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 loop-free path.
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). Now, these faster
convergence times are available as you create STP for each VLAN, which is known
as Per VLAN Rapid Spanning Tree (Rapid PVST+).
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
software can interoperate with legacy 802.1D systems, the system runs Rapid
PVST+ and MST.
Rapid PVST+ is the default STP protocol for Cisco Nexus
uses the extended system ID and MAC address reduction; you cannot disable these
In addition, Cisco
has created some proprietary features to enhance the spanning tree activities.