MPLS-Enabled Applications: Emerging Developments and New Technologies

11.4. PRINCIPLES OF LAYER 2 TRANSPORT OVER MPLS

There are two main approaches to Layer 2 transport over MPLS: one involving LDP signaling [MRT-TRS, PWE3-CON] and the other based on BGP signaling [KOM-BGP]. In the forwarding plane, these approaches are the same, in terms of how Layer 2 frames are encapsulated for transport across the MPLS network. However, the two approaches differ significantly in the control plane. In later sections, we will discuss how each approach operates and then compare and contrast the two.

A single point-to-point Layer 2 connection provided over an MPLS network is sometimes called a pseudowire, to convey the principle that as far as possible the MPLS network should be invisible to the end customer, in such a way that the two CEs interconnected by the pseudowire appear to be directly connected back to back. An MPLS-based L2VPN is composed of a collection of pseudowires that interconnect a customer's CEs in different locations, in a topology chosen by the customer, for example a full-mesh or hub-and-spoke arrangement.

One of the problems with traditional Layer 2 VPNs is the administrative burden of adding a new site to an existing VPN, and the associated lead-times. If the sites are fully meshed, when a new site is introduced a new circuit must be provisioned between the new site and every other site in the network, and hence extra configuration at every site in the network is required. Indeed, often this administrative burden has forced customers to adopt ...