The Cisco Systems GigaStack Gigabit Interface Converter (GBIC) is a versatile, low-cost, Gigabit Ethernet stacking GBIC that offers high-speed interconnectivity between Catalyst 3500 series XL and modular Catalyst 2900 series XL switches.Equipped with two 1000BaseX Gigabit Ethernet ports, the GigaStack GBIC provides a hardware-based, independent stack bus with 1-Gbps forwarding bandwidth in a daisy-chained configuration, or up to 2-Gbps forwarding rate in a point-to-point, full-duplex configuration. For customers who want a low-cost stacking solution, the GigaStack GBIC delivers a shared 1-Gbps stacking bus to connect up to nine Catalyst 3500XL and gigabit-enabled Catalyst 2900XL switches in a daisy-chain configuration. By inserting secondary GigaStack GBICs, users can create a redundant loopback connection from the top to the bottom of the stack. For the highest performance stacking, the Catalyst 3508G XL can be deployed with GigaStack GBICs to aggregate up to eight Catalyst 3500XL and gigabit-enabled Catalyst 2900XL switches in a Gigabit Ethernet star topology.