H3C Technologies H3C S7500E Series Switches - Manual (Page 46)

2-4
z
Egress LSRs for removing labels from packets and IP forwarding the packets to their destination
networks.
In a word, transit LSRs perform MPLS forwarding based on labels of packets, while the ingress and
egress LSRs deal with the switchover between MPLS and IP forwarding.
LSP Establishment and Label Distribution
LSP establishment
Establishing LSPs is to bind FECs with labels on each LSR involved and notify its adjacent LSRs of
the bindings, so as to establish the LFIB on each LSR. LSPs can be established through manual
configuration, or be established dynamically through label distribution protocols.
1) Establishing a static LSP through manual configuration
To establish a static LSP, you need to assign a label to the FEC on each LSR along the packet
forwarding path. Establishment of static LSPs consumes fewer resources than dynamic LSP
establishment. However, static LSPs cannot adapt to network topology changes. Therefore, static
LSPs are suitable for small-scale networks with simple, stable topologies.
2) Establishing an LSP through a label distribution protocol
Label distribution protocols are MPLS signaling protocols. They can classify FECs, distribute labels,
and establish and maintain LSPs. Label distribution protocols include protocols designed specifically
for label distribution, such as the Label Distribution Protocol (LDP), and protocols extended to support
label distribution, such as BGP and RSVP-TE.
This document introduces LDP only.
In this document, the term “label distribution protocols” represents all protocols for label distribution,
and the term “LDP” refers to the Label Distribution Protocol defined in RFC 5036. For detailed
information on LDP, see
LDP .
As shown in Figure 2-4 , a dynamic LSP is established in the following procedure:
A downstream LSR classifies FECs according to destination addresses, assigns a label to a FEC, and
distributes the FEC-label binding to its upstream LSR, which then establishes an LFIB entry for the
FEC according to the binding information. After all LSRs along the packet forwarding path establish a
LFIB entry for the FEC, an LSP is established for packets of this FEC.