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Community Member

Basic Question on Vlan Tagging

Hi Experts,

I would like to have a little clarity on Vlan tagging.

My understanding is, Switches tags the traffic received on a port (access port) when it passes it over a trunk link and strips off at the other end when it receives them.

The traffic between the host and the Switch is not tagged. but just curious to know what would happen if the host somehow receives the tagged vlan traffic? will it accept it or drop it?

-Vijay

9 REPLIES
Community Member

Basic Question on Vlan Tagging

I'm pretty sure if the traffic is soemhow not tagged it will be put in the native VLAN and tagged with VLAN 1 (if this is your native vlan).  ISL trunks encapsulate the entire frame, but 802.1q doesn't tag the native VLAN.

Hope that helps.

Community Member

Basic Question on Vlan Tagging

Hi Andy,

Thanks for your time. But my question was little different. sorry if I wasn;t clear.

My understanding is the traffic gets tagged with the VLAN when it passes over a trunk link . Correct me if I techincally go wrong.

Question is what happens if an access port receives tagged traffic? what does it do?  because the traffic on the access port is not tagged at all.

correct me if I technically misunderstood.

Thanks

-Vijay

Super Bronze

Basic Question on Vlan Tagging

Disclaimer

The  Author of this posting offers the information contained within this  posting without consideration and with the reader's understanding that  there's no implied or expressed suitability or fitness for any purpose.  Information provided is for informational purposes only and should not  be construed as rendering professional advice of any kind. Usage of this  posting's information is solely at reader's own risk.

Liability Disclaimer

In  no event shall Author be liable for any damages whatsoever (including,  without limitation, damages for loss of use, data or profit) arising out  of the use or inability to use the posting's information even if Author  has been advised of the possibility of such damage.

Posting

A host presented with an unknown tagged frame should reject the frame.  NB: some hosts can support tagged frames.

Community Member

Basic Question on Vlan Tagging

Thanks Joseph..

One final question though.

Host doesn't understand the vlans right? so how come this statement is valid?

A host presented with an unknown tagged frame should reject the frame.  NB: some hosts can support tagged frames.

-Vijay

Re: Basic Question on Vlan Tagging

Vijay,

Some hosts, generally servers, can tag frames depending on the type of NIC they have. Some NICs support tagging, but most workstation NICs do not. Workstations that don't would send their traffic untagged.

In response to your other question about tagging the traffic on an untagged port, it would drop the traffic. If you make a port an access port of vlan 10, and the host is tagging traffic for vlan 10, the switch is expecting the traffic to come into the port untagged. It will drop this traffic.

HTH,

John

HTH, John *** Please rate all useful posts ***
Community Member

Re: Basic Question on Vlan Tagging

Thanks John.,

one final question.

Lets say that I have a host connected to a switch and that port is configured as a Trunk port.

PC -------(trunk)-------------SW1 --------(trunk) --------------- SW2

so when the switch receives the tagged traffic and want to send it to the other switch SW2 does it now remove the tag (on receipt) and tag it again(while sending out to SW2)

-Vijay

Super Bronze

Re: Basic Question on Vlan Tagging

Disclaimer

The   Author of this posting offers the information contained within this   posting without consideration and with the reader's understanding that   there's no implied or expressed suitability or fitness for any purpose.   Information provided is for informational purposes only and should not   be construed as rendering professional advice of any kind. Usage of  this  posting's information is solely at reader's own risk.

Liability Disclaimer

In   no event shall Author be liable for any damages whatsoever (including,   without limitation, damages for loss of use, data or profit) arising  out  of the use or inability to use the posting's information even if  Author  has been advised of the possibility of such damage.

Posting

Lets say that I have a host connected to a switch and that port is configured as a Trunk port.

PC -------(trunk)-------------SW1 --------(trunk) --------------- SW2

so when the switch receives the tagged traffic and want to send it to the other switch SW2 does it now remove the tag (on receipt) and tag it again(while sending out to SW2)

First it depends on the switch, i.e. does it use tags internally.

Second, whether it actually removes and retags, or passes the original tag, it needs to verify the sent tagging as some trunks support "native" (i.e. untagged) frames and which VLAN is native can vary between ports.

Basic Question on Vlan Tagging

A look at an 802.1q tagged frame may help understand better.

http://www.cisco.com/en/US/tech/tk389/tk689/technologies_tech_note09186a0080094665.shtml#topic2

Note that the 802.1q tag is positioned immediately after the source MAC address.  The first field in the tag is the TPID (tag protocol identifier - 0x8100).  Also note what field would have been here were the frame not tagged - the Ethertype/length field.

Community Member

Re: Basic Question on Vlan Tagging

Thanks Summers.

I understand the concept of Tagging but just trying to under how does the switch behaves on some adverse scenario's.

I have put forth this table along with my undrestanding. Just wanted to confirm if my understanding is okay.

Lets assume a case that the switch is connected to a packet generator which places some frames on the link towards switch:

Switch   Port Operational Status:  Access
Vlan : 10
My understading:


case 1: The switch gets a traffic on that   port with a tag : 10Since the Switch is configured as an Access port , it would expect a   untagged traffic on this port. Since the port receives a tagged traffic, it   would drop the packet
Case 2:    The switch gets a traffic on this port with a tag : 2Since the Switch is configured as an Access port , it would expect a   untagged traffic on this port. Since the port receives a tagged traffic, it   would drop the packet
Case 3: The switch gets a traffic on this   port with no tagThe Switch process the traffic and attaches the tag Vlan10 when sent over   trunk link


Switch   Port Operational Status:  Trunk


case 1: The switch gets a traffic on that   port with a tag The switch removes the tag upon receiving the tagged traffic and   processes the traffic (re-tags if it has to be sent over another trunk link   or sends as it is if had to be sent to an access port)
Case 2: The switch gets a traffic on this   port with no tagThe switch takes this traffic as part of Native Vlan and process them.

Please let me know if I'm okay wiht my understading.

Thanks.

-Vijay

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