Why VPLS was born
For more than two decades, Ethernet switching technology has dominated local area networks, while IP routing technology has dominated operator networks. During this period, Ethernet continued to expand bandwidth, and with its simplicity and cost-effectiveness, it established itself as the preferred infrastructure technology for metropolitan and wide area networks. Users and operators have shown strong demand for Ethernet, prompting Ethernet to firmly develop in the direction of ruling LAN and WAN infrastructure.
Users like it because Ethernet can meet their requirements. These requirements include:
To increase productivity, the entire enterprise needs higher bandwidth;
The greater geographical distance caused by globalization must be connected through a faster network;
Provide Ethernet applications and services in all locations within the enterprise to minimize the need for IT and training costs;
Require the network to quickly adapt to the service flexibility required by the changing business;
Support non-IP-based key transport streams;
A package of services that consumers need: bandwidth, telephone, broadcast video, video on demand.
Similarly, operators like Ethernet because it can meet their business needs. These requirements include:
Increase user loyalty by meeting the needs of end users;
Reduce capital expenditure, because the Ethernet port has a higher cost performance than other technologies;
Reduce operating expenses because Ethernet is more flexible and easier to configure;
Implement new services to earn more revenue and increase profits.
The combination of the above user needs and the business needs of operators is a new trend in the development of Ethernet services, which is the background of the birth of VPLS.
The emergence of the Internet and the increased productivity brought about by the adoption of new technologies have led to the demand for higher bandwidth and services. Enterprises need customized services with higher bandwidth, while consumers need services such as broadband connections and video on demand. Operators must strike a balance between their target market needs and the reality of their business.
The analysis of the system for deploying new services proves that traditional technologies such as ATM and SONET / SDH are familiar to people, but they do not match the operators' assumptions about their own networks. They are built on expensive infrastructure, which requires huge investments to expand the infrastructure. In addition, complex management and multiple switching between all these technologies will cause huge operating expenses.
The market opportunities for Ethernet services are obvious and have already emerged. Feedback from companies, users, and research shows a fast-growing market. Many early adopters have been successful in deploying these services. Operators face how to respond to the challenges of these market changes. Together, VPLS and Ethernet infrastructure provide the best solution for operators to launch new services and achieve profitability. The benefits of lower capital and operating expenses will quickly improve the operator's income. In addition, using the capabilities of existing networks to ensure that operators ’investments in older technologies can create more returns.
VPLS overview
VPLS is a solution that can provide many of the above services on Ethernet. It uses a combination of Ethernet and MPLS to meet the needs of operators and users. VPLS enables user networks scattered in different geographic locations to communicate with each other as if they were directly connected to each other, that is, the WAN becomes transparent to all user locations. This function is implemented by the MPLS Layer 2 VPN solution.
In the VPLS network, each user location is connected to a node on the MPLS network. For each user or virtual private network, the complete network composed of logical point-to-point connections is established on the backbone MPLS network. This allows a user location to directly see all other locations that belong to that user. The only MPLS label is used to isolate the transport stream of one user from the transport stream of another user, and to separate one service from another.
This segmentation allows users to obtain multiple services from the provider, and each service is customized for the final application. For example, a user's service set may consist of VoIP, Internet access, and possibly two or more VPN services. The first VPN service can provide "wide data" connectivity between all corporate locations and can be used by all employees. The second VPN service can be limited to certain financial transactions between a subset of locations. All these services are uniquely configured through VPLS, so they have unique quality assurance and security attributes.
In a VPLS network, a full-mesh MPLS LSP (Label Switching Path) is established between each PE (operator network edge), the Layer 2 Ethernet frame is encapsulated by MPLS, and user Ethernet traffic is sent to each through MPLS switching Forward between PEs and connect with CE (user edge equipment) to establish a point-to-multipoint Ethernet VPN.
The PE device encapsulates the customer's Ethernet frame into an MPLS packet. The MPLS header contains two layers of labels. The outer label Tunnel Label is used to carry the MPLS LSP. The inner label VC Label represents different virtual circuits, that is, different VPLS. Traffic, this is a pseudo wire encapsulation format. Therefore, after the destination PE (provider edge router) device terminates the LSP and pops up the outer label, it will determine which VPLS instance belongs to the traffic according to the inner VC Label.
VPLS technology includes two levels: signaling control layer and data forwarding layer.
The main function of the signaling control layer is to establish a corresponding virtual circuit between PEs by using a signaling protocol, in other words, to exchange VC tags that identify VPLS instances, so that each PE device can map the VC tags to Different VPLS instances to identify the received MPLS encapsulated traffic.
At the data forwarding layer, each PE maintains a forwarding information base (FIB) for each VPLS service instance, and adds the known MAC address to the corresponding FIB table. All traffic is exchanged based on the MAC address, and unknown packets (such as the destination MAC address is still unknown) will be broadcast to all PEs participating in the VPN until the destination station responds and the PE related to the VPN learns the MAC address.
The basic reference model of VPLS is shown in Figure 1. Customer sites are connected through a service provider network, which is like a Layer 2 switch that can learn. All PEs in the network are connected by a full grid of tunnels, and each tunnel carries multiple dotted lines. The dotted line is a point-to-point connection structure established for each service provided between a pair of PEs. According to the location and customer site data, the number of dotted lines established for customers / services can range from one (for customers with only two locations) to a full grid (for customers with locations connected to each PE).
Each PE in the network can establish a tunnel connecting each other PE, and can establish a dotted line through these tunnels through signaling. When the network provides Layer 2 services to end users, each PE can learn all locally connected MAC addresses and associate the learned remote address with a dotted line. All unknown unicast, multicast and broadcast packets are transmitted to all PEs participating in the customer VPN. Multicast packets are treated like broadcast packets and are propagated to all PEs in the customer VPN.
This network model assumes that all PEs in the service (or VPLS instance) are connected together with a full grid of dotted lines. This full grid eliminates the need to keep the network loop-free.
To ensure that there is no loop in this topology, a concept similar to split horizon is required: no PE forwards the data packets it receives from another PE to one PE. This approach breaks the scalability limitations encountered by other spanning tree-based networks. Currently this network model is being standardized as part of the VPLS draft by the IETF L2VPN working group.
For operators, the benefits of combining Ethernet and MPLS are many. They can immediately benefit from the lower capital expenditure of deploying Ethernet infrastructure. However, a simple Ethernet switching network has limitations in service scalability (due to the limitation of the V LAN ID) and reliability (spanning tree cannot scale well). These limitations are resolved by MPLS.
MPLS provides multiple solutions that not only provide large-scale scalability and multiple reliability options, but also bring other benefits. For example, MPLS dynamic signaling is beneficial for changing and reconfiguring services more quickly. Its traffic engineering capabilities enable providers to support service level assurance across the entire network. Therefore, it not only meets the needs of scalability and reliability, but also provides operational advantages that can further reduce costs.
VPLS standard
The deployment of VPLS is proceeding like mushrooms all over the world and is widely supported by operators and manufacturers. In this process, the role of standards is particularly important.
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