Spanning Tree - Cisco Nexus 9000 Series Configuration Manual

Spanning Tree

The devices support 4095 VLANs in accordance with the IEEE 802.1Q standard. These VLANs are organized
into several ranges, and you use each range slightly differently. Some of these VLANs are reserved for internal
use by the device and are not available for configuration.
Inter-Switch Link (ISL) trunking is not supported on the Cisco NX-OS.
Note
Spanning Tree
This section discusses the implementation of the Spanning Tree Protocol (STP) on the software. Spanning
tree is used to refer to IEEE 802.1w and IEEE 802.1s. When the IEEE 802.1D Spanning Tree Protocol is
referred to in the publication, 802.1D is stated specifically.
STP Overview
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 tree topology.
Although the 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 devices.
Note 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.
Rapid PVST+
Rapid PVST+ is the default spanning tree mode for the software and is enabled by default on the default
VLAN and all newly created VLANs.
A single instance, or topology, of RSTP runs on each configured VLAN, and each Rapid PVST+ instance on
a VLAN has a single root device. You can enable and disable STP on a per-VLAN basis when you are running
Rapid PVST+.
Cisco Nexus 9000 Series NX-OS Layer 2 Switching Configuration Guide, Release 7.x
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Overview