H3C Technologies H3C SR8800 - Manual (Page 13)

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As shown in Figure 1 , a label is encapsulated between the Layer 2 header and Layer 3 header of a
packet. A label is four bytes in length and consists of the following fields:
• Label—20 bits in length. Label value for identifying a FEC.
• Exp—Three bits in length. Reserved field, usually used for CoS.
• S—One bit in length. MPLS supports multiple levels of labels. This field is used to indicate whether
a label is at the bottom of the label stack. 1 indicates that the label is at the bottom of the label
stack.
• TTL—Eight bits in length. Like the homonymous IP header field, it is used to prevent loops.
LSR
A label switching router (LSR) is a fundamental component on an MPLS network. LSRs support label
distribution and label swapping.
LER
A label edge router (LER) is an LSR that resides at the edge of an MPLS network and is connected to
another network.
LSP
A label switched path (LSP) is the path along which packets of a FEC travel through an MPLS network.
An LSP is a unidirectional path from the ingress of an MPLS network to the egress. On an LSP, in the
packet transfer direction, two neighboring LSRs are called the “upstream LSR” and “downstream LSR”
respectively. In
Figure 2 , LSR B is the downstream LSR of LSR A; LSR A is the upstream LSR of LSR B.
Figure 2 Diagram for an LSP
LFIB
On an MPLS network, labeled packets are forwarded according to the Label Forwarding Information
Base (LFIB), which is like the FIB for IP packet forwarding on an IP network.
Control plane and forwarding plane
An MPLS node consists of two planes, control plane and forwarding plane.
• Control plane—Assigns labels, selects routes, establishes the LFIB, establishes and removes LSPs
• Forwarding plane—Forwards packets according to the LFIB