Try MTU sizes until you reach one that doesn’t work!!
NOTE: Cisco IOS has a feature built-in to do an MTU sweep!
function Global:Find-MTU {
[CmdletBinding(SupportsShouldProcess=$True,ConfirmImpact='Low')]
Param(
[parameter(Mandatory = $true, Position = 0)]
[System.Net.IPAddress]$IPaddress
)
$Ping = New-Object System.Net.NetworkInformation.Ping
$PingOptions = New-Object System.Net.NetworkInformation.PingOptions
$PingOptions.DontFragment = $true
[int]$Timeout = 1000
[int]$SmallMTU=1
[int]$LargeMTU=35840
[byte[]]$databuffer = ,0xAC * $LargeMTU
#action
While (-not ($SmallMTU -eq ($LargeMTU - 1))) {
[int]$xTest= ($LargeMTU - $SmallMTU) / 2 + $SmallMTU
$PingReply = $Ping.Send($IPaddress, $Timeout, $($DataBuffer[1..$xTest]), $PingOptions)
Write-Host "testing $($xTest + 28) byte transmission unit size"
if ($PingReply.Status -match "Success"){
$SmallMTU = $xTest
Write-Host "Good" -ForegroundColor Green
}
else{
$LargeMTU = $xTest
Write-Host "Bad" -ForegroundColor Red
}
Start-Sleep -Milliseconds 50
}
If($SmallMTU -eq 1){
Write-Error "The IP address $IPaddress does not respond."
}else{
Write-Host ""
$SmallMTU = $SmallMTU + 28
Write-Host "Your Max MTU is... $SmallMTU" -ForegroundColor Green
Write-Host ""
Write-Host "(Plz Note: 28 bytes were added to this number " -ForegroundColor Yellow
Write-Host "because 20 bytes are reserved for the IP header " -ForegroundColor Yellow
Write-Host "and 8 bytes must be allocated for the ICMP Echo " -ForegroundColor Yellow
Write-Host "Request header.)" -ForegroundColor Yellow
Write-Host ""
#pause
}
}
while($true)
{
#-- Get IP
$ipaddress = Read-Host -Prompt 'Enter IP/Hostname '
if($ipaddress -eq $null){ $ipaddress = "127.0.0.1" }
if($ipaddress -eq ""){ $ipaddress = "127.0.0.1" }
Find-MTU -IPaddress $ipaddress -Verbose
}