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bigcw
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Path MTU discovery

Fri Sep 15, 2017 2:16 pm

Is there a built in tool to discover the path MTU?

If not, has anyone written a script to do it?

(At present I do it manually by doing /ping <ip> do-not-fragment count=1 size=1500 then decrementing size until I see a ping returned)

Chris
Ecom International Network - Operators of AS61337 with POPs in Europe and North America - www.ecomltd.co.uk
Colocker Data Centre - The data centre with a difference! - www.colocker.com
 
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saaremaa
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Re: Path MTU discovery

Fri Sep 15, 2017 3:29 pm

https://www.iea-software.com/products/mtupath/
MTU Path is a utility to assist operators in troubleshooting MTU related network configuration problems by finding the largest one-way path MTU (PMTU) to a remote host or router.
The software works by sending a small number of messages toward a host analyzing feedback from host and intermediate routers to determine largest packet size the network path will accept.
CMDR Saaremaa (Gutamaya Sierra Alpha Alpha)
 
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bigcw
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Re: Path MTU discovery

Fri Sep 15, 2017 3:38 pm

That's great, but I want to do it on the router itself.
Wrote a script...
:local yes
:local no
:local test

#Set upper and lower test limits below
:set yes 1000
:set no 1550

while (($no-$yes) > 1) do={

  :set test (($no-$yes)/2 + $yes)
  if ([/ping **YOUR TEST IP HERE** do-not-fragment count=1 size=$test] = 1) do={ :set yes $test } else={ :set no $test }
  
}

:put $yes
There might be a simpler way of doing it, but if not I hope it is useful to someone.

Chris
Ecom International Network - Operators of AS61337 with POPs in Europe and North America - www.ecomltd.co.uk
Colocker Data Centre - The data centre with a difference! - www.colocker.com

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