MPLS EXP

The label information is added in the MPLS packet. The label length is four bytes. Figure 3-3 shows the encapsulation structure.

Figure 3-3 Encapsulation structure of the label

http://localhost:7904/pages/NE20en/01%20%282010-01-30%29/NE20en/01%20%282010-01-30%29/resources/Feature%20Description/01-03%20Traffic%20Classification.files/image003.gif

The label has four fields:

l           Label: 20 bits. The label field is the pointer used for forwarding the data.

l           Exp: 3 bits. It is reserved for experiment before. Now, it is used as Class of Service (CoS).

l           S: 1 bit. It is the identifier at the end of the stack. MPLS supports the hierarchical structure of the label, that is, the multiple labels. When the S is 1, it represents the label of the lowest layer.

l           TTL: 8 bits. It has the same meaning with the Time To Live (TTL) in the IP packets.

The label is a connection identifier, similar to the VPI/VCI of ATM and the DLCI of the frame relay.