what is internal frame tagging in a switch

[uddipantunga]

"When a frame comes into a switch port, the frame is tagged internally within the switch with the VLAN number of the port. When it reaches
the outgoing port, the internal tag is removed. If the exit port is a trunk port, then its VLAN is identified in either the ISL encapsulation or the
802.1Q tag. The switch on the other end of the trunk removes the ISL or 802.1Q information, checks the VLAN of the frame, and adds the
internal tag. If the exit port is a user port, then the original frame is sent out unchanged, making the use of VLANs transparent to the user."

The above lines are taken from Cisco  BCMSN Quick Reference Sheet .

My questions are

1> What is a internal tag in a switch as above line crearly say it is not dot1q tag because it is only usend in trunk links.

2> if we have the below network

    PC1|-----|SW1|---------|SW2|-------|SW3|-----|PC2

The PC1 & PC2 is in vlan 10 . all interswitch links are trunk links.

In this senario do SW2 removes ISL or dot1q information fames that are comming from SW1 and again tag or encap the frame before sending to SW3 , or just forward it to SW3.

[Richard Ege]

Mr. Tunga,

1. Internal tag is a VLAN number that tags the frame of which port belongs within a which.

2. ISL and dot1q can only be used in traversing in a trunk port, between switches. If the vlan 10 is ur native vlan, it is untagged. But if it is not a native vlan, it will use an encapsulation while traversing between switches. The outgoing port encapsulates the frame and the receiving port de-encapsulates.

I hope it clears your mind.

[mohannad.daradkeh1]

Being such a common question; and it has no answer here goes

Internal tag is switch marking for the VLAN the port the packet was received is associated with. lets assume port is access/untagged port of VLAN 10. when the port receives a frame on that port it is received as a standard Ethernet frame. The switch marks the frame according to the port VLAN subscription "VLAN-10"  and internally forward it to the egress port which might be a trunk/tagged connected to a switch or it might be an access/untagged connected to an end point.

The egress port removes the internal marking and now has two option based on the port link type being access/untagged or trunk/tagged.

A. if the outgoing port is acess/untagged; The internal marking is already stripped and the frame is send as a standard Ethernet frame.

B. If the outgoing is a trunk/tagged port; the internal marking is stripped and switch will add 802.1Q tag and forward the frame as 802.1Q frame, the next switch/hop will receive the frame; strip the 801.2Q tag then internally add marking denoting the "VLAN-10" the port was recived on, lookup the destination mac address and forward the frame to that port, the egress port receives the frame, removes the internal marking and again will send either as a 802.1Q or standard Ethernet based on the link type.   

So in the example above the first hop will receive standard Ethernet frame will mark it internally as "VLAN-10" based on the port subscription of the ingress port; it will then lookup the outgoing port based on the destination mac address. the egress port will strip the internal VLAN-10 marking and add 802.1Q tag and forward as 802.1Q frame to the next hop; next hop receives the 802.1Q Frame removes the tag and mark it internally to VLAN-10 based on destination address forward it to the outgoing port, the egress will remove the internal marking of VLAN-10 adds 802.1Q tag and forward it to the third hop,third hop ingress port will remove the 802.1Q tag, mark internally forward to egress, outgoing port will remove the internal marking and lastly forward this to PC2 as standard ethernet frame.

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