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MTU size problem

E

Exterous

Super Moderator
He are in the process of acquiring another company that only has Bell Canada as an ISP provider. I've been having some problems with their service though:

Already I have adjusted the MTU size down three times (1492, 1438 and now 1400) on our Sonicwall firewall to deal with fragmentation issues. Looking at our connection sometimes a higher MTU works fine but sometimes it doesn't. I haven't dealt with a PPoE corporate setup before so it this normal? Am I just stuck at 1400 and hopefully that will continue to work?

Thanks
 
Have you tried 1500 ? In most cases 1500 will work trouble free & then some of'em will give better speeds with 1492 but as I said 1500 should do you no harm in the meantime !
 
R0H1T said:
Have you tried 1500 ? In most cases 1500 will work trouble free & then some of'em will give better speeds with 1492 but as I said 1500 should do you no harm in the meantime !
Click to expand...

Yeah - sorry - 1500 is the default and the first adjustment was down to 1492 after I started to encounter problems
 
The problem with adjusting network parameters is that you'll have to adjust other values, besides MTU, in conjunction with the ones that you've modified. For instance TTL, RWIN may also need to be adjusted accordingly !
 
There have been very few times in my career where I've had to reduce MTU to resolve a problem.

What problem are you have that makes you believe adjusting the MTU is a solution?
 
theevilsharpie said:
There have been very few times in my career where I've had to reduce MTU to resolve a problem.

What problem are you have that makes you believe adjusting the MTU is a solution?
Click to expand...

High latency and web traffic issues. The latency will stay at 70ms for days and web traffic will be fine. Then the latency jumps up in the 800+ms range and no websites can be reached. (They take forever to load or it just sits on 'waiting for...' for a long time before timing out). A 'ping www.google.com –f –l 14xx' will show that the packets are getting fragmented. I'll go through the motions of finding the MTU that doesn't fragment, adjust the Sonicwall and the problem instantly goes away - for several days at least

I've also occasionally see high latency (500+ms) on a hop between Chicago and Toronto and other times at our modem - but I can't find a pattern to that

R0H1T said:
The problem with adjusting network parameters is that you'll have to adjust other values, besides MTU, in conjunction with the ones that you've modified. For instance TTL, RWIN may also need to be adjusted accordingly !
Click to expand...

I'll take a look but wouldn't an issue there be more consistent? We've previously gone for 8 days with a lowered MTU and no issues
 
Last edited:
Packet fragmentation is not going to cause the latency that you see unless something is seriously broken between your host and the destination.

If switching the MTU is resolving your problem (if only temporarily), you may be running into a situation where fragmented packets are being sent out of order and ultimately dropped at the host. To prevent this without having to manually adjust your MTU downward, make sure that path MTU discovery is functioning by ensuring that your router (as well as your internal hosts) are allowing inbound ICMP Destination Unreachable messages.
 
you remember old RDP would freak OUT with fragmented packets?

quite honestly, leave mtu at default, do not lower it no matter what. I'd go jumbo and lower it down to keep the original packet size over the headaches you can create with mtu changes
 
Exterous said:
He are in the process of acquiring another company that only has Bell Canada as an ISP provider. I've been having some problems with their service though:

Already I have adjusted the MTU size down three times (1492, 1438 and now 1400) on our Sonicwall firewall to deal with fragmentation issues. Looking at our connection sometimes a higher MTU works fine but sometimes it doesn't. I haven't dealt with a PPoE corporate setup before so it this normal? Am I just stuck at 1400 and hopefully that will continue to work?

Thanks
Click to expand...


Use this guide to adjust your MTU setting:

http://www.dslreports.com/faq/695
 
Have you perhaps thought that you might be having a line/net connection issues?

I remember back when I used to work for Bell a case quite similar to what your explaining, and all in all it was an isp related problem due to a bad connection...
 
buffhr said:
Have you perhaps thought that you might be having a line/net connection issues?

I remember back when I used to work for Bell a case quite similar to what your explaining, and all in all it was an isp related problem due to a bad connection...
Click to expand...

Thats what I am leaning towards. Unfortunately their support is less than stellar.
 
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