MPLS LSPs VPN between PE

One thing to note is that with any VPWS (unlike a VPLS or IPLS), it is necessary for the end-points of the virtual private wires to be configured on the CE-devices, which must be capable of switching data on to the correct wire.  In terms of the criteria laid out in section 2, this means that a VPWS will generally require greater ongoing management effort from the VPN user than a VPLS or IPLS. 5.2.1 MPLS-based VPWS One of the simplest ways to create a VPWS is to use ATM or Frame Relay VCs between the PE devices and CE devices, and to cross-connect each of these to separate MPLS circuits (Label Switched Paths or LSPs) through the provider network, as illustrated in the diagram below.  Note that LSPs are uni-directional, and so two LSPs are required for each bi-directional connection. CE1 PE1CE3PE2PE3CE2VCs between the PEand CELSPs between PEdevicesThis is a relatively straightforward approach, and MPLS traffic engineering can be used to provide quality of service if this is required by the customer.  However, when used for multiple VPNs, it does not scale well in the provider network for the following reasons.   •  Firstly, each LSP through the provider network needs to be configured individually and then cross-connected to the specified VC at each end, which requires considerable management effort from the service provider.   •  Secondly, a large number of LSPs may be needed in the provider network, which uses large (and, compared with later solutions, wasteful) amounts of resource in the service provider's routers. 5.2.2 PWE3 VPWS An improvement on this approach is to use the PWE3 extensions to MPLS that are currently being standardized by the IETF in the PWE3 working group.   These extensions improve scalability by using a fixed number of MPLS LSPs VPN between PE devices in the provider network.  Emulated, point-to-point layer 2 connections (known as pseudo-wires or Martini pseudo-wires, after the author of the original draft) are then created between pairs of PE devices by tunneling through such an LSP. Copyright © 2003-2004 Data Connection Limited.  All Rights Reserved. Page 15 http://www.dataconnection.comThe signaling for these pseudo-wires is defined in draft-ietf-pwe3-control-protocol.  The encapsulation required for forwarding data across these pseudo-wires is defined for several layer 2 protocols, including ATM, Frame Relay and Ethernet (draft-ietf-pwe3-atm-encap, draft-ietf-pwe3-frame-relay and draft-ietf-pwe3-ethernet-encap). Therefore, an alternative to the MPLS based VPNs described above is to cross-connect layer 2 PE-CE connections with pseudo-wires using the appropriate layer 2 encapsulation.  Since each pseudo-wire only consumes resources in the PE devices, this is an improvement on the method described in 5.2.1, which also requires additional state in intermediate P devices.