stations. If you enable Port Fast on a port connecting to another switch, you risk creating a spanning-tree loop. If your switch is running PVST or MSTP, you can enable this feature by using the 搒panning-tree portfast?interface configuration or the 搒panning-tree portfast default?global configuration command
Cross Stack UplinkFast (CSUF)
Cross-stack UplinkFast (CSUF) provides a fast spanning-tree transition (fast convergence in less than 1 second under normal network conditions) across a stack of switches that use the GigaStack GBICs connected in a shared cascaded configuration (multidrop backbone). During the fast transition, an alternate redundant link on the stack of switches is placed in the forwarding state without causing temporary spanning-tree loops or loss of connectivity to the backbone. With this feature, you can have a redundant and resilient network in some configurations. You enable CSUF by using the 搒panning-tree stack-port?interface configuration command. The CSUF feature is supported only when the switch is running PVST.
PVST+
The IEEE 802.1Q standard for VLAN trunks imposes some limitations on the spanning-tree strategy for a network. The standard requires only one spanning-tree instance for all VLANs allowed on the trunks. However, in a network of Cisco switches connected through 802.1Q trunks, the switches maintain one spanning-tree instance for each VLAN allowed on the trunks. When you connect a Cisco switch to a non-Cisco device through an 802.1Q trunk, the Cisco switch uses per-VLAN spanning tree+ (PVST+) to provide spanning-tree interoperability. It combines the spanning-tree instance of the 802.1Q VLAN of the trunk with the spanning-tree instance of the non-Cisco 802.1Q switch. However, all PVST+ information is maintained by Cisco switches separated by a cloud of non-Cisco 802.1Q switches. The non-Cisco 802.1Q cloud separating the Cisco switches is treated as a single trunk link between the switches. PVST+ is automatically enabled on 802.1Q trunks, and no user configuration is required. The external spanning-tree behavior on access ports and Inter-Switch Link (ISL) trunk ports is not affected by PVST+.
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