Dell PowerSwitch S4112F-ON EMC SmartFabric OS10 User Guide Release 10.5.0 - Page 453

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 BPDU guard 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. Blocks the L2 bridged ports and LAG ports connected to end hosts and servers from receiving any BPDUs. When you enable BPDU guard and when the BPDU frames are being received on the interface, the bridge or LAG is placed in the blocking state. In case of a LAG, ports are either STP blocked or shutdown based on the error disable command action. The data traffic is dropped but the port continues to forward BPDUs to the CPU that are later dropped. To prevent further reception of BPDUs, configure a port to shut down using the error disable command. For more information on this command. Layer 2 453