(T) MPLS has strongly emerged as the best technology to provide initially IP VPNs also called Layer 3 VPNs and now PWS (Pseudo Wires Services) and VPLS (Virtual Private LAN Services) also called Layer 2 VPNs.

MPLS Pseudo-Wires Services

A pseudo wire service (PWS) provides a Layer 2 point-to-point service. The purpose of an MPLS PW is to emulate a legacy or a new service over an MPLS LSP. Present services supported by MPLS PW include Ethernet/VLAN, PPP/HDLC, Frame Relay and ATM services.

 All services are emulated like virtual circuits (VCs). The VC provides all the functions required to fully emulate the original service (in particular for FR and ATM operations).

PW uses the Martini encapsulation to carry the service over MPLS. The encapsulation provides two label layers: one for the emulated service and, another one for the LSP underlying tunnel.

The PW accomplishes mainly three functions:

• Encapsulation of circuit data or PDUs at the ingress; 
• Carrying the encapsulated data across the tunnel; 
• Managing the signaling, timing, order, OAM and specific aspects of the service.

The end-user does not have to change its previous layer 2 protocol from its CE to the PE to access the PW.

PW technology is defined in the IETF Working Group Pseudo Wire Emulation Edge to Edge (PWE3).  

PW Network Reference Design

MPLS Layer 2 VPNs Services: Virtual Private LAN Services (VPLS)

The primary goal of VPLS is to provide connectivity between dispersed enterprise sites across an MPLS metro network as if they were connected using a LAN. VPLS can be seen as if the MPLS metro network operates as a switch or bridge.

VPLS emulates the various LANs services over an MPLS transport network. It creates a Layer 2 broadcast domain basically through an Ethernet learning bridge model provided by the MPLS network.

Broadcast and multicast are two important LANs services used by Ethernet but are not supported by MPLS. VPLS extends the Martini encapsulation for transporting Ethernet and VLANs traffic across multiple sites that belong to the same LAN by providing in particular broadcast and multicast capabilities.

In order to do so, PE devices are required to dynamically learn MAC addresses on physical ports and on VC LSPs. And, MAC address is learned and aged on a per LSP basis. To support standard Ethernet bridging, a packet is replicated across LSPs for broadcast and multicast traffic, and for flooding of unknown unicast traffic.

VPLS is a Layer 2 MPLS VPN and an alternative to layer 3 MPLS VPN for two business reasons:
First, a number of customers, especially large ones mainly for security reasons, do not want to outsource their routing tables as required per RFC 4364;
Second, Ethernet as a replacement service to legacy DS1 and DS3 services is more and more demanded by enterprise customer to service providers. Most Service Providers have been offering Ethernet through Transparent
LAN Services (TLS). Unfortunately, TLS is a point-to-point service. VPLS, on the other hand, is a multipoint-to-multipoint service;

VPLS are defined presently through multiple drafts in the Layer 2 VPN working group of the IETF (L2VPN).

VPLS Network Reference Design

MPLS Layer 3 VPNs Services: BGP/MPLS IP VPNs (RFC 4364)

Service providers have been supplying IP VPNs to their commercial enterprise customers using their MPLS backbones.

Enterprise VPN routes are communicated from the CE to the PE using an Interior Gateway Protocol (IGP) such as the Open Shortest Path Protocol (OSPF) or an Exterior Gateway Protocol such as Exterior Border Gateway Protocol (eBGP).

The service provider’s PE propagates the VPN routes, called VPN routing and forwarding (VRF) to its PE peers using Interior BGP (iBGP). The enterprise VPN traffic is forwarded between the PEs connected to the enterprise site of the customer’s VPN using MPLS Label Switched Paths (LSPs) in a mesh topology.

Since enterprise networks can use private addresses that cannot be routed over the service provider Internet network, PE routers need to create a new address format using a route distinguisher and the end-user IPv4 prefix address. Multi-Protocol Extensions to iBGP (MP-BGP) is used to carry those created addresses.

BGP/MPLS IP VPNs are defined in RFC 4364 and part of the Layer 3 VPN working group of the IETF (L3VPN).

IP VPNs Network Reference Design

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