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Find your correct MTU size

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Find your correct MTU size

Finding the largest MTU size for better connection speeds.
The best value for MTU is that value just before your packets get fragmented. How do you find out that? By using Ping at an MSDOS command prompt.

Go to Start/ Programs/ MSDOS-PROMPT, and type....

ping -f -l 1472 www.plus.net

Press Enter. Then reduce 1472 by 2 until you no longer get the
"Packet needs to be fragmented but DF set.”
Then increase by 1 until you are 1 less from getting
"Packet needs to be fragmented but DF set." message again.
You know when you are there when it pings successfully.
“Reply from 212.159.8.137: bytes=1472 time=78ms TTL=248”

Add 28 more to this (since you specified ping packet size, not including IP/ICMP header of 28 bytes) And this is your MaxMTU.

Note:If you can ping through with the number at 1472, you are done! Stop right there. Add 28 and your MaxMTU is 1500.(MAX size).

If not and you get this:

Pinging portal.plus.net [212.159.8.137] with 1472 bytes of data:

Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.

Ping statistics for 212.159.8.137:
Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),

If you can not ping through with the number 1472, try 1464 if this works then stop there. Add 28 and your MaxMTU is 1492

Result:
Pinging portal.plus.net [212.159.8.137] with 1464 bytes of data:

Reply from 212.159.8.137: bytes=1464 time=78ms TTL=248
Reply from 212.159.8.137: bytes=1464 time=76ms TTL=248
Reply from 212.159.8.137: bytes=1464 time=77ms TTL=248
Reply from 212.159.8.137: bytes=1464 time=89ms TTL=248

Ping statistics for 212.159.8.137:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 76ms, Maximum = 89ms, Average = 80ms

Finally change your MTU settings on your router,(Netgear DG834GT) on WAN setup window.
7 REPLIES
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Find your correct MTU size

My MTU was/is already at 1500 and unfortunately I disconnect regularly. But I'm using a Cisco router.
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Find your correct MTU size

I would check your filters and substitute a brand new filter for your old ones, i found my netgear supplied filters were not very good, even the new netgear ones were faulty.
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Find your correct MTU size

Hi,

Thanks for the suggestion but i've replaced every item of cabling and hardware and even tested both of my routers against a known working connection, but, the issue remains.

I wish your MTU suggestion was a solution but alas...
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Find your correct MTU size

if you do this from the pc it only gives you the mtu for the pc not the router. You need to do this on the router if you can so you canget the routers mtu.

Chances are they are both the same but its not garenteed
Community Veteran
Posts: 1,229
Thanks: 1
Registered: 30-07-2007

Find your correct MTU size

Sometimes you get "request timed out" instead of the fragmentation message when pinging through a router (or at least that's what happens here) so that could confuse the unwary
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Find your correct MTU size

Quote
Is your cup half full or half empty?


I can guarantee that this works if you follow the instructions properly, I am running this from a PC running Win XP, This is from a technical document for testing purposes, i have propelled my speed to 4Mbps and is constant.
Also my filters & router has reduced the line noise at can support 8Mbps with a little help from some technical white papers covering wiring.
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Find your correct MTU size

any Ideas on how to change the MTU on a BT1250 wless router?