MPLS Multi-Protocol Label Switching is a must-have network technology for carrier backbone networks. Learn more in one minute

1. Understanding MPLS Background

Traditional IP data forwarding is based on a hop-by-hop approach. Each router that forwards data must look up the routing table based on the destination address in the IP packet header to obtain the next hop exit. This is a cumbersome and inefficient task for two main reasons:

• Some route queries must perform multiple lookups in the routing table, which is called a recursive search;
• Since route matching follows the longest match principle, the switching engines of almost all routers must be implemented in software. The switching engines implemented in software and the switching engines implemented in hardware on ATM switches cannot compete in efficiency.

Today, the demand for Internet applications is increasing day by day, and the requirements for bandwidth and latency are also getting higher and higher. In order to improve forwarding efficiency, various router manufacturers have done a lot of improvement work, such as Cisco providing CEF (Cisco Express Forwarding) function on routers, modifying the routing table search algorithm, etc. However, these patches cannot completely solve the problems currently facing the Internet.

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IP and ATM used to be two opposing technologies. Each IP equipment manufacturer and ATM equipment manufacturer tried to eat up the other, wanting IP to dominate the world, or ATM to be the only one!

But in the end, the fusion of these two technologies led to the birth of MPLS (Multi-Protocol Label Switching) technology! MPLS technology combines the advantages of simple signaling of IP technology and high efficiency of ATM switching engine!

2. MPLS Labels

1. Tag structure

IP equipment and ATM equipment manufacturers implement MPLS technology based on their own original foundations. For IP equipment manufacturers, they modify the original specification of directly encapsulating IP packets in the Layer 2 link frame and insert a label between the Layer 2 and Layer 3 packet headers. ATM equipment manufacturers utilize the concept of VPI/VCI on the original ATM switches and use Label instead of VPI/CVI. Of course, the signaling control part of the ATM switch must also be modified, and the routing protocol must be introduced. ATM switching uses the routing protocol to exchange Layer 3 routing information with other devices.

• The 20-bit LABEL field is used to represent the label value. Since the label is fixed-length, the router can analyze the fixed-length label to forward the data packet. This is the biggest advantage of label switching. The fixed-length label means that data forwarding can be implemented by hardware. This hardware forwarding method is much more efficient than the longest match forwarding method that must be implemented by software!
• 3 bits of EXP are used to implement QOS
• The 1-bit S value is used to indicate whether the label stack has reached the bottom. For applications such as VPN and TE, more than two labels will be inserted between the layer 2 and layer 3 headers to form a label stack.
• 8-bit TTL value is used to prevent data from looping on the network.

2. LSR equipment and MPLS architecture

A router that supports label switching is called LSR (Label Switch Router)

The LSR architecture is divided into two parts:

(1) Control Plane

The function of this module is to exchange layer 3 routing information with other LSRs to build routing tables; and exchange label-to-route binding information to build Label Information Table (LIB). At the same time, the Forwarding Information Table (FIB) and Label Forwarding Information Table (LFIB) are generated based on the routing table and LIB. The control plane is what we generally call the routing engine module!

(2) Data Plane

The function of the data plane is mainly to forward IP packets and label packets according to the FIB table and LFIB table generated by the control plane. For the routing protocols used in the control plane, any previous one can be used, such as OSPF, RIP, BGP, etc. The main function of these protocols is to exchange routing information with other devices and generate routing tables. This is the basis for implementing label switching. A new protocol, LDP, is introduced in the control plane. The function of this protocol is to generate a local label for each routing entry in the local routing table, thereby generating a LIB table, and then notifying the binding of the routing entry and the local label to the neighboring LSR, and at the same time receiving the routing entry and label binding informed by the neighboring LSR and putting it in the LIB table. Finally, when the network routing converges, the FIB table and LFIB table are generated with reference to the information in the routing table and LIB table.

(3) MPLSVPN

1) In order to allow the PE router to distinguish which local interface the VPN user route is sent from, a large number of virtual routers are created on the PE router. Each virtual router has its own routing table and forwarding table, which are collectively referred to as VRF (VPN Routing and Forwarding instances). A VRF defines the VPN members connected to the PE router. VRF contains the IP routing table, IP forwarding table (also called CEF table), the interface set using the CEF table, routing protocol parameters, and route import and export rules, etc. The two important parameters related to VPN services defined in VRF are RD (Route Distinguisher) and RT (Route Target). With virtual routers, the routes between different VPN users can be isolated, and the problem of overlapping IP address spaces between different VPNs can be solved.

2) BGP/MPLS IP VPN is a L3VPN (Layer 3 Virtual Private Network). It uses BGP (Border Gateway Protocol) to publish VPN routes on the service provider backbone network and MPLS (Multiprotocol Label Switch) to forward VPN packets on the service provider backbone network. The IP here means that the VPN carries IP (Internet Protocol) packets.

3) The basic model of BGP/MPLS IP VPN consists of three parts: CE, PE and P.

• CE (Customer Edge): A device at the edge of a customer network that has an interface directly connected to the service provider network. A CE can be a router or a switch, or a host. Usually, a CE cannot "sense" the existence of a VPN and does not need to support MPLS.
• PE (Provider Edge): is the edge device of the service provider network, directly connected to the CE. In an MPLS network, all VPN processing occurs on the PE, which places high performance requirements on the PE.
• P (Provider): Backbone equipment in the service provider network, not directly connected to the CE. P equipment only needs to have basic MPLS forwarding capabilities and does not maintain VPN information.

PE and P devices are managed only by the service provider; CE devices are managed only by the user, unless the user delegates the management rights to the service provider.