I recently moved from cable modem to DSL. The move was because of poor customer service provided by Charter the local cable company. When I made the move, I ran into some unexpected problems. These problems had me stumped and confused. This doesn’t happen much when it comes to computers.
I spent hours on hours trying to figure out why I could not have an IPSec tunnel to my office via the network topology that I had configured. All I have is a LAN -> Cisco -> DSL modem.
After 6 hours and 5 Cisco configs later, someone responded to my post on dslreports.com about the issues I was having. Everyone said “sounds like an MTU problem”.
For those that don’t know what MTU is, it stands for Maximum Transmission Unit and it refers to the size of the largest packet that a given layer can pass onwards.
Anyhow, the problem with a DSL modem is that you actually are NATING it even though you may have a public IP address. The modem actually has the IP address bound to it and it forwards it to your machine (or router in this example). Well, if you have a router before your computers, you have to again, NAT it (Nating is where the router re-writes the source/destination address as a packat passes through a router).
As it does this, the packet overhead changes. This is when you need to adjust your MTU on your machine.
I had read an article about it on dslreports.com and people recommended to me that I adjust my MTU to 1458. I did this, restarted my adapter, and everything worked like a charm.
I never though it would be handy in the hosting field, but I recently just helped with a customer with the same Cisco -> Modem configuration. None of the machines in his network could send any emails so I exhausted all my resources trying to figure out why. Once I heard about his network configuration, I felt it was the last chance to fix this guys network.
I told him to download DrTCP (http://www.dslreports.com/drtcp) and run it. When he opened it, I had him select his network adapter in the network settings and type in 1458 and click “save”. I next had him go to his control panel -> network settings and disable and enable his network adapter ( a restart would have done the same thing ). He went into outlook and clicked send / receive and all 14 messages that were waiting to go out went out successfully.
I never though the MTU would ever be back in my life again.. but now I know that this is an important thing to know about if you have a more advanced network then a typical home would.
I hope this article helps you as much as it has helped me.