Dell C1048P Port Extender Networking Configuration Guide for the C9000 Series - Page 959

Important Points to Remember • RSTP is disabled by default on the switch. • The system 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 RST topology. • Adding a group of ports to a range of VLANs sends multiple messages to the RSTP task, avoid using the range command. When using the range command, Dell Networking recommends limiting the range to five ports and 40 VLANs. RSTP and VLT Virtual 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 changes are distributed to the entire Layer 2 network, which can cause a networkwide 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 potential loops caused by non-system issues such as cabling errors or incorrect configurations. RSTP is useful for potential loop detection but to minimize possible topology 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 on both 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 or server racks. Ports connected directly to Layer 3-only routers not 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 the secondbest bridge ID in the network. If the primary VLT peer node fails, the secondary VLT peer node becomes the root bridge, avoiding problems with spanning tree port state 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 to other layer 2 switches, spanning tree topology changes can still be detected after VLT node recovery. To avoid this scenario, ensure that you configure any non-VLT ports as edge ports or have RSTP disabled. Configuring Interfaces for Layer 2 Mode To 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 mode no ip address 2 Place the interface in Layer 2 mode. Rapid Spanning Tree Protocol (RSTP) 959