Figure 27: Vll Resilience With Pseudowire Redundancy And Switching - Alcatel-Lucent 7210 SAS M Service Manual

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Pseudowire Switching

PW switching

Figure 27: VLL Resilience with Pseudowire Redundancy and Switching

Note: 7210 SAS M supports only T-PE functionality. It does not support S-PE functionality.

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.

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 pseudowires

is the same, T-PE1 will normally not switch the VLL path onto the secondary pseudowire upon

receipt of a pseudowire status notification indicating the remote SAP is down since the status

notification is sent over both the primary and secondary pseudowires. However, the status

notification on the primary pseudowire may arrive earlier than the one on the secondary

pseudowire due to the differential delay between the paths. This will cause T-PE1 to switch the

path of the VLL to the secondary standby pseudowire and remain there until the status notification

is cleared. At that point in time, the VLL path is switched back to the primary pseudowire due to

the revertive behavior operation. The path will not switch back to a secondary path when it

becomes up even if it has a higher precedence than the currently active secondary path.

Metro area B

Metro area A

PW switching

7210 SAS M Services Guide

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