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MPLS - A Buyer's Guide

1. Introduction

Until recently, it was difficult to envisage that IP-based networking would overtake established connection-orientated technologies like ATM and Frame Relay in the delivery of time-sensitive information. Yet today, Multi protocol label switching (MPLS) is having a major impact for this very reason and is seen as a natural fit for organisations deploying wide area applications in the Internet age.

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1. Introduction
2. Business Drivers
3. What is MPLS?
4. MPLS Layer 2 Point-to-Point
5. MPLS Layer 3 IP/VPN
6. MPLS Layer 2 VPLS
7. Layer 2 Vs Layer 3
8. Comparing MPLS, IPSec and SSL
9. Costs of setting up an MPLS Network
10. Selecting an MPLS service provider
11. Frequently asked questions

2. Business Drivers

The primary business drivers for MPLS are the need for convergence and prioritisation as companies strive to lower their latency as more and more of today’s business critical applications help firms achieve a vital competitive edge.

3. What is MPLS?

Multi Protocol Label Switching (MPLS) is an IETF (Internet Engineering Task Force) government approved wide area networking standard created to: -

Help carriers and large corporates scale their networks as increasingly large routing tables become more complex to manage

Combine flexible any-to-any communication found on PSTN or Internet with the reliability and security delivered by Private Line, Frame Relay or ATM services

Offer diferentiated performance levels and prioritisation of delay and non-delay sensitive traffic as well as voice and multimedia applications, all on a single network

Address traffic management issues by prioritising time sensitive applications

MPLS is available in three types:

	Layer 2 point to point
	Layer 3 IP VPN
	Layer 2 VPLS

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4. MPLS Layer 2 Point to Point

Suitable for companies that require high bandwidth between a small number of sites.

Layer 2 point to point is a cost effective way and flexible alternative to high bandwidth leased lines. Many wholesale network operators have based their core network infrastructure on Ethernet and use Layer 2. This type of transport is protocol agnostic and allows anything running over the LAN to be sent over the WAN without having to use routers to convert packets up to Layer 3, the network layer.

Advantages of MPLS Layer 2 point to point: -

	It is no longer necessary to managed complex routing tables.
	Customers save money by terminating their WAN connections straight in to a Layer 2 switch as opposed to buying expensive routers to convert Layer 3 back down to Layer 2.

Disadvantages of MPLS Layer 2 point to point: -

	Providers tend to provide only high bandwidth circuits, i.e 10 Mbps and above
	Point to multipoint is not supported.

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5. MPLS Layer 3 IP/VPN

Particularly suitable for large multi-site enterprise, i.e. retail chains, that deploy a large number of low bandwidth sites or large corporates with a offices deployed globally. This type of service is a natural progression away from legacy Frame Relay and ATM services. It is a perfect fit for companies that are: -

	in the process of merging: IP/VPNs are extremely scalable for fast deployment
	require ‘any to any’ connectivity: a shorter hop count between two local sites is more efficient than ‘tromboning’ back into a central point. This is especially relevant for global networks where latency is increased as packets travel over long geographical distances.
	preparing for voice and data convergence: to implement a blanket ‘class of service’ prioritization based on traffic type is made simple across multi-site networks.
	be migrating from traditional ATM to IP: ATM is extremely expensive to maintain
	be migrating from frame relay to IP due to its inflexible nature and management restrictions.
	have low bandwidth requirements at small branch offices: in the UK sub 2 Mbps circuit delivery is available over Kilostream circuits which a cheaper alternative to Megastream & Ethernet lines.
	Need only a secure dial-up capability in smaller locations

Advantages of MPLS Layer 3 IP/VPNs

	Offers the same service guarantees services as Frame Relay or ATM without requiring the use of any permanent virtual circuits (PVC).
	Supports Class of Service (COS) for traffic type differentiation

Disadvantages of MPLS Layer 3 IP/VPNs

	Layer 3 IP/VPN’s are proprietary; one provider, one network.
	Changes to the network configuration. For example adjusting COS has to be requested and typically takes up to 5 days. Change control is also chargeable.
	Not suitable for small networks; instead of buying one circuit between two points, customers have to buy two circuits to be brought into the private IP/VPN cloud. IP VPN are only cost effective for 5 circuits and above
	If using BT, Internet access circuits have to be purchased separately.
	IP only; other standards cannot be used over the network without conversion to Layer 3.
	MPLS can be expensive as larger carriers charge for prioritizing traffic in their annual cost.
	Change control: carriers usually reserve the right to charge for manual changes to network configuration.

How do MPLS Layer 3 IP/VPNs work?

Instead of having routers at every node or routing point of a network, traffic goes to the edge of a network and each packet has a label. Incoming packets or ingress as they are known are assigned a label by a Label Edge Router (LER). Packets are forwarded along a Label Switch Path (LSP) where each Label Switch Router (LSR) makes forwarding decisions based solely on the contents of the label. At each hop, the LSR strips off the existing label and applies a new label, which tells the next hop how to forward the packet. In this way, a label determines how it will be transported over a network, simplifying the routing problem and giving label switching routers (LSR) high performance.

The labels, which are underlying protocol-specific identifiers, are distributed using Label Distribution Protocol (LDP) or or piggybacked on routing protocols like border gateway protocol (BGP) and OSPF. Each data packet encapsulates and carries the labels during their journey from source to destination. High-speed switching of data is possible because the fixed-length labels are inserted at the very beginning of the packet or cell and can be used by hardware to switch packets quickly between links.

Diagram 1: How MPLS Layer 3 IP/VPN works

Resource: Network World Fusion (http://www.nwfusion.com)

Diagram 2: Class of Service bandwidth is prioritised into 6 levels of service. EF is used to carry voice, AF is used to carry priority data, i.e ERP applications such as SAP, Siebel, Citrix, Oracle and video conferencing. DE is used to carry all other data, i.e. email, browsing.