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Beginner

802.1Q frame minimum Size

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.htmlNV32-min.jpg

 

IEEE implementation says 64 bytesNV31-min.jpg

dot1q header.PNG

 

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


Thanks

Siva

8 REPLIES 8
VIP Mentor

Re: 802.1Q frame minimum Size

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/
Beginner

Re: 802.1Q frame minimum Size

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?

VIP Mentor

Re: 802.1Q frame minimum Size

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.
Beginner

Re: 802.1Q frame minimum Size

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.

 

 

VIP Expert

Re: 802.1Q frame minimum Size

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.
Beginner

Re: 802.1Q frame minimum Size

Thank you Joseph W. Doherty 

can you confirm cisco is still following this or not?

Highlighted
Hall of Fame Expert

Re: 802.1Q frame minimum Size

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

 

Beginner

Re: 802.1Q frame minimum Size

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!! 

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