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The Label Switching Paradigm

The label switching paradigm is the central mechanism that is employed in the data forwarding plane for advancing packets from source to destination while in the MPLS network. Primarily based on the ATM and FR model and earlier proprietary tag-based techniques, labels are assigned to packets as they enter the network, get swapped as the packets traverse the network, and are then removed as the packets exit the MPLS portion of the network.

Figure 1–7 on the next page shows the basic label swapping model. In a network, there is an entry point called the ingress node, intermediate nodes called transit nodes, and an exit point named the egress node for a particular path. The set of nodes that participate in the label swapping is called the label swapping domain.

Figure 1–7 The label switching paradigm.

It should be noted that the values of the labels can be assigned and distributed before any label swapping of the data occurs, or the labels can be generated as packets belonging to a specified flow or class enter the network. These two types of label assignment are called control-driven and traffic-driven, respectively. After the label switching domain has been configured for handling the packet traffic that is to be forwarded by label swapping, packets are all processed in the same simple and straightforward manner.

A label is a short, fixed-length value that is carried in the packet header. The label identifies which forwarding path a packet will traverse based on a configurable set of input parameters (destination address, prefix, QoS, etc.). A process running at the ingress node determines any incoming packet's label value based on the information of how the packet maps to its forwarding path set. The ingress node assigns an initial label value to each packet and sends it into the network. Labels have only local significance, and the actual value of a label is only understood between any two communicating nodes. After a label is added by the ingress node, it is swapped at the transit nodes based on its configured label map until the packet reaches its destination at the egress node. Remember, the label maps that direct the packet through its LSP have been set up before the packet has begun its journey. Labeled paths are, therefore, roughly equivalent to a virtual circuit (VC). Also, a labeled path is always unidirectional. If you want packet traffic to flow in the opposite direction on the same route, you must set up two labeled paths.

Label swapping has several advantages over the hop-by-hop routing employed by conventional IP routers. It is simple and efficient. Packet analysis is done only once—at the ingress node. Label swapping within the label swapping domain is quick because the operation simply recognizes the label and swaps it for the packet's next label value. The egress node recognizes that the packet is at the edge of the domain and then removes the label and forwards the packet based on other information, such as the packet's network layer header, into the destination domain.

While label swapping is the heart of MPLS, a quick introduction reveals several more concepts that are necessary to begin to understand the technology more completely and to know why MPLS is more than just label swapping.

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