Pseudowires and Layer 2 VPNs
The scale and flexibility of MPLS makes it an ideal vehicle to carry a diverse range
of services, all sharing the same underlying MPLS transport. This greatly simplifies
a service provider's infrastructure. Layer 2 services are described here.
Other services that can be carried using MPLS include
Carrier Ethernet and
Layer 3 VPNs.
Different standards bodies use different terminology in this area.
- The IETF use "pseudowires" or "PWE3" for an emulated
bi-directional point-to-point circuit, "Virtual Private Wire Service" (VPWS)
for a point-to-point L2 service, and "Virtual Private LAN Service" (VPLS)
for a multipoint-to-multipoint service.
- The Metro Ethernet Forum (MEF) concentrates only on Ethernet L2 services and uses
"E-LINE" for point-to-point, "E-TREE" for point-to-multipoint
and "E-LAN" for multipoint-to-multipoint.
All variants are built using a basic pseudowire for transport across the
wide-area MPLS network. A pseudowire is flexible, being able to carry almost any
kind of traffic, such as Ethernet frames, ATM cells, Frame Relay, TDM circuits etc.
Pseudowires are widely used in Mobile Backhaul,
allowing 2G and 3G traffic to be carried across a single network by using
multiple pseudowires.
L2 VPNs are often used to provide Ethernet services such as
Carrier Ethernet. L2 VPNs are constructed
using pseudowires between Provider Edge (PE) routers. The PE routers are administered
by the service provider to connect Customer Edge (CE) Ethernet ports or VLANs
to the appropriate pseudowire, and hence provide an Ethernet Service between
VPN Site 1 and Site 2 in the above diagram.
L2 VPNs support all the point-to–point, point-to-multipoint and multipoint-to-multipoint
architectures above. For the most sophisticated case (VPLS), a mesh of pseudowires
between the relevant PE routers is established and L2 switching is used to inject packets
into the correct pseudowire. The PE devices then emulate the function of a LAN switch
or bridge to connect all of the customer LANs to create a single bridged (Ethernet) LAN.
DC-MPLS provides full pseudowire, VPWS and VPLS signaling support
using DC-LDP, which includes a comprehensive implementation of LDP
and its VPLS extensions, including RFC 4762. DC-MPLS can also be used to signal
the underlying tunnels using DC-LDP or DC-RSVP.
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