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Copyright © 2010, Juniper Networks, Inc.
Bidirectional Switching Mode
In bidirectional switching mode, the router switches both ends of an APS pair to the same
working interface or to the same protect interface when either end determines that a
switch is required.
Possible reasons for initiating a bidirectional switch include:
Detection of a signal failure
Receipt of an "aps force" on page 92 or "aps manual" on page 92 from the local end
of an APS pair
Reversion to the working interface after a failure has been corrected and the timeout
value specified in "aps revert" on page 89 has expired
The devices at both the local and remote ends of an APS pair must support bidirectional
switching for the router to implement bidirectional switching mode. Otherwise, the router
implements unidirectional switching mode at both ends of the APS pair.
The router detects support for bidirectional switching by interpreting the values of the
K1 and K2 bytes in the SONET/SDH frame. For details about the meanings of the values
of K1 and K2 bytes, see "Communication Methods" on page 73.
Unidirectional Switching Mode
In unidirectional switching mode, the router switches only one end of an APS pair to the
working interface or to the protect interface when that end determines that a switch is
required. Possible reasons for initiating a unidirectional switch are the same as those
described in "Bidirectional Switching Mode" on page 73 for initiating a bidirectional switch.
Reversion After Switchover
A failed interface automatically reverts from the protect interface to the working interface
after the router detects that the working interface is operational and the timeout value
specified in "aps revert" on page 89 has expired. Reversion applies only to recovery from
failures.
You can configure the router to revert to the working interface at a specified time after
it recovers. This feature enables you to use the protect interface as a redundant connection
that functions only when the working interface is not available.
Communication Methods
The router communicates with the remote device by using the K1 and K2 bytes in the line
overhead of the SONET/SDH frame. The values of these bytes determine the switching
and protect actions. Table 7 on page 74 and Table 8 on page 74 list the meanings of the
values of the K1 and K2 bytes. The bytes are defined in Telcordia document
GR-253—Synchronous Optical Network (SONET) Transport Systems: Common Generic
Criteria, Revision 3 (September 2000). See requirement objects R5-56 [179] and R5-58
[181] for information about bit ordering and meaning for the K1 byte; see R5-67 [190v2]
for information about the K2 byte.
Chapter 3: Configuring Unchannelized OCx/STMx Interfaces
73
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