2. Edge Services and Border Leafs

For two-tier and three-tier data center topologies, the role of the border leaf in the network is to provide external connectivity to the data center site. In addition, since all traffic enters and exits the data center through the border leaf switches, they present the ideal location in the network to connect network services like firewalls, load balancers, and edge VPN routers. The border leaf switches connect to the WAN edge devices in the network to provide external connectivity to the data center site. As a design principle, two border leaf switches are recommended for redundancy. The WAN edge devices provide the interfaces to the Internet and DCI solutions. For DCI, these devices function as the Provide Edge (PE) routers, enabling connections to other data center sites through WAN technologies like Multiprotocol Label Switching (MPLS) VPN and Virtual Private LAN Services (VPLS). The Brocade validated design for DCI solutions is discussed in a separate validated design document.

There are several ways that the border leafs connect to the data center site. In three-tier (super-spine) architectures, the border leafs are typically connected to the super-spines as depicted in Figure 2 (3tier) below. In two-tier topologies, the border leafs are connected to the spines as depicted in Figure 1 (2tier) above. Certain topologies may use the spine as border leafs (known as a border spine), overloading two functions into one. This topology adds additional forwarding requirements to spines—they need to be aware of the tenants, VNIs, and VXLAN tunnel encapsulation and de-encapsulation functions.

3. Optimized 5-Stage Layer 3 Clos Topology (Three-Tier)

Multiple PoDs based on leaf-spine topologies can be connected for higher scale in an optimized 5-stage folded Clos (three-tier) topology. This topology adds a new tier to the network, known as a super-spine. This architecture is recommended for interconnecting several EVPN VXLAN PoDs. Super-spines function similar to spines: BGP control-plane and IP forwarding based on the outer IP header in the underlay network. No endpoints are connected to the super-spine. Figure 2 shows four super-spine switches connecting the spine switches across multiple data center PoDs.

The connection between the spines and the super-spines follows the Clos principles: