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MTU settings

douglasG
Dabbler
Posts: 23
Registered: 10-07-2012

MTU settings

Anyone know what the optimum MTU setting is for routers on PN please?
I know that 1500 is the default setting on the majority of newish routers and I believe most ISPs tend to trim the value for their servers.
Thanks in advance
4 REPLIES
Community Veteran
Posts: 26,718
Thanks: 931
Fixes: 10
Registered: 10-04-2007

Re: MTU settings

With the router MTU set at 1500 run the following command
ping -f -l 1472 ntp.plus.net

If that works, 1500 is the correct setting for you.
jelv (a.k.a Spoon Whittler)
   Why I have left Plusnet (warning: long post!)   
Broadband: Andrews & Arnold Home::1 (FTTC 80/20)
Line rental: Pulse 8 Home Line Rental (£13/month)
Mobile: iD mobile (£4/month)
Community Veteran
Posts: 4,868
Thanks: 126
Fixes: 24
Registered: 14-07-2009

Re: MTU settings

My router MTU was set at 1492 and the reply came back
"Packet needs to be fragmented but DF set"
I then changed it to 1500 at got
"Destination net unreachable"
What's the right answer?
douglasG
Dabbler
Posts: 23
Registered: 10-07-2012

Re: MTU settings

thanks jelv.
I'll give that a try.
Community Veteran
Posts: 26,718
Thanks: 931
Fixes: 10
Registered: 10-04-2007

Re: MTU settings

@ReedRichards
"Destination net unreachable" is not the right answer!
You should get (note that because of the larger packet size, latency will be increased from a normal ping):
C:\Users\John>ping -f -l 1472 ntp.plus.net
Pinging ntp.plus.net [212.159.13.50] with 1472 bytes of data:
Reply from 212.159.13.50: bytes=1472 time=37ms TTL=250
Reply from 212.159.13.50: bytes=1472 time=36ms TTL=250
Reply from 212.159.13.50: bytes=1472 time=36ms TTL=250
Reply from 212.159.13.50: bytes=1472 time=35ms TTL=250
Ping statistics for 212.159.13.50:
    Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
    Minimum = 35ms, Maximum = 37ms, Average = 36ms

If you reduce the 1472 in steps of two you will find the maximum packet size that will be accepted without fragmenting. The router MTU should then be set 28 higher than that figure.
We have in the past seen issues where particular Plusnet gateways have had a problem with MTU 1500. I'd have hoped that these were long behind us, but it might be worth seeing if you get the same result on a different gateway when using 1500.
(Note this guidance relates to PPPoA connections only, for PPPoE connections the maximum possible MTU is 1492)
jelv (a.k.a Spoon Whittler)
   Why I have left Plusnet (warning: long post!)   
Broadband: Andrews & Arnold Home::1 (FTTC 80/20)
Line rental: Pulse 8 Home Line Rental (£13/month)
Mobile: iD mobile (£4/month)