Unanswered Question
sundar.palaniappan Tue, 04/17/2007 - 10:02
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See the responses in line.

1)what is the Vlan Tag

A quick search on google spotted this link with explanation and example. The tag references the VLAN ID of the frame.


VLAN Tagging, also known as Frame Tagging, is a method developed by Cisco to help identify packets travelling through trunk links. When an Ethernet frame traverses a trunk link, a special VLAN tag is added to the frame and sent across the trunk link.

As it arrives at the end of the trunk link the tag is removed and the frame is sent to the correct access link port according to the switch's table, so that the receiving end is unaware of any VLAN information.

2)how Mpls works

Here's MPLS FAQ that I hope answers your queries.


MPLS is a packet-forwarding technology which uses labels to make data forwarding decisions. With MPLS, the Layer 3 header analysis is done just once (when the packet enters the MPLS domain). Label inspection drives subsequent packet forwarding. MPLS provides these beneficial applications:

* Virtual Private Networking (VPN)

* Traffic Engineering (TE)

* Quality of Service (QoS)

* ATM over MPLS (AToM)

Additionally, it decreases the forwarding overhead on the core routers. MPLS technologies are applicable to any network layer protocol.

3)What is the P

P?? Do you mean the P in the M(P)LS. P stands for protocol. That is any network layer protocol.



ankbhasi Tue, 04/17/2007 - 10:06
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Hi Ali,

Vlan TAG is giving an identity to your frame/packets when they cross switches. When frames leave an access port it never knows which VLAN it belongs to so it does not have any identity which vlan it belongs to but when you want your frame to carry its vlan identity across switches then you configure port as a trunk port where you configure encapsulation dot1q or ISL.

When it's dot1q/isl trunk and any frame leaves a trunk port its gets and carry away its vlan information. Now when it is dot1q it is termed as tagging because it is inside a frame and when it is ISL it is encapsulation which encapsulates frame.

Now how MPLS work may be some champ from MPLS backgroud may help you with the same and your 3rd question is "P" which I am not able to understand what exactly it is?




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