Users Guide

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 ush of learned media access control (MAC)
and address resolution protocol (ARP) addresses, requiring these addresses to be relearned. However, enabling RSTP can detect
potential loops caused by nonsystem issues such as cabling errors or incorrect congurations. RSTP is useful for potential loop
detection but to minimize possible topology changes after link or node failure, congure it using the following specications.
The following recommendations help you avoid these issues and the associated trac loss caused by using RSTP when you enable
VLT on both VLT peers:
• Congure 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
congured 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 conguration, if the node has non-VLT ports using RSTP that are not congured 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 congure any non-VLT ports as edge ports or have RSTP disabled.
Conguring Interfaces for Layer 2 Mode
To congure 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.
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Rapid Spanning Tree Protocol (RSTP)