MPLS: What is it? What can it do for me?

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MPLS: What is it? What can it do for me?

Author: J.D. Wegner

Abstract

Multiprotocol Label Switching (MPLS) began as a way for routers to short-cut the time-consuming process of treating each packet independently, looking up the target IP address each time and in every router along the path. MPLS streamlines the packet forwarding process by introducing a label, assigned by the ingress edge router, into the packet. The beauty of MPLS is that the label itself has no meaning other than what the software gives it. Therefore, the label can be used to implement any forwarding treatment that comes to mind. This white paper will teach you what MPLS is and how it can help provide your customers with flexible networking services.

Introduction

Multiprotocol Label Switching (MPLS) began life as a way for routers to short-cut the time-consuming process of treating each packet independently, looking up the target IP address each time and in every router along the path. In an MPLS network, the ingress router does a standard lookup and assigns a numeric label to the packet. Core routers then examine the label and forward the packet according to the label. All packets with the same label are forwarded the same way. This relieves the core routers of much processing, making the overall network more efficient.

Because the labels are just numbers, they can be assigned according to any criteria the router software supports. This feature give MPLS an extraordinary ability to support many networking applications. Some examples of MPLS applications, in addition to basic IP destination-based routing, including

VPN membership
QoS
Traffic Engineering
Emulation of any Layer-2 technology

Service providers and enterprises are implementing MPLS to provide their customers with flexible networking services.

What Is MPLS?

Traditional routers forward packets by examining the network-layer header (typically the destination IP address), searching for the best matching entry in the routing table, and forwarding the packet through the specified interface to the next hop router. This process is time-consuming and is repeated for each packet at each router along the path. Because no state is maintained from packet to packet, the system is highly scalable, but inefficient.

MPLS streamlines the packet forwarding process by introducing a label into the packet. The MPLS label is assigned by the ingress edge router based on a Forwarding Equivalence Class (FEC) which represents a series of packets to be forwarded in the same manner, over the same path, to the same destination. In basic IP routing, for example, a label is assigned to each target IP network that the MPLS domain knows about. An FEC could also be associated with specific classes of service for QoS processing, or with a VPN. The beauty of MPLS is that the label itself has no meaning other than what the software gives it. Therefore, the label can be used to implement any forwarding treatment that comes to mind. By allowing multiple labels to be stacked within a packet, MPLS permits multiple applications, such as QoS and VPNs, to be combined. Once a packet has entered the MPLS domain, the routers use a simple and fast label lookup process to forward the packet to its next hop. In the Cisco implementation, MPLS leverages the Cisco Express Forwarding (CEF) feature to optimize label lookup and forwarding.

For MPLS to operate, labels must be generated, stored, and distributed. The Label Distribution Protocol (LDP) handles the generation and distribution of labels for basic IP forwarding and other applications, and the Label Information Base (LIB) stores the labels generated locally and received from LDP neighbors. LDP is generally enabled on each MPLS interface. Other labels, as required by various applications, are generated and distributed through other protocols. For example, Multi-Protocol Border Gateway Protocol (MP-BGP) assigns and distributes labels used for VPN forwarding, and the Resource Reservation Protocol (RSVP) does the same for traffic engineering.

What Does MPLS Do for Me?

MPLS began life as Tag Switching, a method to decrease processing load and therefore, latency, in the core routers of a network. Since then, router vendors have implemented technologies such as Cisco's Express Forwarding (CEF) to allow for wire-speed processing at layers two and three. Today, these technologies provide much more of a performance boost than Tag Switching or MPLS can give. The main reason to implement MPLS today is to provide a foundation upon which you can build services to offer your customers.

Service Providers

For networking service providers, moving to MPLS is a clear win.

Simplified Architecture. The service providers' networks can be simplified to a two-level hierarchy. Edge routers are the work-horses of the network. They exchange routing updates with the customer edge routers, communicate those customer routes to other edge routers using BGP, and exchange internal routes typically using OSPF or IS-IS with the core routers. When encountering IP traffic, they perform traditional routing table lookups and either impose or strip off the MPLS labels. The core routers have a simpler task. They merely exchange internal routes with other core and edge routers. For traffic forwarding, they use pure MPLS label lookup.