Multiprotocol Label Switching (MPLS)

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Definition of Multiprotocol Label Switching (MPLS) in The Network Encyclopedia.

What is Multiprotocol Label Switching (MPLS)?

Multiprotocol Label Switching, also known as MPLS, is a proposed standard from the Internet Engineering Task Force (IETF) for a switching protocol for backbone routers in large TCP/IP internetworks such as the Internet.

Multiprotocol Label Switching (MPLS)
Multiprotocol Label Switching (MPLS)

How It Works

Multiprotocol Label Switching (MPLS) is an outgrowth of switching protocols such as Layer 3 switching, tag switching, and Internet Protocol (IP) switching. MPLS enables routers to exchange information about the common paths taken by IP packets with different addresses. Routers can identify these paths as label-switched paths (LSPs) and enable routers to perform more efficient lookups in the relatively small label-swapping table (LST) that they maintain, instead of having to perform slower lookups in large IP address tables.

MPLS is particularly useful in the Internet infrastructure, where core routers are often connected in fully meshed topologies using permanent virtual circuits (PVCs) through Asynchronous Transfer Mode (ATM) backbones. MPLS also supports quality of service (QoS), which makes IP more suitable for sending multimedia and other streamed information.

Why Should I Care About MPLS?

Multiprotocol Label Switching (MPLS) was originally developed to speed up the routing of packets through the WAN network. Since its development, the speed of traditional routing has sped up considerably, but MPLS still has many benefits.

MPLS enables service providers (SP) to offer additional services to its enterprise customers, including VPNs, improved traffic engineering, QoS, Layer 2 tunneling, and multiprotocol support. MPLS can be deployed like a multiservice-based network, providing an IP-based alternative to Frame Relay, ATM, and leased line. This presents a cost-saving to service providers. Rather than building separate networks for IP, Frame Relay, and ATM users, MPLS allows the SP to build a single MPLS network and support them all.

MPLS Security

An additional benefit of MPLS is a small measure of security (as compared to Frame Relay or ATM). As illustrated in the figure, as soon as the packet or cell from a company enters the SP network, the label assigned essentially keeps that packet segregated from all other customers’ packets/cells.

MPLS security
MPLS security

Because there is no place where one customer can view another customer’s packet/cells, there is no danger of having someone outside the SP network snoop for packets. Obviously, this would not stop someone bent on illegally accessing a company’s information, but it does remove the possibility of someone claiming that he “accidentally” received the information. Unfortunately, the number of incidents of people or groups intentionally stealing or monitoring data has been on the rise over the past several years. Because of this trend, many people no longer consider MPLS to be “inherently secure,” as it was once billed.

Many companies opt for encryption using technologies such as IPsec (IP Security) to provide data security for their traffic traversing MPLS networks (and, in general, any WAN type). This is especially true where companies have offices with connections in developing and emerging countries, where the trust level of in-country providers may be lower than in the U.S. and Europe.

How Does the Router Know Where to Send Stuff?

The routers in an MPLS network forward packets based on labels, but the router must know the relationship between a label and path through the network. This relationship is established and communicated throughout the network using
Forwarding Equivalence Classes (FEC). A FEC is a specific path through the network of LSRs and is equal to the destination network, stored in an IP routing table. The LSRs simply look at the label and forward the packet based on the contents of the FEC. This is much simpler, faster, and more flexible than traditional IP routing. Sometimes a packet arrives at a router without a label (if it came from a non-MPLS network). When this happens, it is the router’s job to add a label so that the packet can be properly forwarded through the MPLS network.

Other MPLS Features

Traffic engineering is the ability to dynamically define routes based on known demand or alternative available routes. Traffic engineering can also be used to optimize network usage. Intelligent rerouting refers to an MPLS network’s ability to reroute based on network congestion. Rather than changing the route on a packet-bypacket basis, MPLS can reroute on a flow-by-flow basis.

MPLS is particularly well suited to support VPNs. With a VPN, the packets from one enterprise are transparent to all other enterprise VPNs. The labels and the FECs effectively segregate VPN traffic from other packets on the MPLS network. MPLS Layer 2 tunneling, also known as Any Transport over MPLS (AToM), allows an SP to transport Frame Relay and ATM over an MPLSbased network. This increases the range of services that the SP can offer.

MPLS Trafic Separation (Figure from "Cisco Networking Simplified (2008). See References Section
MPLS Trafic Separation

ATM and Frame Relay

Label switching is also implemented in other technologies such as ATM and frame relay networks. MPLS proposes to bring the same traffic management features to IP internetworks, and MPLS-enabled routers will treat ATM and frame relay switches as peers within the label-switched IP internetwork.

MPLS in a Nutshell

Multiprotocol Label Switching explained in a nutshell

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