Pseudowire Switching
Page 132 7210 SAS-X, R6 OS Services Guide
from T-PE2 to T-PE1. S-PE1 and S-PE2 will effect 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
towards 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.
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