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Software Configuration Guide—Release IOS XE 3.6.0SG and IOS 15.2(2)SG
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Chapter 6 Configuring the Cisco IOS In-Service Software Upgrade Process
About ISSU
ISSU requires additional information to determine compatibility between software versions. A
compatibility matrix is defined, containing information about other images relative to the one in
question. This compatibility matrix represents the compatibility of two software versions, one running
on the active and the other on the standby supervisor engine, and to allow the system to determine the
highest operating mode it can achieve. Incompatible versions cannot progress to SSO operational mode.
Compatibility Matrix
You can perform the ISSU process when the Cisco IOS software on both the active and the standby
supervisor engine is capable of ISSU and the old and new images are compatible. The compatibility
matrix information stores the compatibility among releases as follows:
• Compatible—The base-level system infrastructure and all optional HA-aware subsystems are
compatible. An in-service upgrade or downgrade between these versions succeeds with minimal
service impact. The matrix entry designates the images to be compatible (C).
• Base-level compatible—One or more of the optional HA-aware subsystems is not compatible. An
in-service upgrade or downgrade between these versions succeeds; however, some subsystems
cannot always maintain state during the transition from the old to the new version of Cisco IOS. The
matrix entry designates the images to be base-level compatible (B).
However, you should be able to perform an ISSU upgrade without any functionality loss even if the
matrix entry is B. The downgrade may experience some functionality loss if the newer image had
additional functionality.
• Incompatible—A core set of system infrastructure exists in Cisco IOS that must be able to
interoperate in a stateful manner for SSO to function correctly. If any of these required features or
subsystems is not interoperable, then the two versions of the Cisco IOS software images are declared
to be incompatible. An in-service upgrade or downgrade between these versions is not possible. The
matrix entry designates the images to be incompatible (I). The system operates in RPR mode during
the period when the versions of Cisco IOS at the active and standby supervisor engines are
incompatible.
If you attempt to perform ISSU with a peer that does not support ISSU, the system automatically
uses RPR instead.
The compatibility matrix represents the compatibility relationship a Cisco IOS software image has with
all of the other Cisco IOS software versions within the designated support window (for example, all of
those software versions the image “knows” about) and is populated and released with every image. The
matrix stores compatibility information between its own release and prior releases. It is always the
newest release that contains the latest information about compatibility with existing releases in the field.
The compatibility matrix is available within the Cisco IOS software image and on Cisco.com so that
users can determine in advance whether an upgrade can be done using the ISSU process.
To display the compatibility matrix data between two software versions on a given system, enter the
show issu comp-matrix stored command.
Note This command is useful only for verification purposes because it is available only after the ISSU process
has started. You might want to check the compatibility matrix prior to starting ISSU. Use the Feature
Navigator to obtain the needed information:
http://tools.cisco.com/ITDIT/CFN/jsp/index.jsp
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