Which statement correctly describes unified in-service software upgrades (ISSU)?
A. Unified ISSU is built upon graceful-restart extensions to allow upgrades with no impact to the data plane.
B. Unified ISSU is supported on systems with single or dual Routing Engines.
C. Unified ISSU permits both upgrades and downgrades with no control plane disruption and is based on the graceful Routing Engine switchover (GRES) feature.
D. Unified ISSU is based on nonstop-routing (NSR) and graceful Routing Engine switchover (GRES), and permits software upgrades only.
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