Pseudowire switching scales VLL and VPLS services over a multi-area network by removing the need for a full mesh of targeted LDP sessions between PE nodes. The following figure shows the use of pseudowire redundancy to provide a scalable and resilient VLL service across multiple IGP areas in a provider network.
In the network in the preceding figure, PE nodes act as leading nodes and pseudowire switching nodes act as followers for the purpose of pseudowire signaling. 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 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 affect the spoke-SDP cross-connect only when both directions of the pseudowire have been signaled and matched.
The pseudowire switching TLV is useful in a few situations. First, it allows for troubleshooting of the path of the pseudowire especially if multiple pseudowire switching points exist between the two T-PE nodes. Secondly, it helps in loop detection of the T-LDP signaling messages where a switching point receives back a label mapping message it already relayed. Finally, it can be inserted in pseudowire status messages when they are sent from a pseudowire switching node toward a destination PE.
Pseudowire status messages can be generated by the T-PE nodes. Pseudowire status messages received by a switching node are processed and 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, it means the message was originated by a T-PE node and the S-PE should process and pass the message without changes except for the VCID value in the FEC TLV.