said by HarveyTheRabbit :

Whatever device is providing PPPoE authentication should be adjusting the MTU to 1492. This is common with not only the 4200 but most routers (like Linksys, D-Link, etc). So you do not need to change your MTU on a per PC basis.

It is also not a good idea to have the modem doing PPPoE and NAT, and then a router behind it doing NAT. Double NAT can cause problems with VPNs, gaming, etc... Ensure your router is doing PPPoE and the modem is a bridge.

If your router more advanced, like a Cisco, you will have to configure your router's MTU per interface. You may need to ensure that your TCP packets MSS values are set lower than your interface's MTU. I would recommend 1452. Under the interface configuration of your WAN Interface:

ip mtu 1492
ip tcp adjust-mss 1452

said by bsc :

If your clients are set to 1500 and they try to send through your router at 1492, every single large packet/segment/frame that goes out your WAN port will fragment!

Set every client to 1492 as well as the router if you want 100% efficiency!

said by GVG :

OK, so I am going to set all my computers and my router to 1492, that way when it hits the DSL modem everything is the right size from the start.

said by bsc :

Use your head dude. OK I wasn't specific enough with exact numbers but clearly you are smart enough to know what I meant.

If your hosts are sending 1493-1500 bytes packets THEY WILL FRAGMENT. If you are downloading large files, etc then pretty much everything is going to send at its MAX of 1500. So for normal web surfing and the like, sure it won't have an effect. But for downloads you will see inefficiency.

Will it absolutely destroy your speeds? Not in most situations, no, but its not optimal either.

said by evilghost :

Sorry you misread my post, I really wasn't trying to be a jerk, I'm not very strong with some of the aspects of networking so I was trying to wrap my head around the MTU and weight setting local MTU to 1492. In testing it apperas I fragment with a packet size greater than 1450 bytes.

How could I use tcpdump to detect fragmentation on live traffic flows?

said by bsc :

I apologize then, I definitely did snap there. I thought you were telling me I was wrong. We can still be friends!

I will elaborate a little.. the number you put in (1450) is essentially the raw data. This is going to be the SEGMENT size. This is a part of Layer 4 in the OSI model, where TCP and UDP live. Before the data gets sent out it gets headers added from Layer 3 (ICMP for a ping) and Layer 2 (Ethernet in most cases and in the case of DSL PPPoE headers get added here).

All in all in a PPPoE environment, you are going to add 48 (40 for ethernet and other header data and 8 for PPPoE) bytes to the SEGMENT SIZE before you send the data out. So the largest SEGMENT SIZE that will not fragment is 1452.

said by bsc :

Correct, fragmentation will not occur because the clients are sending 1492.

As you can imagine you are taking a slight hit on LAN transfer efficiency in order to maximize your WAN transfers. I think it is more than fair due to the high throughput of a LAN anyway.