VLAN:Does the access port drop the frame when it received a frame with tag?

Unanswered Question
Aug 14th, 2013

The tag in the frame(vlan id)is in accord with the access port vlan id. The situation like one site is trunk mode port but another side is access mode port

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Vignesh Rajendr... Wed, 08/14/2013 - 08:46

HI Ti Huang,


Yes offcourse, it gets dropped. The concept of tagging is used so that a single interconnect (trunk) may be used to transport data for various VLANs and tagged frames get dropped at access ports.


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Thanks & Regards,


Vignesh R P

312038477a Thu, 08/15/2013 - 05:41

HI Vignesh R P,


Thank you! Some vender like HuaWei will accept the frame with tag in the access port if the tag vlanID is the same to the access vlanID.So,I am not sure CISCO will drop or not before you reply to me.


Best Regards,


Ti Huang

Peter Paluch Thu, 08/15/2013 - 06:14

Hi Vignesh,


I respectfully disagree.


Access ports, under circumstances, do accept tagged frames. The reason for this is the presence of 802.1p CoS priority bits in the 802.1Q VLAN tag. If a sender needs to perform Layer2 QoS marking, the only way to do it on Ethernet is to tag the outgoing frame and express the frame's priority in the CoS bits.


Older Catalyst switches such as 2950 happily accept tagged frames on access ports if the VLAN ID in the tag is either 0 or equal to the access VLAN of the port. Newer Catalysts such as 2960 or 3560 accept tagged frames on access ports only if there is a voice VLAN configured on the port, and in such case, tagged frames are accepted if the VLAN ID in the tag is set either to 0, access VLAN or the voice VLAN.


This behavior is not well documented - it may be necessary to test it in a lab.


Best regards,

Peter

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