MPLS Overview (part one)

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Multiprotocol Label Switching MPLS (Multiprotocol Label Switching) is an IP (Internet Protocol) backbone network technology. Introduction of MPLS over IP networks label switched connectionless connection-oriented concept, the combination of the third layer and the second layer routing exchange technology, a simple full flexibility of IP routing and L2 switching.


MPLS originated in IPv4 (Internet Protocol version 4), the core technology can be extended to multiple network protocols, including IPv6 (Internet Protocol version 6), IPX (Internet Packet Exchange) and CLNP (Connectionless Network Protocol) and so on. MPLS in "Multiprotocol" refers to support multiple network protocols.


Thus, MPLS not a service or application, it is actually a tunnel technology. This technology supports multiple upper layer protocols and services, and to some extent, can guarantee the security of information transmission.

The basic structure of MPLS


Network architecture

The typical structure of MPLS network as shown in FIG. MPLS label-based forwarding, the MPLS label switching network and a packet forwarding device called Label Switch Routers LSR (Label Switching Router) in the figure; network region LSR is referred to as a MPLS domain (MPLS Domain). Located on the edge of the MPLS domain, another network connected to the edge router LSR called LER (Label Edge Router), referred to as a core inner region LSR LSR (Core LSR).


FIG: MPLS network structure of FIG.

When IP packets MPLS network, the MPLS LER inlet IP packet content analysis and adding suitable labels for the IP packets, all the MPLS LSR forwarding data in the network according to the label. When the IP packet leaves the MPLS network, the label pop export LER.


IP packets in the MPLS network through a path referred to as a label switched path LSP (Label Switched Path). LSP is a unidirectional path, consistent with the direction of data flow.


FIG above, LSP ingress LER is the inlet (the Ingress); LSP located in the middle of the intermediate node is called LSR (Transit); LSP is referred to as an egress LER node (Egress). An LSP can have zero, one or more intermediate nodes, but there is only one ingress and egress and a.


The direction of the LSP, MPLS packets sent by the Ingress to Egress, the Ingress node upstream of Transit, Transit Ingress is the downstream node. Similarly, Transit Egress is an upstream node, the downstream node is a Transit the Egress.



MPLS architecture from the control plane (Control Plane) and a forwarding plane (Forwarding Plane) composition.

Control plane: responsible for generating and maintaining routing information and label information.

Routing information table RIB (Routing Information Base): generated by the IP routing protocols (IP Routing Protocol), for routing.

Label Distribution Protocol LDP (Label Distribution Protocol): responsible for the distribution of labels, label forwarding table of the establishment of information, establishing a label switched path, demolition and other work.

Label information table LIB (Label Information Base): generating a label distribution protocol, used to manage the tag information.


Forwarding plane: the data plane (Data Plane), responsible for general IP packet forwarding, and forwarding packets with MPLS label.

Forwarding information base FIB (Forwarding Information Base): extracting the necessary routing information is generated from the RIB, responsible for ordinary IP packet forwarding.

Label forwarding information table LFIB (Label Forwarding Information Base): referred to as the label forwarding table established by the LFIB on LSR label distribution protocol, is responsible for forwarding packets with an MPLS label.


MPLS label


MPLS packets will return with the same characteristics as a class, is called a FEC FEC (Forwarding Equivalence Class). Belonging to the same FEC packets during forwarding LSR is processed in the same manner.


FEC may be divided according to the source address, destination address, source port, destination port, VPN and other factors. For example, in the conventional longest match algorithm IP forwarding, all packets to the same route is a forwarding equivalence class.


Label (Label) is a short, fixed length identifier has local significance only, is used to uniquely identify a FEC packet belongs. In some cases, such as load balancing, there may be corresponding to a plurality of FEC into labels, but on a device, a label can only represent one FEC.


MPLS packet with common IP packets compared to an increase of MPLS label information, MPLS label length is 4 bytes. MPLS label encapsulated between the link layer and the network layer, link layer may support any protocol.


A label contains four fields:


Label: 20bit, label range.

Exp: 3bit, for expansion. Now commonly used as CoS (Class of Service), when the device is blocked, sending high-priority priority packets.

S: 1bit, identification bottom of the stack. MPLS supports tags, i.e. tags nested. S is a value of 1 indicates that the bottom of the label.

TTL: same meaning 8bit, and IP packets TTL (Time To Live).


TTL: same meaning 8bit, and IP packets TTL (Time To Live).


Label stack (Label Stack) refers to the ordered set of labels. Layer header label close called MPLS label stack or outer MPLS label (Outer MPLS label); near IP header tag called MPLS label stack bottom or inner MPLS label (Inner MPLS label). In theory, MPLS label can be infinitely nested. Currently nested MPLS labels are used in MPLS VPN, TE FRR (Traffic Engineering Fast ReRoute) in.


Label stack LIFO organized label, the label starts processing from the top of the stack.


MPLS packet capture Example:

FIG: MPLS packet capture exemplary


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