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New ISP Issues

New ISP Issues

New ISP Issues

I don't know if I'm in the correct forum. Please pardon me if I'm not.

We recently changed ISP's. They setup PPPoE which we were not running before. Now some of our software loads very slowly. QuickBooks in particular. Just after the ISP change we also loaded VPN client software (Shrew) on a couple of desktops (Windows XP Pro SP3). That is running super slow too. While troubleshooting that I noticed fragmented packets our MTU is 1464. I tried to edit the windows registry:


and that didn't help.

Our ISP's Tech told me to change the SonicWall TZ170 MTU setting (now 1500) to 1480, but I get an error:

Error: The MTU must be a value between 68-1500 in increments of 8 (e.g., 1492).

I'm not sure what to do or where to go to find an answer.


RE: New ISP Issues

If he said to change it to 1480, change it to any number that does not exceed 1480. 1480 is your maximum, if you like.

RE: New ISP Issues

I made the change to the sonic wall router this morning. I changed the MTU to 1476 and there seems to be no change. I'm continueing to monitor this issue, but the ISP tech is convinced that our sonic wall is the problem so he is bringing out an RB751U-2HnD to replace it free for one week. If that fixes the issue we will purchase the new router for $60.00. I don't know what a $60.00 router can do that the $300 one wouldn't or couldn't, but as long as it solves my issues I don't really care all that much. I'll continue to post any changes for the next person that has this issue.

RE: New ISP Issues

If you suspect MTU issues, you can quickly and easily rule them out as follows:
Assuming your max MTU is 1476, do the following pings:
ping with a size of 1477 with the DNF bit set (on Windows from memory something like ping -f -l 1477 www.google.com)
You should get "Packet needs to be fragmented but DF set." That's what you want to see.
Now, ping at your maximum MTU ping -f -l 1476 www.google.com
You should get a reply
Reply from bytes=1371 time=60ms TTL=56
Try different values and ensure that above a certain value, you get
"Packet needs to be fragmented but DF set"
and 1 byte less than that value gives you a proper ping response.
If, for example, you get "Packet needs to be fragmented but DF set" at 1477 bytes, then NO RESPONSE AT ALL at 1476 (and all values down to 1453) before finding you get a response at 1452, this indicates a problem. They call it a "black hole".
If you don't find a black hole, then you probably don't have MTU issues.

RE: New ISP Issues

Thanks Vince. As I alluded to in my initial post I did that and got a max of 1464. That's why I questioned the ISP tech's answer of putting in 1480. 1480 is what his router is set to though ours won't use that number. The closest without going over is 1476.

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