802.1Q frame minimum Size

[sivam siva]

Hello all

Which option is the minimum frame size for an 802.1Q frame?
A. 64 bytes

B. 68 bytes

C. 1518 bytes

D. 1522 bytes

Cisco implementation says 68 bytes  

Frame Size

The 802.1Q tag is 4 bytes. Therefore, the resulting Ethernet frame can be as large as 1522 bytes. The minimum size of the Ethernet frame with 802.1Q tagging is 68 bytes.

https://www.cisco.com/c/en/us/support/docs/lan-switching/8021q/17056-741-4.html

IEEE implementation says 64 bytes

I believe 64 bytes is enough to detect the collision, no matter with/without Tag.

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Thanks

Siva

[Mark Malone]

Hi
A collision can even be detected before the full 64 is sent , good read on co;collisions in this link below

There are two types of collision:
– “ordinary” – collision that detected before 512 bits of frame were sent
– late – collision that detected after 512 bits of frame were sent

https://yurmagccie.wordpress.com/2017/06/21/ethernet-frame-and-csmacd/

[sivam siva]

Hello Mark 

Thanks for the reply,

My question is does cisco realy follows minimum required frame size with the tag as 68?

I should answer 64 or 68?

[Mark Malone]

Hi
yes Cisco support 64 as its IEEE standard , all vendors would as its the global standard that's set to work off so there is some consistence globally ,the 68 is just when the tag is added as a 4 byte extra

even the doc in a way states it , the minimum size with tagging is 68 , so without tagging which Cisco supports too is 64

The 802.1Q tag is 4 bytes. Therefore, the resulting Ethernet frame can be as large as 1522 bytes. The minimum size of the Ethernet frame with 802.1Q tagging is 68 bytes.

[sivam siva]

Thank you @Mark Malone

But if you see my question there I have attached a 64 bytes frame with tag,

That is actually taken from cisco switch cat 3550.

[Joseph W. Doherty]

Good question!

From an Ethernet collision detection viewpoint, frame should only need to be 64 bytes (512 bits) regardless for VLAN tag or not.

From a Cisco implementation standpoint, their hardware might not "expect" a frame to be smaller than 64 bytes when that hardware strips off the VLAN tag. If Cisco does have this requirement, it might not be required on all their hardware. If not, more likely required on "early" hardware.

[sivam siva]

Thank you Joseph W. Doherty 

can you confirm cisco is still following this or not?

[Giuseppe Larosa]

Hello Siva,

your understanding is correct.

the minimum size is 64 bytes because an 802.1Q tagged frame is STILL a standard ethernet frame.

This was more put in evidence when 802.1Q was compared with Cisco proprietary ISL. (30 bytes overhead 26 bytes header and 4 byte CRC, an ISL frame is NOT a standard ethernet frame but carries an ethernet frame in its payload using a tunnel encapsulation).

ISL does not support multiple Vlan tagging (like it is used in QinQ or other forms like provider backbone bridging ).

So this question was easier to answer many years ago when both 802.1Q and ISL were studied and compared.

Note: compliments for you questions they are becoming a challenge to answer.

Hope to help

Giuseppe

[sivam siva]

Thank you very much @Giuseppe Larosa

I'll never forget your help.

I'm still confusing what cisco expect me to answer in the exam!!