Core issue
STP does not allow loops to be formed when switches or bridges are interconnected through multiple paths. STP implements the 802.1D IEEE algorithm, as Bridge Protocol Data Unit (BPDU) messages are exchanged with other switches to detect loops. Then, selected bridge interfaces are shutdown in order to remove the loop. This algorithm guarantees that there is one and only one active path between two network devices.
Refer to Understanding Spanning-Tree Protocol Topology Changes for more information.
Resolution
Sun Fire B1600 Blade System switches do not support STP mode in VLAN Spanning Tree Plus (PVST+) mode, which is the default mode for Cisco switches. Only rapid-PVST is compatible with Sun Fire B1600 Blade System switches in STP mode.
