Manual

Cisco IOS In Service Software Upgrade Process
Information About Performing ISSU
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Cisco IOS Release 12.2(28)SB
Versioning Capability in Cisco IOS Software to Support ISSU
Before the introduction of the ISSU capability, the SSO mode of operation required each RP to be
running like versions of Cisco IOS software. The operating mode of the system in a redundant HA
configuration is determined by exchanging version strings when the standby RP registers with the active
RP.
The system entered SSO mode only if the versions running on the both RPs were the same. If not, the
redundancy mode was reduced to ensure compatibility. With ISSU capability, the implementation allows
two different but compatible release levels of Cisco IOS images to interoperate in SSO mode and enables
software upgrades while packet forwarding continues. Version checking done before ISSU capability
was introduced is no longer sufficient to allow the system to determine the operating mode.
ISSU requires additional information to determine compatibility between software versions. Therefore,
a compatibility matrix is defined that contains information about other images with respect 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 RP, and to allow the system to determine the highest operating
mode it can achieve. Incompatible versions will not be able to progress to SSO operational mode.
The Cisco IOS infrastructure has been internally modified and redesigned to accommodate subsystem
versioning with ISSU. Cisco IOS subsystems correspond to feature sets and software component
groupings. Features or subsystems that maintain state information across RPs are HA-aware or SSO
clients. A mechanism called ISSU Framework, or ISSU protocol, allows subsystems within Cisco IOS
software to communicate RP to RP and to negotiate the message version for communication between
RPs. Internally, all NSF- and SSO-compliant applications or subsystems that are HA-aware must follow
this protocol to establish communication with their peer across different versions of software. (For
further information on operating modes, see the Stateful Switchover document.)
Compatibility Matrix
You can perform the ISSU process when the Cisco IOS software on both the active and the standby RP
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 will succeed 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 will succeed; however, some subsystems
will not be able to maintain state during the transition. The matrix entry designates the images to be
base-level compatible (B).
• Incompatible—A core set of system infrastructure exists that must be able to interoperate in a
stateful manner for SSO to function correctly. If any of these required features or protocols 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).
If you attempt to perform ISSU with a peer that does not support ISSU, the system automatically
uses Fast Software Upgrade (FSU) 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