Planet XGS3-24042 User Manual download pdf (Page 648)

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79-2

Figure 79-1 The Encapsulation Structure of a Label

There are 4 fields in a label:

 Label：The label value, whose length is 20 bits, a pointer for forwarding.

 Exp：3bits, used by QoS.

 S：1bit, the label’s layered structure supported by MPLS, that is, there are multiple label layers. The

value 1 represents the bottom-most layer of label.

 TTL：8bits, serves the same purpose as the TTL (Time To Live) in IP packets.

The label, like VPI/VCI of ATM and DLCI of Frame Relay, is identification for connections. If there is a label

field in the link-layer protocol, such as VPI/VCI of ATM and DLCI of Frame Relay, the label will be

encapsulated in these fields, otherwise, in a transitional layer between the link layer and the IP layer. Thus,

labels can be supported by any link layer protocol.

Label Space

LSR can distribute a different label for a FEC according to its ingress port. As a result, packets from different

ports can be forwarded independently, which is the basic foundation of VPN. To enhance the utilization

efficiency of labels, MPLS provides the concept of label space, which is a label prefix. By allocating FECs

belonging to different label spaces with the same label, the boundary of label is actually expanded. The label

space is only meaningful when allocating labels, but not when forwarding them.

Label Switching

There is no need to analyze packet header in non-edge LSRs, instead, the label will be used as a pointer to

the next-hop egress port and a new label. The label packet will replace the old label with the new one and

then be forwarded through the specified egress port. Label switching will simplify and accelerate the

forwarding process, and realize applications like VPN, QoS, traffic engineering and etc.

Label Switching Router

LSR (Label Switching Router) is the basic element of a MPLS network, with all LSRs supporting MPLS

technology.

LSR is a device able to forwarding packets according to their label value. A LSR connecting an IP route

network and a MPLS switching network is called an Edge LSR. Such a LSR is able to adding labels to IP

messages and forwarding data according to LSP, or deleting MPLS packet labels and forwarding data

according to the IP routes. Each LSR must be distributed a global-alone LSR ID, usually get an interface IP

address of LSR. Assume that, LSR Ru and Rd agree on the map between the label L and the FEC F. Packets

can be forwarded from Ru to Rd based on the label L, in which case, Ru is the upstream LSR, and Rd the

downstream LSR, that is to say, the forwarding of packets id always from the upstream LSR to the

downstream one.

Label Switched Path

The path a FEC follows in the MPLS network is called a LSP (Label Switched Path). Two adjacent LSRs in a

TTL

Exp

Label

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Planet XGS3-24042 User Manual Page 648

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