What is the function of NSF?

A.
forward traffic simultaneously using both supervisors

B.
forward traffic based on Cisco Express Forwarding

C.
provide automatic failover to back up supervisor in VSS mode

D.
provide nonstop forwarding in the event of failure of one of the member supervisors

Explanation:
VSS is network system virtualization technology that pools multiple Cisco Catalyst 6500 Series Switches into one virtual switch, increasing operational efficiency,
boosting nonstop communications, and scaling system bandwidth capacity to 1.4 Tbps. Switches would operate as

a single logical virtual switch called a virtual switching system 1440 (VSS1440). VSS formed by two Cisco Catalyst 6500 Series Switches with the Virtual Switching
Supervisor 720-10GE. In a VSS, the data plane and switch fabric with capacity of 720 Gbps of supervisor engine in each chassis are active at the same time on
both chassis, combining for an active 1400-Gbps switching capacity per VSS. Only one of the virtual switch members has the active control plane. Both chassis are
kept in sync with the inter-chassis Stateful Switchover (SSO) mechanism along with Nonstop Forwarding (NSF) to provide nonstop communication even in the
event of failure of one of the member supervisor engines or chassis.
http://ciscorouterswitch.over-blog.com/article-cisco-catalyst-6500-series-vss-1440- 124536783.html