MPLS Transport Profile (MPLS-TP)
Page 38 7210 SAS M, T, X, R6, Mxp MPLS Configura-
tion Guide
RFC5586 specifies mechanisms for implementing the G-ACh, relying on the combination of a
reserved MPLS label, the 'Generic-ACH Label (GAL)', as an alert mechanism (value=13) and
Generic Associated Channel Header (G-ACH) for MPLS LSPs, and using the Generic Associated
Channel Header, only, for MPLS PWs (although the GAL is allowed on PWs). The purpose of the
GAL is to indicate that a G-ACH resides at the bottom of the label stack, and is only visible when
the bottom non-reserved label is popped. The G-ACH channel type is used to indicate the packet
type carried on the G-ACh. Packets on a G-ACh are targeted to a node containing a MEP by
ensuring that the GAL is pushed immediately below the label that is popped at the MEP (for
example, LSP endpoint or PW endpoint), so that it can be inspected as soon as the label is popped.
A G-ACh packet is targeted to a node containing a MIP by setting the TTL of the LSP or PW label,
as applicable, so that it expires at that node, in a similar manner to the SROS implementation of
VCCV for MS-PWs.
Figure 13: Label for LSP and PW G-ACh Packets
7210 SAS supports the G-ACh on static pseudowires and static LSPs.
MPLS-TP Operations, Administration and Maintenance (OAM)
This section details the MPLS-TP OAM mechanisms that are supported.
On-Demand Connectivity Verification (CV) using LSP-Ping
MPLS–TP supports mechanisms for on demand CC/CV as well as route tracing for LSPs and
PWs. These are required to enable an operator to test the initial configuration of a transport path,
or to assist with fault isolation and diagnosis. On demand CC/CV and route tracing for MPLS-TP
is based on LSP-Ping and is described in RFC6426. Three possible encapsulations are specified in
that RFC:
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