router is unaware of the redundant configuration, ensuring that traffic is always switched
using just the active pseudowire.
•
Alternatively, you can configure two active pseudowires, one to each of the PE routers.
Using this approach, control plane signalling is completed and active pseudowires are
established with both the primary and backup neighbors. However, the data plane
forwarding is done only over a one of the pseudowires (designated as the active
pseudowire by the local device). The other pseudowire is on standby. The active
pseudowire is preferably established with the primary neighbor and can switch to the
backup pseudowire if the primary fails.
The decision to switch between the active and standby pseudowires is controlled by
the local device. The remote PE routers are unaware of the redundant connection, and
so both remote PE routers send traffic to the local device. The local device only accepts
traffic from the active pseudowire and drops the traffic from the standby. In addition,
the local device only sends traffic to the active pseudowire. If the active pseudowire
fails, traffic is immediately switched to the standby pseudowire.
The two configurations available for pseudowire redundancy have the following
limitations:
•
For the single active pseudowire configuration, it takes more time (compared to the
two active pseudowire configuration) to switchover to the backup pseudowire when
a failure is detected. This approach requires additional control plane signalling to
complete the pseudowire with the backup neighbor and traffic can be lost during the
switchover from primary to backup.
•
If you configure two active pseudowires, bandwidth is lost on the link carrying the
backup pseudowire between the remote PE router and the local device. Traffic is always
duplicated over both the active and standby pseudowires. The single active pseudowire
configuration does not waste bandwidth in this fashion.
Pseudowire Failure Detection
The following events are used to detect a failure (control and data plane) of the
pseudowire configured between a local device and a remote PE router and initiates the
switch to a redundant pseudowire:
•
Manual switchover (user initiated)
•
Remote PE router withdraws the label advertisement
•
LSP to the remote PE router goes down
•
LDP session with the remote PE router goes down
•
Local configuration changes
•
Periodic pseudowire OAM procedure fails (Layer 2 circuit-based MPLS ping to the PE
router fails)
When you configure a redundant pseudowire between a CE device and a PE router, a
periodic (once a minute) ping packet is forwarded through the active pseudowire to
605Copyright © 2017, Juniper Networks, Inc.
Chapter 20: Configuring MPLS and Pseudowires
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