2.1.h Describe chassis virtualization and aggregation technologies

2.1.h [i] Multichassis

A VSS combines a pair of Catalyst 6500 series switches into a single logical network element. The VSS manages the redundant links, which externally act as a single port channel. The VSS simplifies network configuration and operation by reducing the number of Layer 3 routing neighbors and by providing a loop-free Layer 2 topology.

http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst6500/ios/12-2SX/configuration/guide/book/vss.html