In the network in Figure: VLL resilience with pseudowire redundancy and switching, PE nodes act as leading nodes and pseudowire switching nodes act as followers for the purpose of pseudowire signaling. This is because a switching node needs to pass the SAP interface parameters of each PE to the other. T-PE1 sends a label mapping message for the Layer 2 FEC to the peer pseudowire switching node; for example, S-PE1. It includes the SAP interface parameters, such as MTU, in the label mapping message. S-PE1 checks the FEC against the local information and, if a match exists, appends the optional pseudowire switching point TLV to the FEC TLV in which it records its system address. T-PE1 then relays the label mapping message to S-PE2. S-PE2 performs similar operation and forwards a label mapping message to T-PE2.
The same procedures are followed for the label mapping message in the reverse direction; for example, from T-PE2 to T-PE1. S-PE1 and S-PE2 affect the spoke-SDP cross-connect only when both directions of the pseudowire are signaled and matched.
Pseudowire status messages can be generated by the T-PE nodes and, or the S-PE nodes. Pseudowire status messages received by a switching node are processed, then passed on to the next hop. An S-PE node appends the optional pseudowire switching TLV, with its system address added to it, to the FEC in the pseudowire status notification message, only if it originated the message or the message was received with the TLV in it. Otherwise, the message was originated by a T-PE node and the S-PE should process and pass the message without changes, except for the VC-ID value in the FEC TLV.
The merging of the received T-LDP status notification message and the local status for the spoke-SDPs from the service manager at a PE complies with the following rules:
When the local status for both spoke-SDPs is up, the S-PE passes any received SAP or SDP binding generated status notification message unchanged; for example, the status notification TLV is unchanged but the VC-ID in the FEC TLV is set to value of the pseudowire segment to the next hop.
When the local operational status for any of the spokes is down, the S-PE always sends an SDP-binding down status bits regardless of whether the received status bits from the remote node indicated SAP up or down or SDP-binding up or down.