Virtual Leased Line Services
7210 SAS M Services Guide Page 141
Pseudowire Switching
Note: The 7210 SAS devices cannnot be configured as S-PE nodes. They can be configured only
in a T-PE nodes. The discussion in the following section is about T-PE applies to 7210 only.
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. In addition, the user configures a VLL service on the pseudowire switching node (the S-PE
node) using the two SDPs.
The pseudowire switching node acts in a passive role with respect to signalling 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 signalling 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 towards 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 to 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 Figure 21, the user configures a regular Epipe VLL service PE1 and PE2. These services
consist each of a SAP and a spoke SPD. However, the target destination of the SDP is actually not
the remote PE but the pseudowire switching node. In addition, the user configures an Epipe VLL
service on the pseudowire switching node using the two SDPs.
Figure 21: Pseudowire Service Switching Node
|7210 PE1 (Epipe)|---sdp 2:10---|7210 PW SW (Epipe)|---sdp 7:15---|7210 PE2 (Epipe)
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