Pseudowire Switching
Page 126 7750 SR OS Services Guide
Pseudowire Switching
The pseudowire switching feature provides the user with the ability to create a VLL service by
cross-connecting two spoke SDPs. The objective of this feature is to allow 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 7750 SR pseudowire switching node instead of what is normally the
remote PE. In addition, 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 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 7750 SR
pseudowire switching node to relay end-to-end pseudowire status notification messages between
the two PEs. The 7750 SR 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 7750 SR PE to identify the origin of the pseudowire status notification
message.
In the Figure 22, the user configures a regular Epipe VLL service in 7x50 PE1 and 7x50 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 7x50 pseudowire switching node. In addition, the user
configures an epipe VLL service on the 7x50 pseudowire switching node using the two SDPs.
Figure 9: Pseudowire Service Switching Node
Configuration examples can be found in Configuring Two VLL Paths Terminating on T-PE2 on
page 209.
|7x50 PE1 (Epipe)|---sdp 2:10---|7x50 PW SW (Epipe)|---sdp 7:15---|7x50 PE2 (Epipe)
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