
    2. Otherwise, after the vpn has connected, adjust the mtu on the tunnel interface that was created (in this example the tunnel was tun0)  
      1. ifconfig tun0 mtu 1380

*How to tell if you are connected to the VPN over IPv6

First, check to see what IP address(es) your computer has.  Go to and see what it says.  If you only have an IPv4 address, you can't be connecting over IPv6.  If you only have an IPv6 address, then you are definitely connecting over IPv6.
If you have both an IPv4 and an IPv6 address and you aren't able to connect at all, it's hard for you to tell what address you're using to connect with to the VPN. As a general rule of thumb, if you are using the Cisco AnyConnect software it will always use IPv4 if it has one. If you are using openconnect or some other free client, it is likely using IPv6 - most open source software will try IPv6 first.  If you absolutely need to know, contact the Technology Services Help Desk, they can look up your connection in the logs and see what IP address you connected with.
If you are able to connect, but things aren't working, you can see what the IP address of the server you're connected to is in the VPN application. This will let you know if it is IPv4 or IPv6.  Open the statistics window (on Mac click on the graph icon on the connection window, on Windows click on the gear icon on the connection window, then select the statistics tab). Then look for the line named "Server" to find the server's IP address.  
If the address is in IPv6 format (up to 8 hex numbers, separated by colons - note that there can be fewer than 8 if there is a double colon - such as 2620:0:e00:3a::2) you are connected to the server over IPv6.
client window showing IPv6 address
If the address is in IPv4 notation (4 decimal numbers separated by periods such as you are not connecting over IPv6. 
Client window showing IPv4 address

KeywordsVPN IPv6 problems MTU MSS   Doc ID85269
OwnerNetwork E.GroupUniversity of Illinois Technology Services
Created2018-08-27 13:43:16Updated2023-08-14 09:22:31
SitesUniversity of Illinois Technology Services
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