Inspecting An Mpls Lsp; Ldp Overview; Ldp Basic Concepts - H3C S9500 Series Operation Manual

Operation Manual – MPLS Basics
H3C S9500 Series Routing Switches
For an MPLS packet with only one level of label, the ICMP response message travels
along the IP route when the TTL expires.

1.2.4 Inspecting an MPLS LSP

In MPLS, the MPLS control plane is responsible for establishing an LSP. However, it
cannot detect the error when an LSP fails to forward data. This brings difficulty to
network maintenance.
MPLS LSP ping and traceroute provide a mechanism for detecting errors in LSP and
locating nodes with failure in time. Similar to IP ping and traceroute, MPLS LSP ping
and traceroute use MPLS echo requests and MPLS echo replies to check the
availability of LSPs. The MPLS echo request message carries FEC information to be
detected, and is sent along the LSP like other data packets of the same FEC. Thus, the
LSP can be checked.
MPLS LSP ping is a tool for checking the validity and availability of an LSP. It uses
messages called MPLS echo requests. In a ping operation, an MPLS echo
request is forwarded along an LSP to the egress, where the control plane
determines whether the LSR itself is the egress of the FEC and responds with an
MPLS echo reply. When the ping initiator receives the reply, the LSP is considered
perfect for forwarding data.
MPLS LSP traceroute is a tool for locating LSP errors. By sending MPLS echo
requests to the control plane of each transit LSR, it can determine whether the
LSR is really a transit node on the LSP.
Note:
When an MPLS echo request reaches the egress, the destination address in the IP
header is set to an address on 127.0.0.0/8 (loopback address of the LSR) and the TTL
is set to 1, so as to prevent further forwarding of the request.

1.3 LDP Overview

1.3.1 LDP Basic Concepts

An LDP dictates the messages to be used in label distribution and the related
processes.
Chapter 1 MPLS Basics Configuration
1-12