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What does MPLS do?

Dear students, let’s briefly discuss MPLS today. Before learning MPLS VPN, many students will ask, what exactly is MPLS? And Why do we need MPLS?

First, what is MPLS? It is short for: Multi-Protocol Label Switching. It is a new technology that uses tags to guide the high-speed and efficient transmission of data on open communication networks. The meaning of multi-protocol means that MPLS can not only support multiple protocols at the network layer level, but also be compatible with multiple data link layer technologies at the second layer. To understand this problem, we must first understand the traditional IP data forwarding.

Traditional network layer packet forwarding (such as forwarding IP packets on the Internet) relies on network layer routing protocols (such as OSPF or other IGP protocols, BGP) or static routing protocols to provide forwarding information. Each hop device on the network makes forwarding decisions independently. Forwarding decisions are made solely based on the goals of unicast forwarding. Routers forward data for IP packets, participate in Layer 3 information exchange, and then make path selections. As the devices on the path need to perform table lookup, route selection and forwarding according to the Layer 3 information of the IP data packet, this means that when the data packet reaches a router, it must be decapsulated, re-encapsulated, and forwarded at Layer 3. Reduced the efficiency of forwarding. For a simple example, this is like if we go to a certain place by car, if we use the traditional forwarding method, then we need to get off at each stop between the destinations, and then get on the car according to the information on the station board. It will be very troublesome.

 

How can we make forwarding easier and more efficient? In simple terms, it is to establish a forwarding line from the source to the destination. This is equivalent to us going to a certain place, as long as we take a fixed bus line from source to some place, instead of having to get off at each stop halfway, and then get on again according to the information of the station sign. The original intention of MPLS was based on this idea. This technology combines the advantages of packet forwarding technology based on layer 2 switching and layer 3 routing technology. MPLS assigns labels to packets so that packets can be transmitted in a packet-based or cell-based network. The forwarding mechanism throughout the network is label swapping. In this technology, data units (such as packets or cells) carries a fixed-length label tells the switching nodes on the packet path how to process and forward data. Label switching is used to construct forwarding paths, that is, LSPs. Each LSP can be analogized to a bus line in real life, so as to avoid going through the three-layer encapsulation and decapsulation process to achieve data forwarding through labels.

 

As for how to form an LSP, we will talk about next time …

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