H3C Technologies H3C S7500E Series Switches - Manual (Page 45)

2-3
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 2-2 , LSR B is the downstream LSR of LSR A, while LSR A is the upstream LSR
of LSR B.
Figure 2-2 Diagram for an LSP
LSR A
LSR B LSR C
LSR D
LSP
LSP
LS
P
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.
z
Control plane: Assigns labels, selects routes, establishes the LFIB, establishes and removes
LSPs.
z
Forwarding plane: Forwards packets according to the LFIB.
Structure of the MPLS Network
Figure 2-3 Diagram for the MPLS network structure
LER
Ingress LSR
LSP
LER
Egress LSR
Transit LSR
IP network
IP network
MPLS domain
As shown in Figure 2-3 , the element of an MPLS network is LSR. LSRs in the same routing or
administrative domain form an MPLS domain.
An MPLS domain consists of three types of LSRs:
z
Ingress LSRs for receiving and labeling packets coming into the MPLS domain.
z
Transit LSRs for forwarding packets along LSPs to their egress LERs according to the labels.