Specifications

Mediatrix 4100 175
C HAPTER
19
Maximum Transmission Unit
(MTU)
This chapter describes the MTU (Maximum Transmission Unit) requirements of the Mediatrix 4100.
What is MTU?
The Maximum Transmission Unit (MTU) is a parameter that determines the largest packet than can be
transmitted by an IP interface (without it needing to be broken down into smaller units). Each interface used
by TCP/IP may have a different MTU value specified.
The MTU should be larger than or equal to the largest packet you wish to transmit unfragmented. Note that
this only prevents fragmentation locally. Some other link in the path may have a smaller MTU: the packet will
be fragmented at that point, although some routers may refuse packets larger than their MTU.
Mediatrix 4100 MTU
The Mediatrix 4100 MTU is 1500 bytes, which is the Ethernet typical value.
Possible Hardware Problem
The implementation of the IEEE Standard 802.1q in the Mediatrix 4100 may have a minor problem because
of hardware limitations.
802.1q increases the Ethernet frame header by 4 bytes, adding a Virtual LAN ID and a user_priority. This is
useful to limit broadcasts that cross bridges, and it may also prioritize frames in the queuing algorithm of
switches. However, it also increases the maximum possible size of Ethernet frames from 1518 to 1522 bytes,
and this might not be handled adequately by every hardware.
A workaround is available for PCs running Windows to avoid sending 1522 bytes packets (note that this
happens only in special and rare cases). The workaround is to reduce the MTU of the interface (the one that
sends packets with 802.1q framing) by 4 bytes.
1. Use the registry editor (regedt32) and go to the key:
Windows 2000 and later:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces
\<ethernet adapter>
Windows NT4 and 98:
\HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\<ethernet
adapter>\Parameters\Tcpip
where <Ethernet adapter> can be found by using the command “ipconfig /all”.
2. Add (or modify) a value named MTU of type REG_DWORD. Set it to 1496 (instead of 1500), in
decimal. Restart the computer to have those changes in effect.
In Windows 2000 and later this value is under the following key:
•Key: Tcpip\Parameters\Interfaces\ID for Adapter2