Pseudowire Switching
Page 136 7210 SAS-X, R6 OS Services Guide
VLL Resilience with Two Destination PE Nodes
Figure 16 illustrates the application of pseudowire redundancy to provide Ethernet VLL service
resilience for broadband service subscribers accessing the broadband service on the service
provider BRAS.
Figure 16: VLL Resilience
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.
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.
IP/MPLS
7750 SR
PE 2
PE 3
PE 1
Eth SAP
Eth SAP
(standby)
Eth SAP
(primary)
Primary Eth
Standby
backup Eth
PW
DSLAM
BRAS 1
7210 SAS
7750 SR
OSSG115-7210M
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