Traditionally, VLANs have been the standard method for providing network segmentation in campus networks. VLANs use loop prevention
techniques such as Spanning Tree Protocol (STP), which impose restrictions on network design and resiliency. Further, there
is a limitation with the number of VLANs that can be used to address layer 2 segments (4094 VLANs). Therefore, VLANs are a
limiting factor for IT departments and cloud providers who build large and complex campus networks.
VXLAN is designed to overcome the inherent limitations of VLANs and STP. It is a proposed IETF standard [RFC 7348] to provide the same Ethernet Layer 2 network services as VLANs do, but with greater flexibility. Functionally, VXLAN is
a MAC-in-UDP encapsulation protocol that runs as a virtual overlay on an existing Layer 3 network.
However, VXLAN by itself does not provide optimal switching and routing capabilities in a network because it uses a “flood
and learn” mechanism that limits scalability. "Flood and learn" mechanism is where the host’s information is flooded across
the network for it to be reachable. To provide optimal switching and routing capabilities, a VXLAN overlay requires:
An underlying transport network that performs data plane forwarding to allow unicast communication between end points connected
to the fabric.
A control plane that is capable of distributing Layer 2 and Layer 3 host reachability information across the network.
To meet these additional requirements, Internet drafts submitted by the bess workgroup (draft-ietf-bess-evpn-overlay-12) proposed MP-BGP to carry Layer 2 MAC and Layer 3 IP information simultaneously. MP-BGP incorporates Network Layer Reachability
Information (NLRI) to achieve this. With MAC and IP information available together for forwarding decisions, routing and switching
within a network is optimized. This also minimizes the use of the conventional "flood and learn" mechanism used by VXLAN and
allows for scalability in the fabric. EVPN is the extension that allows BGP to transport Layer 2 MAC and Layer 3 IP information.
This deployment is called a BGP EVPN VXLAN fabric (also referred to as VXLAN fabric).