Specific VxRail networks, including the VxRail external management network and any external-facing end-user networks that are configured for VxRail, must have routing services that are enabled to support connectivity to external services and applications, as well as end-users.
A leaf-spine network topology in the most common use case for VxRail clusters. A single VxRail cluster can start on a single pair of switches in a single rack. When workload requirements expand beyond a single rack, expansion racks can be deployed to support the additional VxRail nodes and switches. The switches at the top of those racks, which are positioned as a ‘leaf’ layer, can be connected together using switches at the adjacent upper layer, or ‘spine’ layer.
If you choose to use a spine-leaf network topology to support the VxRail cluster or clusters in your data center, enabling Layer 3 routing services at either the spine layer or the leaf layer can both be considered.
Establishing routing services at the spine layer means that the uplinks on the leaf layer are trunked ports, and pass through all the required VLANs to the switches at the spine layer. This topology has the advantage of enabling the Layer 2 networks to span across all the switches at the leaf layer. This topology can simplify VxRail clusters that extend beyond one rack, because the Layer 2 networks at the leaf layer do not need Layer 3 services to span across multiple racks. A major drawback to this topology is scalability. Ethernet standards enforce a limitation of addressable VLANs to 4094, which can be a constraint if the application workload requires a high number of reserved VLANs, or if multiple VxRail clusters are planned.
Enabling routing services at the leaf layer overcomes this VLAN limitation. This option also helps optimize network routing traffic, as it reduces the number of hops to reach routing services. However, this option does require Layer 3 services to be licensed and configured at the leaf layer. In addition, since Layer 2 VxRail networks now terminate at the leaf layer, they cannot span across leaf switches in multiple racks.
Note: If your network supports VTEP, which enables extending Layer 2 networks between switches in physical racks over a Layer 3 overlay network, that can be considered to support a multi-rack VxRail cluster.