Pseudowire Control Word; Pseudowire Redundancy - Alcatel-Lucent 7705 Service Manual

VLL Service Considerations

Pseudowire Control Word

The PW control word (CW) is a 32-bit field that is inserted between the VC label and the
Layer 2 frame. The presence of the control word is indicated by the C bit of the FEC element
used in LDP signaling. The PW control word is described in RFC 4385.
The PW control word is supported for all implemented PW types (ATM N-to-1 cell mode,
Ethernet VLLs, SAToP, CESoPSN, and IP PW).
The following points describe the behavior of the 7705 SAR when it receives a Label
Mapping message for a PW. It is assumed that no Label Mapping message for the PW has
been sent to the next PW router yet. The 7705 SAR operating system does the following.

Pseudowire Redundancy

Pseudowire (PW) redundancy protects a PW and any services on the PW against endpoint
failures. This differs from LSP redundancy and FRR, which offer protection against link and
node failures within the backhaul network.
As shown in
to two endpoints at the MTSO (7x50-1 and 7x50-2), which is done using two spoke SDPs on
the 7705 SAR. This configuration removes any single point of failure from a given network.
If 7x50-1 loses all of its connectivity to the network or to the RNC, then the 7705 SAR can
reroute the PW traffic to 7x50-2, which switches traffic to the RNC.
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• If the received Label Mapping message has C = 0 (where C refers to the C bit of the
FEC element), a Label Mapping message with C = 0 is sent forward to the next
router (or hop). In this case, the control word is not used.
• If the received Label Mapping message has C = 1 and the PW is locally configured
such that the use of the control word is mandatory, then the 7705 SAR sends a Label
Mapping message with C = 1. In this case, the control word is used. (Note: SAToP
and CESoPSN are the only services that require the control word.)
• If the received Label Mapping message has C = 1 and the locally configured PW
does not support use of an optional control word (that is, Ethernet or ATM N-to-1
cell mode PWs), then the 7705 SAR sends a new Label Mapping message in which
the C bit is set to correspond to the locally configured preference for use of the
control word (that is, C = 0).
Figure 35, in order to provide redundant PWs, the 7705 SAR must signal PWs
7705 SAR OS Services Guide