Alcatel-Lucent 7450 - Figure 15: Example Usage of PW Identifiers; Generic Associated Channel

MPLS-TP Maintenance Identifiers
Page 68 7450 ESS MPLS Guide
Figure 15: Example usage of PW Identifiers
Generic Associated Channel
MPLS-TP requires that all OAM traffic be carried in-band on both directions of an LSP or PW.
This is to ensure that OAM traffic always shares fate with user data traffic. This is achieved by
using an associated control channel on an LSP or PW, similar to that used today on PWs. This
creates a channel, which is used for OAM, protection switching protocols (e.g. LSP linear
protection switching coordination), and other maintenance traffic., and is known as the Generic
Associated Channel (G-ACh).
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 (e.g. 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.
al_0229
VC-Switching
Spoke-SDPSpoke-SDP
7750 T-PE
global_id: 1
node_id: 10.0.0.10
ac_id: 1
7750 T-PE
global_id: 2
node_id: 10.0.0.30
7750 S-PE
global_id: 1
node_id: 10.0.0.20
SAP SAP
Epipe
vll
pw-path-id:
agi: 0
SAII: 1:10.0.0.10:1
TAII: 2:10.0.0.30:2
pw-path-id:
agi: 0
SAII: 2:10.0.0.30:2
TAII: 1:10.0.0.10:1
pw-path-id:
agi: 0
SAII: 1:10.0.0.10:1
TAII: 2:10.0.0.30:2
pw-path-id:
agi: 0
SAII: 2:10.0.0.30:2
TAII: 1:10.0.0.10:1