Virtual Private LAN Service

Virtual Private LAN Service (VPLS) emulates a LAN segment across the MPLS backbone across pseudowires or virtual circuits. VPLS creates one or more LANs for each customer who is using the service from the service provider. Each LAN, of course, is completely separate from the other emulated LAN segments—hence the "P" for "Private" in VPLS. When the customer with different Ethernet sites connects to an MPLS backbone where VPLS is deployed, it appears as if all the sites are interconnected through a virtual Ethernet switch. Two options are available to interconnect these Ethernet sites: either Spanning Tree Protocol (STP) bridge protocol data units (BPDU) are not allowed to pass through the virtual switch, or they are allowed to pass. In the first case, the spanning tree in each Ethernet site terminates at the provider edge (PE) router. In the second case, the spanning tree crosses the MPLS backbone (the virtual switch), and one STP runs through all sites. An Ethernet LAN is a Layer 2 domain. As such, Ethernet frames are transported across the MPLS backbone. This is the same as for Ethernet over MPLS (EoMPLS). However, EoMPLS is a service that is point-to-point in nature, whereas VPLS—emulating a LAN—is point-to-multipoint in nature and as such must support replicating broadcast and multicast frames. Finally, VPLS must perform some features that are inherent to an Ethernet switch—such as MAC address learning and aging—if the virtual switch is to be emulated.

The Need for VPLS

VPLS is a service that emulates an Ethernet LAN. The need for VPLS arose because MPLS VPN is a service that is IP centric. No other Layer 3 traffic can be carried across the MPLS backbone with this service. Any Transport over MPLS (AToM) allows you to carry all Layer 3 protocols as AToM carries the Layer 2 frames across the MPLS backbone; thus, AToM is not limited to carrying IP. The disadvantage of AToM is that it is point-to-point. Between each pair of PE routers is a pseudowire (two LSPs, one for each direction) that carries the Layer 2 frames. Metro Ethernet networks have seen a tremendous rise in popularity in the past few years because Ethernet is cheap, flexible, omnipresent, and easy to provision.

If a customer wants to connect his Ethernet segments from different sites across an MPLS backbone from a service provider, he could use the EoMPLS service, but that would connect the segments in a point-to-point fashion. If the different Ethernet sites are located in proximity, the customer could connect them by deploying an Ethernet switch between the segments. The

Ethernet switch would forward the unicast frames and replicate the packets to different outgoing ports for the forwarding of multicast and broadcast frames. If the different sites are not in close proximity, a switch could not be put directly between the different sites to interconnect the sites at Layer 2. VPLS would provide that functionality by emulating an Ethernet LAN or acting as a logical bridge over MPLS.

Figure 11-1 shows some Ethernet sites from one customer in different cities. The different LAN segments are interconnected by the service provider that runs the VPLS service. The VPLS service that runs over MPLS emulates an Ethernet switch that has different ports leading to the different Ethernet sites. A port can be a physical Ethernet port or a pseudowire.

Figure 11-1 VPLS: Logical Bridge

Figure 11-1 VPLS: Logical Bridge

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