Administrator 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 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.
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.
INTERFACE mode
switchport
3. Enable the interface.
INTERFACE mode
no shutdown
To verify that an interface is in Layer 2 mode and enabled, use the show config command from INTERFACE mode. The bold lines
indicate that the interface is in Layer 2 mode.
DellEMC(conf-if-gi-1/1)#show config
!
interface GigabitEthernet 1/1
no ip address
switchport
no shutdown
DellEMC(conf-if-gi-1/1)#
Enabling Rapid Spanning Tree Protocol Globally
Enable RSTP globally on all participating bridges; it is not enabled by default.
When you enable RSTP, all physical and port-channel interfaces that are enabled and in Layer 2 mode are automatically part of the RST
topology.
• Only one path from any bridge to any other bridge is enabled.
• Bridges block a redundant path by disabling one of the link ports.
To enable RSTP globally for all Layer 2 interfaces, use the following commands.
1. Enter PROTOCOL SPANNING TREE RSTP mode.
CONFIGURATION mode
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Rapid Spanning Tree Protocol (RSTP)