Virtual Leased Line Services
7750 SR OS Services Guide Page 227
VLL Resilience for a Switched Pseudowire Path
Figure 50 illustrates the use of both pseudowire redundancy and pseudowire switching to provide
a resilient VLL service across multiple IGP areas in a provider network.
Figure 50: VLL Resilience with Pseudowire Redundancy and Switching
Pseudowire switching is a method for scaling a large network of VLL or VPLS services by
removing the need for a full mesh of T-LDP sessions between the PE nodes as the number of these
nodes grows over time.
Like in the application in VLL Resilience with Two Destination PE Nodes on page 223, the T-PE1
node switches the path of a VLL to a secondary standby pseudowire in the case of a network side
failure causing the VLL binding status to be DOWN or if T-PE2 notified it that the remote SAP
went down. This application requires that pseudowire status notification messages generated by
either a T-PE node or a S-PE node be processed and relayed by the S-PE nodes.
Note that it is possible that the secondary pseudowire path terminates on the same target PE as the
primary, for example, T-PE2. This provides protection against network side failures but not
against a remote SAP failure. When the target destination PE for the primary and secondary
OSSG114
Core area
Metro area B
Metro area A
Access Node
Access Node
7x50 T-PE2
7x50 T-PE1
7x50
7x50
7x50 S-PE3
7x50 S-PE1
7x50 T-PE3
Primary PW
Standby PW
7x50
7x50
7x50
7x50 S-PE4
PW switching
SDP6:600
SDP4:400
SDP1:100
SDP3:300
7x50 S-PE2
PW switching
PW switching
PW switching
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