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ebook-Network Transformation Strategy-60

MPLS became widely implemented around 2001 and today has become the most common solution for WAN infrastructure today. The popularity of MPLS initially arose because routing lookups were difficult to implement efficiently in software. By minimizing them, MPLS offered a significant reduction in latency. The improvement in chipsets have eliminated the lookup issues, but MPLS still brings significant benefits such as traffic engineering and reliable performance

Class of Service and Traffic Prioritizing

MPLS allows for detailed control over how the network routes traffic. The network engineer can prevent traffic congestion, manage circuit capacity, and prioritize services more effectively. Traffic prioritization, and more specifically CoS, is a key method for deploying a successful MPLS network. MPLS-based services can support CoS, which is particularly crucial for companies that are using voice and video. Given the cost of MPLS bandwidth, customers generally must get the most out of narrow pipes, making CoS particularly important to maximize available capacity.


All MPLS CoS implementations use differentiated services (DiffServ).  Routers use three bits, called Class of Service (CoS) or experimental bits, in the MPLS header of each packet transported across the MPLS network to differentiate the traffic. This allows eight traffic classes to be implemented with one usually reserved for default traffic class, leaving seven useable classes. Generally, traffic classification is executed by the service provider edge routers. These routers perform the tasks of measuring the compliance of the customer traffic, sorting the traffic into MPLS traffic classes, and optionally marking the out-of-contract traffic and drop excess traffic.


In most cases, customers that require CoS want to retain their IP Differentiated Services Code Point (DSCP) markings for end-to-end CoS control. To satisfy this request, PE routers have to measure the traffic and set the MPLS experimental bits directly. The MPLS markings are retained from the ingress PE router to the egress PE router, giving this method the name short pipe mode.


CoS can have a profound impact on MPLS performance when configured correctly because applications are prioritized as they travel across an MPLS network. However, application bandwidth requirements should be calculated carefully, and organizations must understand network performance in terms of latency and jitter. Otherwise, the value of CoS will be less effective if the underlying network performance is poor.

High Performance

Because of the any-to-any nature of MPLS services, network architects can reduce the number of hops between network points, which creates increased response time and improved application performance. With an any-to-any configuration, a private network is provisioned between all locations, with each location able connect to the others. This improves the overall performance and reliability of the network.


MPLS differs from traditional IP routing because MPLS does “label switching” instead of an IP lookup at each router hop. The first device does a routing lookup, just like IP routing, but instead of determining a next-hop, it finds the final destination router and creates a predetermined path to that final router.


As customer data enters the MPLS network, a label is attached to each packet. This label identifies specific Virtual Private Networks (VPN) in a shared infrastructure and keeps it private. Upon reaching its destination, the label is removed, and the data packet is back to its original state.


The MPLS Label consists of four sub-parts:

  • The Label: Information for MPLS routers is secured in the label - directing where the packet should be forwarded.

  • Experimental: Experimental bits are used to set the CoS priority for the labeled packet.

  • Bottom-of-Stack: The Bottom-of-Stack informs the MPLS Router if it is the final leg of the journey and there are no more labels, which means the router is an egress router.

  • Time-To-Live: Time-To-Live (TTL) enforces a hop limit of a packet along a path. When the TTL limit is reached, the packet is discarded.


Label switching permits the routes through a network to be subject to better control. For example, a labeled packet coming from router 2 may be intended for an address at router 6. Meanwhile, a labeled packet starting at router 3 may travel through router 1 while a different labeled packet value might travel through router 5 to reach its destination.

This concept creates the ability to engineer the nodes and links to handle traffic with more efficiency and provide various traffic classes to different levels of service. In a functional sense, if an application requires more bandwidth the label can direct the router to route the traffic on a path with the most available bandwidth. This is a concept called traffic engineering (TE), which is a policy-based method that uses label switching to adapt the network to the needs of the traffic classes.


Other benefits of MPLS include greater reliability and predictability of traffic within the network. Because the Label Switched Paths (LSPs) are pre-determined, packets only travel along the paths they have been assigned to take. This is a significant difference from IP routing, where one packet’s path could be completely different from the next based on network conditions at the time.


“It’s almost like on every hop of the way, that router is making a decision based on what he knows for a path forward,” says Joe Whitehouse of Metro Switch. Whereas within MPLS, he adds, “it’s always down the same path and it makes life much easier for the MPLS device, which may be a router. … There’s really no heavy lifting that has to be done from an IP look-up standpoint.”


In the event that the LSP is down, Whitehouse points out that MPLS can to set up pre-programmed fallback paths: “In the case of a network failure, you can switch over the entire path to avoid the failure. You really can’t do that with IP.”


MPLS providers offer service-level agreements (SLA), which indicate performance levels from the perspective of applications, repair times and delivery. Service levels are generally a reflection of the performance of a specific MPLS network and will provide a basis for the type of real-world expectations the provider will be anticipated to deliver throughout the contract.

Downsides – Cost and Deployment Time

Besides being known for reliability and performance, MPLS is also known for lengthy provisioning times for services. MPLS circuits can take anywhere from weeks to months to complete configurations and be ready for turn-up. Requiring long lead times for MPLS services can adversely impact business projects.


Compared to other connection options, such as Metro-E, DSL, or broadband services, MPLS is generally significantly more expensive. For global enterprises, they may find it difficult to find an MPLS service provider who can deliver global coverage. Service providers may piece together this coverage through partnerships with other service providers - which can be costly. Most organizations are willing to spend more for the service as well as alter project planning to accommodate the lengthy provision time-table in order to get maximum benefits and reliability.

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