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, 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 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 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 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 if the received status bits from the remote node indicated SAP up/down or SDP-binding up/down.