Virtual Leased Line Services
7210 SAS-X, R6 OS Services Guide Page 135
The S-PE signals a label mapping message as soon as the local configuration is complete. The
control word C-bit field in the pseudowire FEC is set to the value configured on the static
spokesdp.
When the label mapping for the egress direction is also received from the T-LDP peer, and the
information in the FEC matches that of the local configuration, the static-to-dynamic crossconnect
is effected.
Note that it is possible that end nodes of a static pseudowire segment be misconfigured. In this
case, an S-PE or T-PE node may be receiving packets with the wrong encapsulation. In this case, it
is possible that an invalid payload will be forwarded over the pseudowire or the SAP respectively.
Furthermore, if the S-PE or T-PE node is expecting the control word in the packet encapsulation
and the received packet comes with no control word but the first nibble below the label stack is
0x0001, the packet may be mistaken for a VCCV OAM packet and may be forwarded to the CPM.
In that case, the CPM will perform a check of the IP header fields such as version, IP header
length, and checksum. If any of this fails the VCCV packet will be discarded.
Pseudowire Redundancy
Pseudowire redundancy provides the ability to protect a pseudowire with a pre-provisioned
pseudowire and to switch traffic over to the secondary standby pseudowire in case of a SAP and/or
network failure condition. Normally, pseudowires are redundant by the virtue of the SDP
redundancy mechanism. For instance, if the SDP is an RSVP LSP and is protected by a secondary
standby path and/or by Fast-Reroute paths, the pseudowire is also protected. However, there are a
couple of applications in which SDP redundancy does not protect the end-to-end pseudowire path:
• There are two different destination PE nodes for the same VLL service. The main use case
is the provision of dual-homing of a CPE or access node to two PE nodes located in
different POPs. The other use case is the provision of a pair of active and standby BRAS
nodes, or active and standby links to the same BRAS node, to provide service resiliency to
broadband service subscribers.
• The pseudowire path is switched in the middle of the network and the SR-Series
pseudowire switching node fails.
Pseudowire and VPLS link redundancy extends link-level resiliency for pseudowires and VPLS to
protect critical network paths against physical link or node failures. These innovations enable the
virtualization of redundant paths across the metro or core IP network to provide seamless and
transparent fail-over for point-to-point and multi-point connections and services. When deployed
with multi-chassis LAG, the path for return traffic is maintained through the pseudowire or VPLS
switchover, which enables carriers to deliver “always on” services across their IP/MPLS networks.
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