Dell EMC SmartFabric OS10 User Guide Release 10.5.0

SFS Support for MSTP on Layer3 fabric

The default spanning tree mode in smart fabric services is Rapid-PVST.

The spanning tree behavior for Layer3 fabric is as follows:

• STP is enabled on Cluster control VLAN (VLAN 4000). The spine switches are configured to take over the STP root role.
• STP is disabled on all inter leaf-spine VLANs and leaf-leaf VLAN (VLANs from 4001-4091).
• STP is enabled on all user created VLANs.
• STP is disabled on server facing port.

In case you need to inter-operate switches controlled by SFS to external switches which are running RSTP or MSTP, SFS has an API to change the Global Fabric STP mode to MSTP. In this case SFS will create 2 reserved MSTIs:

• MST with instance id 63: Clustering control VLAN (VLAN 4000) is part of this instance. Spine switches are configured to take over as STP root for this MSTI.
• MST with instance id 62: All the inter switch reserved VLANs (4001-4091) are part of this configuration. On this MSTI, spanning tree is disabled.
• All user created VLANs are part of CST (default MST instance), which inter-operates with RSTP. STP is enabled for this MSTI.
• STP is disabled at port-level for all server facing ports.

By default, the STP mode is rPVST+. You can change the mode to MSTP once the fabric is build out. When you change the mode, the whole fabric goes through a reboot cycle and the new mode will be set as MSTP. This step is traffic impacting.

When the mode is changed, MSTi are created and VLANS are assigned to these MSTi. The CST is configured with STP priority such that SFS controlled switches have lower priority to become a root bridge.

There is no change on existing STP behavior for SFS controlled entities because of this change. All other STP behaviors such as Disabling of STP on server facing ports still holds good.