Please be aware these settings are for informational purposes only and change these values at your own risk. The most common reason to change the MTU is the inability to dial between sites. Often site to site connections will restrict the MTU size to reduce overhead. By default the Mitel MTU auto discovery would correct your MTU value if it is detected correctly. However, sometimes the ICMP request can be blocked causing an MTU value above what is possible.

For reference: Mitel Connect has a default MTU of 1500 and it is adjust upon discovery of a lower value if possible.

  • Step 1 Determine site MTU size
  • Step 2 Check switch MTU
  • Step 3 Change the global MTU value
  • Step 4 Verify MTU changed
Step1: Determining Site MTU
To quickly assess a WAN path’s PMTU, execute a series of ping tests using the following command at a Window’s CDM prompt from the remote end of the WAN tunnel connection; ping <HQ server IP> -f -l XXXX (e.g. ping <HQ server IP> -f -l 1400), where XXXX is the packet size.
ping string.png
Begin increasing or decreasing the packet size from this number in small increments until you find the largest allowable size that does not fragment and successfully pings.

1500 Fail
mtu 1500 fail.png

1400 Fail
mtu 1400 fail.png

1393 Fail
mtu 1393 fail.png

1392 Pass (This is our MAX MTU)
mtu 1392 success.png

Step 2: Checking our switch MTU
Now that we know our site MTU can be no more than 1392 let’s check one of our switches for the MTU value.

From the HQ server open CMD Then Type:
Ipbxctl –telneton <switch IP>
telnet <switch IP>
........Login: anonymous
........Password: ShoreTel

Then type gotoshell
cmd gotoshell.png

Once you see the ->
type ifShow (S must be capitalized)

Now look for the Maximum Transfer Unit size is XXXX
This number is the max MTU for the switch
gotoshell ifShow.png
If you leave this session open during step 3 you will see a notice stating the MTU has been changed.

Step 3: Changing the max MTU
Directly Login into the server and then open the registry aka regedit.exe

From within the registry editor navigate to the following key and edit it:
HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Shoreline Teleworks\Telephony Management Server\Settings Key Name= SwitchDebug

Add this parameter and the appropriate value to the end of the string ether_mtu_size max mtu here. Use your max value that you found from our previous ping tests.

Default Value= debug_options onhook_dialing_for_IP110 1.
New Value Example= debug_options onhook_dialing_for_IP110 1 ether_mtu_size 1472.

regedit mtu value.png
Saving this key instantly changes the MTU value globally.

Step 4: Verify the MTU Value
Repeat step 2 to see if the value has changed. Also note if you never closed the ipbxctl session from step 3 you would see this notice stating the change.
site mtu change verify.png