Figure 1 shows the application of pseudowire redundancy to provide Ethernet VLL service resilience for broadband service subscribers accessing the broadband service on the service provider BRAS.
If the Ethernet SAP on PE2 fails, PE2 notifies PE1 of the failure by either withdrawing the primary pseudowire label it advertised or by sending a pseudowire status notification with the code set to indicate a SAP defect. PE1 receives it and immediately switches its local SAP to forward over the secondary standby spoke-SDP. To avoid black holing of packets during the switching of the path, PE1 accepts packets received from PE2 on the primary pseudowire while transmitting over the backup pseudowire. However, in other applications such as those described in Access Node Resilience Using MC-LAG and Pseudowire Redundancy, it is important to minimize service outage to end users.
When the SAP at PE2 is restored, PE2 updates the new status of the SAP by sending a new label mapping message for the same pseudowire FEC or by sending pseudowire status notification message indicating that the SAP is back up. PE1 then starts a timer and reverts to the primary at the expiry of the timer. By default, the timer is set to 0, which means PE1 reverts immediately. A special value of the timer (infinity) means that PE1 should never revert to the primary pseudowire.
The behavior of the pseudowire redundancy feature is the same if PE1 detects or is notified of a network failure that brought the spoke-SDP status to operationally down. The following events cause PE1 to trigger a switchover to the secondary standby pseudowire.
The T-LDP peer (remote PE) node withdraws the pseudowire label.
The T-LDP peer signals a FEC status indicating a pseudowire failure or a remote SAP failure.
The T-LDP session to peer node times out.
The SDP binding or VLL service goes down as a result of a network failure condition, such as the SDP to the peer node going operationally down, or the SDP binding going operationally down because the VCCV BFD session is going down.
The SDP type for the primary and secondary pseudowires need not be the same. That is, the user can protect an RSVP-TE based spoke-SDP with an LDP or GRE based one. This provides the ability to route the path of the two pseudowires over different areas of the network. All VLL service types, for example, Apipe, Epipe, Fpipe, and Ipipe, are supported on the 7750 SR.
Nokia routers support the ability to configure multiple secondary standby pseudowire paths. For example, PE1 uses the value of the user-configurable precedence parameter associated with each spoke-SDP to select the next available pseudowire path after the failure of the current active pseudowire (whether it is the primary or one of the secondary pseudowires). The revertive operation always switches the path of the VLL back to the primary pseudowire though. There is no revertive operation between secondary paths, meaning that the path of the VLL is not switched back to a secondary pseudowire of higher precedence when the latter comes back up again.
Nokia routers support the ability for a user-initiated manual switchover of the VLL path to the primary or any of the secondary, be supported to divert user traffic in case of a planned outage such as in node upgrade procedures.
On the 7750 SR, this application can make use of all types of VLL supported on SR-series routers. However, if a SAP is configured on an MC-LAG instance, only the Epipe service type is allowed.