Information About VXLAN BGP EVPN
VXLAN is a MAC in IP/UDP overlay that allows layer 2 segments to be stretched across an IP core. All the benefits of layer 3 topologies are thereby available with VXLAN. The encapsulation and decapsulation of VXLAN headers is handled by a functionality embedded in VXLAN Tunnel End Points (VTEPs). VTEPs themselves could be implemented in software or a hardware form-factor.
VXLAN natively operates on a flood-n-learn mechanism where BU (Broadcast, Unknown Unicast) traffic in a given VXLAN network is sent over the IP core to every VTEP that has membership in that network. IP multicast is used to send traffic over the network.The receiving VTEPs decapsulate the packet, and based on the inner frame perform layer-2 MAC learning. The inner SMAC is learnt against the outer Source IP Address (SIP) corresponding to the source VTEP. In this way, reverse traffic can be unicasted toward the previously learnt end host.
Motivations for using an overlay architecture include:
Scalability — VXLAN provides Layer-2 connectivity that allows the infrastructure that can scale to 16 million tenant networks. It overcomes the 4094-segment limitation of VLANs. This is necessary to address today’s multi-tenant cloud requirements.
Flexibility — VXLAN allows workloads to be placed anywhere, along with the traffic separation required in a multi-tenant environment. The traffic separation is done using network segmentation (segment IDs or virtual network identifiers [VNIs]).Workloads for a tenant can be distributed across different physical devices (since workloads are added as the need arises, into available server space) but the workloads are identified by the same layer 2 or layer 3 VNI as the case may be.
Mobility — VMs can be moved from one data center location to another without updating spine switch tables. This is because entities within the same tenant network in a VXLAN/EVPN fabric setup retain the same segment ID, regardless of their location.
One of the biggest limitations of VXLAN flood-n-learn is the inherent flooding that is required ensuring that learning happens at the VTEPs. In a traditional deployment, a layer-2 segment is represented with a VLAN that comprises a broadcast domain, which also scopes BU traffic. With VXLAN, now the layer-2 segment spans a much larger boundary across an IP core where floods are translated to IP multicast (or HER). Consequently, the flood-n-learn based scheme presents serious scale challenges especially as the number of end hosts go up. This is addressed via learning using a control-plane for distribution of end host addresses. The control plane of choice is BGP EVPN.