STP provides a loop-free
network at the Layer 2 level. Layer 2 LAN ports send and receive STP frames,
which are called Bridge Protocol Data Units (BPDUs), at regular intervals.
Network devices do not forward these frames but use the frames to construct a
802.1D is the original
standard for STP, and many improvements have enhanced the basic loop-free STP.
You can create a separate loop-free path for each VLAN, which is named Per VLAN
Spanning Tree (PVST+). Additionally, the entire standard was reworked to make
the loop-free convergence process faster to keep up with the faster equipment.
This STP standard with faster convergence is the 802.1w standard, which is
known as Rapid Spanning Tree (RSTP).
Finally, the 802.1s standard,
Multiple Spanning Trees (MST), allows you to map multiple VLANs into a single
spanning tree instance. Each instance runs an independent spanning tree
Although the software can
interoperate with legacy 802.1D systems, the device runs Rapid PVST+ and MST.
You can use either Rapid PVST+ or MST in a given VDC; you cannot mix both in
one VDC. Rapid PVST+ is the default STP protocol.
Cisco NX-OS uses the extended system ID and MAC address reduction; you
cannot disable these features.
In addition, Cisco has created
some proprietary features to enhance the spanning tree activities.