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Dell EMC Networking OS Configuration Guide for the S5048F–ON System 9.14.2.8

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 network-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 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 second-best 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.

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