Minimum Size of an Ethernet frame

[szaborobert]

I am learning for the CCIE R&S and there is something I don`t understand. The minimum ethernet frame size is 64 bytes. I have just captured some frames and packets with ethereal in promiscous mode and there are some packets, frames which are 60 bytes. How can this happen?

STP

0000 01 80 c2 00 00 00 00 15 63 d4 b2 2b 00 26 42 42 ........ c..+.&BB

0010 03 00 00 00 00 00 80 00 00 08 21 8e 16 c0 00 00 ........ ..!.....

0020 00 13 80 01 00 15 63 d4 b2 00 80 2b 01 00 14 00 ......c. ...+....

0030 02 00 0f 00 00 00 00 00 00 00 00 00 ........ ....

TCP - showed in red, TCP RST

0000 00 14 a9 81 70 fe 00 11 85 69 db 00 08 00 45 00 ....p... .i....E.

0010 00 28 1f fb 00 00 80 06 67 7a c0 a8 19 02 c0 a8 .(...... gz......

0020 19 08 07 d0 c8 ef 00 00 00 00 5e 62 e3 94 50 14 ........ ..^b..P.

0030 00 00 e9 be 00 00 00 00 00 00 00 00 ........ ....

[szaborobert]

I have sent this question too early. I did some calculations and the only answer can be : The checksum is not included in the ethereal capturing. It is 4 bytes so if I add it to the 60 bytes I will have the minimum size.

[andrew.burns · Accepted solution]

Hi,

As you spotted, the checksum isn't shown by ethereal (now wireshark) because if it's wrong then ethereal won't even see the packet, but don't forget that the MAC frame is technically 72 octets minimum as you also have a 7 octet preamble and a 1 octet SFD (start of frame delimiter).

HTH

Andrew.