What Are Pseudo-Wires?
A Pseudo-wire is a mechanism used in data networking and telecommunications that is used
to emulate various services using packet switched networks with technologies such as Ethernet,
MPLS (Multi Protocol Labelled Switching) and IP (Internet Protocol). It is often termed a Pseudo
Wire and abbreviated simply as PW. Telecommunications and Data Network Services such as T1
/ E1 lines, ATM, Sonet, SDH and Ethernet are the common services emulated using Pseudo-wires
The two main types of service emulation are VPWS (Virtual Private Wire Service) and VPLS
(Virtual Private LAN Service)
Point-to-Point connectivity can be provided between customer sites through the use of VPWS,
where the service provider can be thought of as emulating a pair of wires between customer sites.
The second type of service known as VPLS is used to extend a customer LAN (Local Area
Network) as far as the Service Provider edge. The Service Provider then takes up the role of
emulating the function of the LAN switches to connect all the customer remote LANs into a single
bridged Ethernet LAN.
This type of emulated network service is mainly used when a customer is using ATM (Asynchronous
Transfer Mode) or Frame Relay connections. The customer connections remain unchanged, but when
the date reaches the Service Provider edge it is encapsulated inside IP and routed across the SPs
Core IP Backbone. There is little, if any additional cost or configuration changes to the customer.
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.
PWE3 extensions to the MPLS system allow for a number of MPLS LSPs (Label Switched Paths)
between Service Provider PE (Provider Edge) devices. Layer 2 point-to-point Pseudo Wires are
created between the PE devices and generally LDP (Label Distribution Protocol) is used as the
signaling protocol in conjunction with Routing Protocols. Another method actually uses MPBGP
(Multi Protocol Border Gateway Protocol) to advertise the CE (Customer Edge) devices such as
Routers and Ethernet Switches. This allows multiple customer sites to set up VPWS.
Another type of Pseudo Wire layer 2 VPN is a VPLS. In this scenario the Ethernet LAN
at each customer site is extended as far as the edge of the provider network., where the SP
emulates the function of the LAN switches to connect all the customer remote LANs into a single
bridged Ethernet LAN.
One of the drawbacks of a VPWS is in the fact that it only provides Point-to-Point services,
whereas VPLS can provide a point-to-multipoint service. With VPWS the CE devices must perform
Layer 2 switching and select the correct Pseudo Wires, but with VPLS all traffic is sent to
the Service Provider PE routers.
The idea of VPLS is that the SP sets up a full mesh topology network of Pseudo Wires
between the edge devices which can replicate packets and also perform MAC Address learning
which is the basic function of an Ethernet Switch. A number of Auto-Discovery mechanisms allow
the SP PE devices to correctly setup the appropriate Pseudo Wires. The most common of these
are LDP and BGP, although RADIUS (Remote Access Dial In User Service) is another option. There
needs be an Auto-Discover process, particularly in Full-Mesh networks with many sites.
This article on Pseudo Wires was written by David Christie, MD at NSTUK Ltd, Website
Network Systems Training
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