Microsoft KB Archive/314496

= Default MTU Size for Different Network Topology =

Article ID: 314496

Article Last Modified on 12/1/2007

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APPLIES TO


 * Microsoft Windows XP Professional x64 Edition
 * Microsoft Windows XP Professional
 * Microsoft Windows XP Home Edition

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This article was previously published under Q314496



For a Microsoft Windows 2000 version of this article, see 140375.



SUMMARY
The Maximum Transfer Unit (MTU) specifies the maximum transmission unit size of an interface. Each interface used by TCP/IP may have a different MTU value specified. The MTU is usually determined through negotiation with the lower-level driver and by using that lower-level driver value. However, that value may be overridden.



MORE INFORMATION
Each media type has a maximum frame size that cannot be exceeded. The link layer is responsible for discovering this MTU and reporting it to the protocols above the link layer. Network Driver Interface Specification (NDIS) drivers may be queried for the local MTU by the protocol stack. Knowledge of the MTU for an interface is used by upper-layer protocols, such as TCP, which automatically optimizes packet sizes for each medium.

If a network adapter driver, such as an asynchronous transfer mode (ATM) driver, uses local area network (LAN) emulation mode, it may report that it has an MTU higher than is expected for that media type. For instance, the network adapter may emulate Ethernet but report an MTU of 9180 bytes. Windows accepts and uses the MTU size reported by the adapter even when it exceeds the normal MTU for a given media type.

The following is a summary of default MTU sizes for different network media.   Network                  MTU (bytes) ---  16 Mbps Token Ring        17914 4 Mbps Token Ring         4464 FDDI                      4352 Ethernet                  1500 IEEE 802.3/802.2          1492 PPPoE (WAN Miniport)      1480 X.25                       576

Keywords: kbinfo kbnetwork KB314496

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