• Dell Networking OS supports only one Rapid Spanning Tree (RST) instance.• All interfaces in virtual local area networks (VLANs) and all enabled interfaces in Layer 2 mode are automatically added to the RSTtopology.• Adding a group of ports to a range of VLANs sends multiple messages to the rapid spanning tree protocol (RSTP) task, avoid using therange command. When using the range command, Dell Networking recommends limiting the range to five ports and 40 VLANs.RSTP and VLTVirtual link trunking (VLT) provides loop-free redundant topologies and does not require RSTP.RSTP can cause temporary port state blocking and may cause topology changes after link or node failures. Spanning tree topology changesare distributed to the entire Layer 2 network, which can cause a network-wide flush of learned media access control (MAC) and addressresolution protocol (ARP) addresses, requiring these addresses to be re-learned. However, enabling RSTP can detect potential loops causedby non-system issues such as cabling errors or incorrect configurations. RSTP is useful for potential loop detection but to minimize possibletopology changes after link or node failure, configure it using the following specifications.The following recommendations help you avoid these issues and the associated traffic loss caused by using RSTP when you enable VLT onboth VLT peers:• Configure any ports at the edge of the spanning tree’s operating domain as edge ports, which are directly connected to end stations orserver racks. Ports connected directly to Layer 3-only routers not running STP should have RSTP disabled or be configured as edgeports.• Ensure that the primary VLT node is the root bridge and the secondary VLT peer node has the second-best bridge ID in the network. Ifthe primary VLT peer node fails, the secondary VLT peer node becomes the root bridge, avoiding problems with spanning tree portstate changes that occur when a VLT node fails or recovers.• Even with this configuration, if the node has non-VLT ports using RSTP that are not configured as edge ports and are connected toother layer 2 switches, spanning tree topology changes can still be detected after VLT node recovery. To avoid this scenario, ensurethat you configure any non-VLT ports as edge ports or have RSTP disabled.Configuring Interfaces for Layer 2 ModeTo configure and enable interfaces in Layer 2 mode, use the following commands.All interfaces on all bridges that participate in Rapid Spanning Tree must be in Layer 2 and enabled.1 If the interface has been assigned an IP address, remove it.INTERFACE modeno ip address2 Place the interface in Layer 2 mode.INTERFACE modeswitchport3 Enable the interface.INTERFACE modeno shutdownExample of Verifying an Interface is in Layer 2 Mode and EnabledTo verify that an interface is in Layer 2 mode and enabled, use the show config command from INTERFACE mode. The bold linesindicate that the interface is in Layer 2 mode.Rapid Spanning Tree Protocol (RSTP) 783
