A useful command that displays your Windows network stacks calculated Path MTU, Estimated Path Speed and RTT time for your recent IP or IPv6 destinations!

I recently stumbled across this useful windows command to show what MTU value windows will try and use for your IP connections based on the source, next hop and destination address.

 netsh interface ipv4 show destinationcache

Interface 10: Ethernet

PMTU Destination Address                           Next Hop Address

---- --------------------------------------------- -------------------------

1500 4.153.29.52                                   192.168.1.1

1500 13.91.148.88                                  192.168.1.1

You can even add the level=verbose option to display Windows estimated path speed as well as the RTT Mean (average) and RTT Deviation (also known as jitter.)

In this example, I am also passing the address parameter so you can see the syntax if you want to drill down to a specific destination, the level=verbose param works fine with or without a destination specified :)

netsh interface ipv4 show destinationcache address=74.214.24.53 level=verbose

Destination              : 74.214.24.53

Next Hop Address         : 192.168.1.1

Source                   : 192.168.1.11

Interface                : Ethernet

Path MTU                 : 1500

Upper-layer MTU          : 1480

RTT mean                 : 16

RTT deviation            : 6

Path transmit speed (Bps): 10337720

Path receive speed (Bps) : 28962424

Link transmit Speed (bps): 1000000000

Link receive Speed (bps) : 1000000000

Flags                    : 0x0

To get the same info for IPv6, just replace ipv4 with ipv6 in your netsh command.