Broadcom 57XX MTU

Question

We appear to have an issue here with Broadcom 57XX NICs, in that the maximum size ethernet frame we can put on the wire is much less than the 1500 byte ethernet maximum.   Allowing for headers, I really should be able to put 1460 bytes of data in a packet, yet I can only put 1364bytes without fragmentation occurring (verified with ping -f -l 1365 x.x.x.x)   We've no additional encapsulation/framing/tunneling going on, can anyone shed any light on this?   Thanks, Matt

MattR_uk · Answer

Thanks for the suggestion, but the question relates to basic LAN connectivity rather than finding the maximum mtu over a downstream link.  Even with a hard-coded MTU at 1492, Im not able to put anymore than 1364 byts in a packet without fragmentation.   Thx

jmwills · Answer

Go to DSL Reports and run Dr TCPIP and that program will recommend a number for your MTU.  1492 seems to be the popular unit.

NemesisDB · Answer

probably won't help but make sure you have the latest drivers: http://www.broadcom.com/support/ethernet_nic/downloaddrivers.php

MattR_uk · Answer

thanks for the suggestions; found the problem.   Some IPsec Client software does MTU 'tuning' behind the scenes.  Checking with what we have, it seems Cisco defaults to some low value around 1300 and Lucent to around 1400.  These values overide any Windows MTU settings you may put in (since they're higher up the stack) The Broadcom thing was a red-herring caused by incorrect driver bindings.

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