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I'm wondering, because I captured traffic on a network link that handles "jumbo" frames, but the capture NIC I used had the MTU set at the default 1500 value. I looked at the resulting .pcap file, and I do have frames with byte sizes exceeding 1500. So can anyone confirm that the promisc function of the NIC is not subject to the MTU set on the NIC? Or must one set the MTU size on the capture NIC to say 9000 to be able to capture jumbo frames without incident?

asked 22 May '12, 20:49

willdennis's gravatar image

accept rate: 0%

The MTU size does not limit capturing (receiving) large packets, as it's the Maximum Transmission Unit (MTU) ;-)) There might be issues with your nic driver if we talk about jumbo frames, but that's a different problem.

Try yourself:

ifconfig eth0 mtu 500

Now capture some traffic. You will see packets > 500 bytes.

See also this question:


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answered 22 May '12, 23:03

Kurt%20Knochner's gravatar image

Kurt Knochner ♦
accept rate: 15%

edited 23 May '12, 06:37

Sometimes it pays to remember what the acronym stands for :) Thanks, Kurt!

(23 May '12, 05:29) willdennis

My guess would be that the NIC implements Postel's law

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answered 22 May '12, 21:57

Jaap's gravatar image

Jaap ♦
accept rate: 14%

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question asked: 22 May '12, 20:49

question was seen: 6,305 times

last updated: 23 May '12, 06:37

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