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. A switching node must 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, it 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 operations 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 will affect the spoke-SDP cross-connect only when both directions of the pseudowire have been signaled and matched.
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 spokes 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 the 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 SDP-binding down status bits, regardless of whether the received status bits from the remote node indicated SAP up/down or SDP-binding up/down.