native vlans and tagging

Unanswered Question
Nov 16th, 2007

Hi all, I know i have mentioned this in the other forum, but i need a bit more clarity.

If I say have a pc plugged into vlan 2, pvid of 2, i gather this means that if ant frame comes in untagged it gets put into vlan 2 right ? ok, well my confusion is what happens to this when it goes over a trunk port, it I put the trunk as member of vlan 1,2,3,etc, will my originally untagged frame that came in and got put into vlan 2 get tagged along the trunk as vlan 2 ?

I have this problem too.
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bvsnarayana03 Sat, 11/17/2007 - 02:27

If your PC is in vlan 2, then the frame generated by it will be tagged with vlan2 information & passes thru the trunk as tagged(considering vlan2 is not native).

If u make vlan 2 as native on both end of trunks, frame will go untagged.

rajatsetia Tue, 11/20/2007 - 09:18

Hi Carl,

I hope i understand ur question correctly :)

you are asking once farme is tagged with vlan id, what will happen to a frame as it pass through a inter switch trunk port.

then the asnwer is (using 802.1q trunking protocol)-

802.1Q does not actually encapsulate the original frame, it sets the EtherType value in the Ethernet header to Tag Protocol ID (TPID) 0x8100, identifying this frame as an 802.1Q frame. It then inserts an extra two-bytes of Tag Control Information (TCI) after the TPID, followed by another two bytes containing the frame's original EtherType. Together the four bytes of TPID and TCI are called the VLAN Tag.

The format of the TCI is

15:13 12 11:0

user_priority CFI VID

user_priority: a 3-bit field is defined in IEEE 802.1p.

Canonical format indicator (CFI): a 1-bit indicator used for compatibility between Ethernet and Token Ring networks.

VLAN ID (VID): a 12-bit field specifying the VLAN to which the frame belongs. A value of 0 means that the frame doesn't belong to any VLAN; in this case the 802.1Q tag specifies only a priority and is referred to as a priority tag. A value of hex FFF is reserved for implementation use. All other values may be used as VLAN identifiers, allowing up to 4094 VLANs. On bridges, VLAN 1 is often reserved for management.

so ur original vlan tag is retained and other end of trunk port will be able to original vlan id of frame.

HTH

rgds

rajat

s.arunkumar Tue, 11/20/2007 - 20:31

carl,

i think u are right..the frame coming from a pc dont have any vlan id as the pc does not have any information of to which vlan it belongs..once it enter the switch port the switch understands that the untagged frame received on this port is on vlan2(as in ur example)...and then the switch enter the vlan info on the frame and this is used when it traverse along the trunk port..

:)

bvsnarayana03 Tue, 11/20/2007 - 21:11

guys, i'm sorry if my english is so difficult to understand. What i meant was that the frame generated by pc will be tagged on switch, assuming that everyone knows tagging /untagging is done by switch.

rajatsetia Wed, 11/21/2007 - 02:54

we donnt know which direction we are taking this discussion until unless carl confirm if his query is replied :)

still we have discussed a lot which can be helpful..

yup, untagged frame comes from pc

as it hits the switch port , its tagged by vlan id (based on port vlan membership)

then this tagged frame travels down the inter switch trunk port(we took 802.1q as trunking protocol). 802.1q add extra 2 byte of information and retain the original vlan id of frame.

* only exception is if you define any vlan id as native accross the 802.1q trunk , then it will be forwarded as ethernet frame only , no 802.1q header is added to this frame.

** by default vlan 1 is native until unless you manually change it some other vlan id.

hope i was able to summarize all the posts in here ...

HTH

rgds

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