Alcatel-Lucent 7750 SR OS Service Manual page 131

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 will receive it and will immediately switch its local SAP to
forward over the secondary standby spoke SDP. In order to avoid black holing of in-flight packets
during the switching of the path, PE1 will accept packets received from PE2 on the primary
pseudowire while transmitting over the backup pseudowire. Note that in the case of the FT
application shown in
timeout and will be re-established via the standby SAP on the link between PE3 and the BRAS
node. However, in other applications such as those described in
MC-LAG and Pseudowire Redundancy on page
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 back 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) will mean that PE1 should never revert back 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 operational status to DOWN. The following are the
events which will cause PE1 to trigger a switchover to the secondary standby pseudowire:
1. T-LDP peer (remote PE) node withdrew the pseudowire label.
2. T-LDP peer signaled a FEC status indicating a pseudowire failure or a remote SAP failure.
3. T-LDP session to peer node times out.
4. SDP binding and VLL service went down as a result of network failure condition such as the
SDP to peer node going operationally down.
The SDP type for the primary and secondary pseudowires need not be the same. In other words,
the user can protect a RSVP-TE based spoke SDP with a 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.
The 7x50 supports 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 will not be
switched back to a secondary pseudowire of higher precedence when the latter comes back up
again.
The 7x50 supports 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.
7750 SR OS Services Guide
Figure 24, this does not matter as the subscriber PPPoE session will actually
Virtual Leased Line Services
Access Node Resilience using
133, it will be important to minimize service
Page 131