05-14-2012 01:53 AM - edited 03-07-2019 06:40 AM
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
05-14-2012 02:18 AM
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.
05-14-2012 02:33 AM
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
05-14-2012 02:37 AM
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A host presented with an unknown tagged frame should reject the frame. NB: some hosts can support tagged frames.
05-14-2012 03:54 AM
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
05-14-2012 04:31 AM
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
05-14-2012 04:54 AM
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
05-14-2012 05:12 PM
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.
05-14-2012 04:43 AM
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.
05-15-2012 09:14 AM
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 : 10 | Since 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 : 2 | Since 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 tag | The 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 tag | The 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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