Mpls vpn Technical Principles (1)

1. Significance of MPLS

Traditional IP data forwarding is based on the hop-by-hop mode. Each router that forwards data needs to find the route table based on the destination IP address of the IP address header to obtain the next hop egress, this is a tedious and inefficient task, mainly because of two reasons: 1. Some route queries must perform multiple searches on the route table, which is called recursive search; 2. Because route matching follows the longest Match Principle, almost all vro switching engines must be implemented using software, the exchange engine implemented by software cannot compete with the switch engine implemented by hardware on the ATM switch in terms of efficiency.

The demand for Internet applications is increasing today, and the demand for bandwidth and latency is also increasing. To improve Forwarding efficiency, various vro manufacturers have done a lot of improvement work, such as Cisco's CEFCisco Express Forwarding function on the vro, and modifying the route table search algorithm. However, these patches cannot completely solve the problems currently faced by the Internet.

IP and ATM were once two opposite technologies. Each IP device manufacturer and ATM device manufacturer tried to eat each other and wanted IP addresses to dominate the world, or the ATM family was the only one! However, the fusion of the two technologies is the birth of the Multi-Protocol Label Switching technology! The combination of MPLS technology and IP technology, simple signaling, and efficient ATM switching engine advantages!

2. Implementation Details of MPLS Technology

2.1 tag Structure

The implementation of MPLS technology by IP equipment and ATM equipment manufacturers is based on their own original, for IP equipment vendors, it modifies the specification that the original IP package is directly encapsulated in the L2 link frame, instead, a Label is inserted between the two-layer and three-layer headers, while the ATM device manufacturer uses the concept of VPI/VCI on the original ATM switch, the Label is used to replace VPI/CVI. Of course, the signaling control part of the ATM switch must be modified, and the routing protocol is introduced, ATM exchange uses the routing protocol to exchange layer-3 routing information with other devices.

The label structure is as follows:

The LABEL field of 20 bits is used to indicate the LABEL value. Because the LABEL is fixed length, you can analyze the LABEL of a vro to forward data packets. This is the biggest advantage of LABEL exchange, the Fixed Length Label means that data forwarding can be implemented using hardware. This hardware forwarding method is much more efficient than the longest matching forwarding method that must be implemented using software!

3-bit EXP is used to implement QOS

The 1-bit value indicates whether the tag stack is complete. For VPN, TE and other applications, more than two tags are inserted between the Layer 2 and Layer 3 headers to form the tag stack.

The 8-bit TTL value is used to prevent data from forming loops on the Internet.

In this way, the complete layer-2 frame with tags becomes the following form:

In the ATM Cell mode, the cell structure is as follows:

2.2 architecture of LSR Equipment

Through modification, the Router that supports Label switching is LSR (Label Switch Router), and the ATM Switch that supports MPLS is generally called ATM-LSR.

The architecture of the LSR device is as follows:

The architecture of LSR is divided into two parts::

1. Control Plane)

The function of this module is to exchange layer-3 routing information with other LSR to establish a route table, and to exchange the binding information of labels on the route, so as to create a Label

Information Table (LIB) label Information Table. At the same time, Forwarding Information is generated based on the route table and LIB.

Table (FIB) Table and Label Forwarding Information

Table (LFIB) Table. The control plane is also known as the routing engine module!

2. Data Plane)

The data plane function is mainly to forward IP packets and label packets based on the FIB table and LFIB table generated by the control plane.

The routing protocols used in the control plane can use any of the previous methods, such as OSPF, RIP, and BGP. The main function of these protocols is to exchange routing information with other devices, generate a route table. This is the basis for implementing tag exchange. A new protocol-LDP is imported into the control plane to generate a local label for each route entry in the local route table and generate a LIB table, then, the route entry and the local label are bound to the neighbor LSR, and the route entry and label helper notified by the neighbor LSR are received in the LIB table. Finally, when the network route converges, generates FIB and LFIB tables based on the information of the route table and LIB table. The label distribution mode is described as follows.