In Figure 1-1, we have a routed 3-stage Clos Fabric, where all Inter-Switch links are routed point-to-point layer-3 connections. As explained in previous sections, a switched layer-2 network with an STP control plane allows only one active path per VLAN/Instance and VLAN-based traffic load sharing. Due to the Equal Cost Multi-Path (ECMP) supported by routing protocols, a routed Clos Fabric enables flow-based traffic load balancing using all links from the ingress leaf via the spine layer down to the egress leaf. The convergence time for routing protocols is faster and less disruptive than STP topology change. Besides, a routed Clos Fabric architecture allows horizontal bandwidth scaling. We can increase the overall bandwidth capacity between switches, by adding a new spine switch. Dynamic routing protocols allow standalone and virtualized devices lossless In-Service Software Update (ISSU) by advertising infinite metrics or withdrawing all advertised routes.
But how do we stretch layer-2 segments over layer-3 infrastructure in a Multipoint-to-Multipoint manner, allowing tenant isolation and routing between segments? The answer relies on the Network Virtualization Overlay (NVO3) framework.
BGP EVPN, as an NVO3 control plane protocol, uses EVPN Route Types (RT) in update messages for identifying the type of advertised EVPN NLRIs (Network Layer Reachability Information). Besides publishing prefix information with RT-5 (IP Prefix Route), BGP EVPN uses RT-2 (MAC-IP advertisement) for publishing hosts’ MAC/IP addresses NLRI. Among these two fundamental route types, BGP EVPN can create a shared delivery tree for layer-2 Broadcast traffic, such as ARP Request messages, without using a Multicast-enabled underlay network. Besides, BGP EVPN allows us to implement a Tenant Routed Multicast (TRM) solution. We can use a vPC for device multihoming, but BGP EVPN has a built-in ESI multihoming option utilizing RT-1 (Ethernet AD Route) and RT-4 (Ethernet Segment Route). This solution uses a proactive control plane learning, where Leaf switches publish reachability information when a hos joins the network.
Virtual Extensible LAN (VXLAN) encapsulation allows switches to add a Layer-2 Virtual Network Identifier (L2VNI) for Intra-VLAN traffic and L3VNI for Tenant-specific/VRF Inter-VLAN connections. The Generic Protocol Extension for VXLAN (VXLAN-GPE) enables leaf switches to add a Group Policy to data packets.
Finally, adding a new Layer-2 segment to a BGP EVPN fabric requires configuration only in leaf switches. We don’t have to touch Inter-Switch links or spine switches, like we must do in Layer-2 switched infrastructure.
In the upcoming chapter, we delve deeper into the implementation and advantages of the BGP EVPN with VXLAN data center fabric solution.
Figure 1-1: Routed 3-Stage Clos Fabric with BGP EVPN and VXLAN.
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