Important Points to Remember• RSTP is disabled by default.• 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 areautomatically added to the RST topology.• Adding a group of ports to a range of VLANs sends multiple messages to the rapid spanning treeprotocol (RSTP) task, avoid using the range command. When using the range command, DellNetworking 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 nodefailures. Spanning tree topology changes are distributed to the entire Layer 2 network, which can cause anetwork-wide flush of learned media access control (MAC) and address resolution protocol (ARP)addresses, requiring these addresses to be re-learned. However, enabling RSTP can detect potentialloops caused by non-system issues such as cabling errors or incorrect configurations. RSTP is useful forpotential loop detection but to minimize possible topology changes after link or node failure, configure itusing the following specifications.The following recommendations help you avoid these issues and the associated traffic loss caused byusing RSTP when you enable VLT on both VLT peers:• Configure any ports at the edge of the spanning tree’s operating domain as edge ports, which aredirectly connected to end stations or server racks. Ports connected directly to Layer 3-only routersnot running STP should have RSTP disabled or be configured as edge ports.• Ensure that the primary VLT node is the root bridge and the secondary VLT peer node has thesecond-best bridge ID in the network. If the primary VLT peer node fails, the secondary VLT peernode becomes the root bridge, avoiding problems with spanning tree port state changes that occurwhen a VLT node fails or recovers.• Even with this configuration, if the node has non-VLT ports using RSTP that are not configured asedge ports and are connected to other layer 2 switches, spanning tree topology changes can still bedetected after VLT node recovery. To avoid this scenario, ensure that you configure any non-VLTports 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.Rapid Spanning Tree Protocol (RSTP) 761
