The pseudowire switching feature provides the user with the ability to create a VLL service by cross-connecting two spoke-SDPs. This feature allows the scaling of VLL and VPLS services in a large network in which the otherwise full mesh of PE devices would require thousands of Targeted LDP (T-LDP) sessions per PE node.
Services with one SAP and one spoke-SDP are created normally on the PE; however, the target destination of the SDP is the pseudowire switching node instead of what is normally the remote PE. Also, the user configures a VLL service on the pseudowire switching node using the two SDPs.
The pseudowire switching node acts in a passive role with respect to signaling of the pseudowires. It waits until one or both of the PEs sends the label mapping message before relaying it to the other PE. This is because it needs to pass the interface parameters of each PE to the other.
A pseudowire switching point TLV is inserted by the switching pseudowire to record its system address when relaying the label mapping message. This TLV is useful in a few situations:
It allows for troubleshooting of the path of the pseudowire especially if multiple pseudowire switching points exist between the two PEs.
It helps in loop detection of the T-LDP signaling messages where a switching point would receive back a label mapping message it had already relayed.
The switching point TLV is inserted in pseudowire status notification messages when they are sent end-to-end or from a pseudowire switching node toward a destination PE.
Pseudowire OAM is supported for the manual switching pseudowires and allows the pseudowire switching node to relay end-to-end pseudowire status notification messages between the two PEs. The pseudowire switching node can generate a pseudowire status and send it to one or both of the PEs by including its system address in the pseudowire switching point TLV. This allows a PE to identify the origin of the pseudowire status notification message.
In the following example, the user configures a regular Epipe VLL service PE1 and PE2. These services each consist of a SAP and a spoke-SDP. However, the target destination of the SDP is not the remote PE, but the pseudowire switching node. Also, the user configures an Epipe VLL service on the pseudowire switching node using the two SDPs.
|7450 ESS, 7750 SR, and 7950 XRS PE1 (Epipe)|---sdp 2:10---|7450 ESS, 7750 SR, and
7950 XRS PW SW (Epipe)|---sdp 7:15---|7450 ESS, 7750 SR, and 7950 XRS PE2 (Epipe)|
Configuration examples are in Configuring Two VLL Paths Terminating on T-PE2.