Rstp And Vlt; Configuring Interfaces For Layer 2 Mode - Dell S3048-ON Configuration Manual

• 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 are automatically added to the RST
topology.
• Adding a group of ports to a range of VLANs sends multiple messages to the rapid spanning tree protocol (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 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
Example of Verifying an Interface is in Layer 2 Mode and Enabled
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.
Dell(conf-if-gi-1/1)#show config
!
interface GigabitEthernet 1/1
no ip address
switchport
Rapid Spanning Tree Protocol (RSTP) 687