API Guide

Introduction to STP
The spanning-tree protocol is a Layer 2 network protocol that prevents loops in a network topology. Spanning-tree is useful
when more than one network path exists and devices in the network are either competing for or sharing these paths.
By eliminating loops, the protocol improves scalability in a large network and allows you to implement redundant paths, which
can be activated when the active paths fail.
Layer 2 loops occur in a network due to poor network design and without enabling xSTP protocols, can cause high switch CPU
utilization and memory consumption.
Supported STP modes
The following variants of spanning-tree protocols are used in OS10 to provide a loop free layer 2 topology:
● Rapid Spanning Tree protocol can be seen as an evolution of the 802.1D standard. Primarily RSTP is created to address the
slow convergence nature of STP protocol (802.1D).
● Multiple Spanning Tree protocol (MSTP) defined in IEEE standard (802.1s), is an evolution of spanning tree protocols
allowing creation of multiple instance of spanning tree and mapping multiple VLANs to a specific spanning tree instance.
● Rapid per-VLAN spanning-tree protocol (Rapid-PVST) is a variant of RSTP protocol and supports creation of per VLAN
spanning tree instance to isolate link fluctuations only to a particular VLAN segment and also helps in load balancing across
different links.
● 802.1D STP Compatibility mode support. This mode enables the bridge to function as an IEEE Std 802.1D legacy STP
compatible mode while the system is running RSTP or MSTP modes of the spanning tree protocol.
● RSTP and MSTP are backward compatible with STP 802.1D. When an interface receives STP BPDU, the system responds
with the STP version of BPDU.
Change STP modes
The default xSTP variant running in OS10 is Rapid-PVST. You can change the mode to RSTP or MSTP using the spanning-
tree mode {rstp | mst | rapid-pvst} command.
Mode specific functionality
Enable and disable STP
Spanning Tree Protocol (STP) is enabled by default on the switches. You can disable the STP globally on the switch or at the
interface level.
Disabling spanning tree at an instance level causes all the port members of that instance to disable the spanning tree. This
moves the port to the Forwarding / Blocking state based on the operational status of the ports.
Use the spanning-tree disable command to disable the STP.
Backward compatibility and interoperability
Spanning tree modes are backward compatible and inter-operable with STP version.
BPDU extensions
STP extensions provide a means to ensure efficient network convergence by securely enforcing the active network topology.
OS10 supports BPDU filtering, BPDU guard, root guard, and loop guard STP extensions.
The system discards regular data traffic after a BPDU violation.
BPDU filtering
Stops sending or receiving BPDUs from a faulty device, there by protecting the network from unexpected
flooding of BPDUs. Enabling BPDU Filtering on an interface causes the system to stop sending or
receiving BPDUs.
466 Layer 2