Pseudowire Switching
Page 144 7210 SAS M Services Guide
Pseudowire Switching Behavior
In the network in Figure 22, PE nodes act as masters and pseudowire switching nodes act as slaves
for the purpose of pseudowire signaling. This is because a switching node will need 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 will include 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 operation 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 will effect the spoke SDP cross-connect only
when both directions of the pseudowire have been signaled and matched.
Pseudowire status notification messages can be generated by the T-PE nodes and/or the S-PE
nodes. Pseudowire status notification 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 VC ID value in the FEC TLV.
The merging of the received T-LDP status notification message and the local status for the spoke
SDPs from the service manager at a PE complies with the following rules:
• When the local status for both spokes is up, the S-PE passes any received SAP or SDP-
binding generated status notification message unchanged, for example, the status
notification TLV is unchanged but the VC-ID in the FEC TLV is set to value of the
pseudowire segment to the next hop.
• When the local operational status for any of the spokes is down, the S-PE always sends
SDP-binding down status bits regardless if the received status bits from the remote node
indicated SAP up/down or SDP-binding up/down.
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