VPLS is a virtual private network (VPN) technology. In contrast to L2TPv3, which allows only point-to-point layer 2 tunnels, VPLS allows any-to-any (multipoint) connectivity.
In a VPLS, the local area network (LAN) at each site is extended to the edge of the provider network. The provider network then emulates a switch or bridge to connect all of the customer LANs to create a single bridged LAN.
With BGP, one has auto-discovery as well as signaling. The mechanisms used are very similar to those used in establishing Layer-3 MPLS VPNs. Each PE is configured to participate in a given VPLS. The PE, through the use of BGP, simultaneously discovers all other PEs in the same VPLS, and establishes a full mesh of pseudo-wires to those PEs.
With LDP, each PE router must be configured to participate in a given VPLS, and, in addition, be given the addresses of other PEs participating in the same VPLS. A full mesh of LDP sessions is then established between these PEs. LDP is then used to create an equivalent mesh of PWs between those PEs.
An advantage to using PWs as the underlying technology for the data plane is that in case of failure, traffic will automatically be routed along available backup paths in the service provider's network. Failover will be much faster than could be achieved with e.g. Spanning Tree Protocol (STP). VPLS is thus a more reliable solution for linking together Ethernet networks in different locations than simply connecting a WAN link to Ethernet switches in both locations.
VPLS has significant advantages for both service providers and customers. Service providers benefit because they can generate additional revenues by offering a new Ethernet service with flexible bandwidth and sophisticated service level agreements (SLAs). VPLS is also simpler and more cost effective to operate than a traditional service. Customers benefit because they can connect all of their sites to an Ethernet VPN that provides a secure, high speed and homogenous network. Moreover, VPLS provides a logical next step in the continuing evolution of Ethernet from a 10 Mbit/s shared LAN protocol to a multi-Gbps global service.
When a PE receives a frame from a CE, it inspects the frame and learns the CE's MAC address, storing it locally along with LSP routing information. It then checks the frame's destination MAC address. If it is a broadcast frame, or the MAC address is not known to the PE, it floods the frame to all PEs in the mesh.
Ethernet does not have a time to live (TTL) field in its frame header, so loop avoidance must be arranged by other means. In regular Ethernet deployments, Spanning Tree Protocol is used for this. In VPLS, loop avoidance is arranged by the following rule: A PE ''never'' forwards a frame received from a PE, to another PE. The use of a full mesh combined with split horizon forwarding guarantees a loop-free broadcast domain.
For LDP, a method of subdividing a VPLS VPN into two or three tiered hierarchical networks was developed. Called hierarchical VPLS (HVPLS), it introduces a new type of MPLS device: the multi-tenant unit (MTU) switch. This switch aggregates multiple customers into a single PE, which in turn needs only one control and data plane connection into the mesh. This can significantly reduce the number of LDP sessions and LSPs, and thus unburden the core network, by concentrating customers in edge devices.
HVPLS (LDP) may also be used to join two VPLS mesh structures together. Without using HVPLS, every node in each VPLS mesh must become meshed with all nodes in the other VPLS mesh. However, with HVPLS, the two meshes can essentially be joined together at certain locations. Techniques such as redundant pseudo-wires can provide resiliency in case of failures at the interconnection points.
To counter this problem, sites may use a router as the CE device. This hides all MAC addresses on that site behind the CE's MAC address.
PE devices may also be equipped with content-addressable memory (CAM), similar to high-end Ethernet switches.
An alternative mechanism is using MAT (MAC Address Translation). However, at the time of writing this, there aren't vendors providing MAT functionality.
The RADIUS server keeps track of all PEs that requested authentication for a particular VPN, and returns a list of them to the PE requesting authentication. The PE then establishes LDP sessions to every PE in the list.
Category:Network protocols Category:MPLS networking
de:Virtual Private LAN Service es:Virtual Private LAN Service fr:Virtual Private LAN ServiceThis text is licensed under the Creative Commons CC-BY-SA License. This text was originally published on Wikipedia and was developed by the Wikipedia community.
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