As I discussed in previous tips, it is often necessary to adjust the MTU size to accommodate VPN traffic, as the additional headers associated with encapsulation often cause large packets to exceed the MTU size, resulting in fragmentation at some point in the network. If this fragmentation is in any way blocked by firewalls, routers or hosts, connectivity can be disrupted. Reducing the MTU size on the host will cause the host to segment its data into smaller packets that are less likely to be fragmented.
In a relative way, these advanced settings are very rarely changed, so the dialog on most hosts is often obscure, if it exists at all. For instance, in Microsoft's Windows, changing the MTU size usually requires a number of registry edits. If you need to lower your MTU size on a Windows machine, but editing your registry doesn't excite you, try this helpful tool: DrTCP.
As operating systems become more sophisticated, their internal software constructs become more complex. Virtual and sub-adapters are becoming more common. These types of adapters generally have more appropriate default settings than older adapters, but changing them can be a hassle. One example of this is Microsoft's PPPoE, which is commonly used by DSL providers. When you combine PPPoE with Windows XP, you're usually OK, because XP sets the MTU automatically. If that setting doesn't work for you, Microsoft's instructions for lowering the MTU for Window's XP's PPPoE can be found here: www.microsoft.com/windowsxp/pro/using/howto/networking/pppoe.asp.
Thomas Alexander Lancaster IV is a consultant and author with over ten years experience in the networking industry, focused on Internet infrastructure.