PLANET XGS3-24042 - Manual (Page 648)

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 S Exp Label