Alcatel-Lucent 7750 SR OS Service Manual page 134

Pseudowire Redundancy
In this application, a new pseudowire status bit of active or standby indicates the status of the SAP
in the MC-LAG instance in the 7x50 aggregation node. All spoke SDPs are of secondary type and
there is no use of a primary pseudowire type in this mode of operation. Node A is in the active
state according to its local MC-LAG instance and thus advertises active status notification
messages to both its peer pseudowire nodes, for example, nodes C and D. Node D performs the
same operation. Node B is in the standby state according to the status of the SAP in its local MC-
LAG instance and thus advertises standby status notification messages to both nodes C and D.
Node C performs the same operation.
A 7x50 node selects a pseudowire as the active path for forwarding packets when both the local
pseudowire status and the received remote pseudowire status indicate active status. However, a
7x50 in standby status according to the SAP in its local MC-LAG instance is capable of processing
packets for a VLL service received over any of the pseudowires which are up. This is to avoid
black holing of user traffic during transitions. The 7x50 standby node forwards these packets to the
active node via the Inter-Chassis Backup pseudowire (ICB pseudowire) for this VLL service. An
ICB is a spoke SDP used by a MC-LAG node to backup a MC-LAG SAP during transitions. The
same ICB can also be used by the peer MC-LAG node to protect against network failures causing
the active pseudowire to go down.
Note that at configuration time, the user specifies a precedence parameter for each of the
pseudowires which are part of the redundancy set as in the application in
Two Destination PE Nodes on page
forward packet to in case both pseudowires show active/active for the local/remote status during
transitions.
Only VLL service of type epipe is supported in this application. Furthermore, ICB spoke SDP can
only be added to the SAP side of the VLL cross-connect if the SAP is configured on a MC-LAG
instance.
More details on the behavior of this scenario can be found in
LAG and Pseudowire Redundancy on page
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130. A 7x50 node uses this to select which pseudowire to
133.
7750 SR OS Services Guide
VLL Resilience with
Access Node Resilience using MC-