Cisco FlexStack network stacking module Expansion slot. Cisco Catalyst 2960-S switches running FlexStack use a hop-by-hop method of transferring Ethernet packets across the stack. Packets traverse the stack by going from one member to another over the FlexStack links until the packet reaches its destination. This is the same behavior as multiple standalone Ethernet switches forwarding packets from one switch to another. There are multiple physical switches acting as a single unit. This single unit behavior is a logical switch.