Introduction

The function of spanning tree is to prevent loops in Layer 2 networks, which can cause congestion and reach such a high saturation level that network performance is impacted. Spanning Tree enablement is part of the SmartFabric automation process when a switch fabric is created.

Spanning Tree options for OS10 and SFS

Dell Ethernet switches running OS10 in full switch mode support RSTP (Rapid Spanning Tree Protocol), Multiple Spanning Tree (MST) and Rapid Per-VLAN Spanning Tree Plus (RPVST+). RPVST+ is the spanning tree protocol configured by default in SmartFabric mode, with RSTP as an optional configuration setting.

The primary difference with RPVST+ to RSTP is that RPVST+ can create separate spanning tree instances on a per-VLAN instance. This feature makes it more efficient than other static spanning tree protocols that support a single network topology, and why it is the default setting for a SmartFabric-enabled network.

Another similar spanning tree protocol, Per-VLAN Spanning Tree (PVST), also creates a network topology on a per-VLAN basis. However, RPVST+ differs from PVST in that it sends industry standard Bridge Protocol Data Unit (BPDU) packets between the switches. The BPDU packets exchanged across the switches contain information about key metrics like priority and cost, and are used to manage an adaptive network topology. PVST is not currently a supported protocol when running SFS.

Figure 24.          Static RSTP topology compared to per-VLAN support for RPVST+

Spanning Tree guidelines for VxRail and SmartFabric networks

At the time SFS is enabled on a set of switches, the initial spanning tree setting is applied to the switch fabric. SFS will disable spanning tree on the switch ports connected to the VxRail nodes. These switch ports are configured by SFS as trunked ports. When the VxRail cluster is then built on the switch fabric, all the networks created for VxRail are VxLAN-backed virtual networks, and not VLAN-backed networks. The VxLAN protocol does not support the broadcast of BPDU packets, so there is no requirement of switch ports connected to VxRail nodes needing to process BPDU packets as edge ports.

However, once a switch fabric expands to a multirack configuration, the recommendation is that RSTP, if used, should be disabled and RPVST+ enacted for a SmartFabric-enabled switch network. RPVST+ is the default setting in SFS, and is the spanning tree setting configured to the switch fabric. In a multirack topology with leaf and spine switches, RPVST+ will enforce that the links between the leaf and spine switches will not be active/active. This is due to the common VLAN requirements supporting the VxLAN network across racks in a leaf and spine topology.

In a multirack switch fabric, RPVST+ applies updated topology rules if changes are made to the switch fabric as a part of day 2 operations to prevent loops in the network. In addition, SmartFabric also enforces a single path topology for the Layer 2 networks between the ingress and egress points on the switch fabric to prevent looping with the upstream network.

For Layer 3 connectivity, enabling static routing or peering through BGP is the recommended method. Spanning tree settings on the upstream network are not impacted with the Layer 3 routed option. The default RPVST+ setting can stay in effect on the leaf and spine switch network.

Spanning tree rules can come into effect in either of these conditions:

• Layer 2 VLAN-based networks from the leaf switch layer to the upstream network
• Layer 3 properties are assigned to a VLAN for passage to the upstream network.

For Layer 2 VLAN-based networks:

• The uplink created by SFS is a port channel, either LAG or VLTi multi-chassis LAG (MLAG). Traffic distribution follows standard load-balancing principles across all the physical links within the port channel.
• SFS enforces a single logical uplink is active to prevent looping with the upstream network.
• SFS enforces that a Layer 2 network is only allowed to be added on one of the Layer 2 uplinks in the SmartFabric network, negating the requirement for spanning tree to enforce a single path rule.

For Layer 3 VLAN-backed networks, RPVST+ is the only spanning tree option. In these instances:

• The receiving switches upstream must be running RPVST+ to enable spanning tree integration with the SmartFabric-based network.
• Configure root bridges in the upstream network to optimize distribution of the VLAN traffic.

Figure 25.          Per-VLAN root bridges at distribution layer for RPVST+