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Issue of May 2003 
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Vendor Voice: Multiprotocol Label Switching
Can MPLS make a better VPN?

Reflecting on VPN evolution, we see that each type had certain limitations. But VPNs based on MPLS incorporate the best of past versions, along with new features like smart healing. by Vinod Chandran

Multiprotocol label switching based IP-VPN (Internet Protocol-Virtual Private Network) is the latest technology for providing secure VPN connectivity to closed user groups. It is an Internet Engineering Task Force (IETF)-specified framework that uses label switching to forward data through the network. Labels are short fixed-length identifiers that define the data path followed through the network. Such paths are known as label switched paths (LSPs).

MPLS is an integration of Layer 2 and Layer 3 technologies with reference to the OSI model for data communications. By making traditional Layer 2 features available at Layer 3, MPLS enables the service provider to offer a one-tier network. MPLS traffic engineering automatically establishes and maintains a tunnel across the backbone using RSVP (Resource Reservation Setup Protocol).

The path used by a given tunnel at any point in time is determined based on the tunnel resource requirements and network resources such as bandwidth. Tunnel paths are calculated at the tunnel ingress based on a fit between required and available resources (constraint-based routing). The IGP (Interior Gateway Protocol) automatically routes the traffic into these tunnels. Typically, a packet crossing the MPLS Traffic engineering backbone travels on a single tunnel that connects the ingress point to the egress point.

Why is MPLS important ?

MPLS enhances the IP Services offering, with guaranteed Quality of Service and supports traffic engineering as well as multimedia services.

It provides a diversified range of services (Layer-2, Layer-3) to meet the requirements of the entire spectrum of customers, including security, quality of service (QoS), and any-to-any connectivity, with service level agreements and fully managed services.

  • Fully integrates IP Routing & Layer 2 switching
    Conventional IP routers use longest match lookups to perform the routing function. The router matches the IP address on the incoming packet with the address on the routing table having the best match, in terms of number digit matches. This involves a search algorithm and the routing is based on the best match. This process is inherently more time-consuming than the exact-match lookups performed by MPLS routers. But now new route lookup engines have emerged that run just as fast as MPLS lookup engines.
  • Utilizes existing IP infrastructure to deliver multiple services
    By just changing the way packets are assigned to an LSP, new services can be easily offered without changing the MPLS forwarding infrastructure. New services can therefore migrate seamlessly to a common MPLS infrastructure.
  • Optimizes IP networks through traffic engineering
    Traffic Engineering to optimize the flow of traffic through the IP network is made possible by MPLS. Traffic can be segregated and processed differently based on network performance requirements.
  • Seamlessly integrates private and public networks
    The public and private networks can share the same infrastructure without any possibility of mix-up.


The IETF wanted to do a single prefix lookup at the entry into the MPLS network and make forwarding decisions in the network based on fixed label values--with the ability to specify routes.

Historically, ISPs could not cause traffic to dynamically flow on the path they chose; the traffic followed the IGP/PNNI shortest path. Although dynamic protocols are normally very effective at routing traffic, ISPs could not split traffic between diverse routes.

Dynamic ATM algorithms did not permit easy binding of traffic to specific paths. The carriers defined MPLS to allow them to bind specific prefixes to specific paths. In other words, they defined a way for traffic to follow a different path than the IGP (Interior Gateway Protocol) would dictate—even if that administrative path were a sub-optima IGP path.

The IETF Working Group was chartered in the spring of 1997 with the following objectives:

  1. Enhance the performance and scalability of IP routing.
  2. Facilitate explicit routing and traffic engineering
  3. Separate control (routing) from the forwarding mechanism so each can be modified independently
  4. Develop a single forwarding algorithm to support a wide range of routing and switching functionality.

The writer is DGM, BSNL.

Evolution of VPNs

VPNs were initially being set up using physical leased lines hired from service providers by the user organizations. All the hardware and management of the network was from the user companies.

However such VPNs could not meet the highly dynamic connectivity requirements and also in cases of failure of the physical leased line, no protection of traffic was possible.

Internet VPNs then emerged as an alternate solution. In this type of interconnection the nodes are interconnected over the Internet using encryption techniques to provide the necessary security.

However, despite the low cost and wide reach, such VPNs do not meet the complete requirements, since the Quality of Service is not guaranteed when the connections are across the Internet. At present tunneling VPNs are popular among many business units. Secure tunnels are set up end-to-end across the Internet for exchange of information between the source and destination.

Such tunneling VPNs are highly complex and costly especially due to the problem of setting up squared tunnels for connecting locations. The performance of these tunneling VPNs is limited by the delay and bandwidth congestion seen on the Internet.

VPNs based on MPLS provides the best of both worlds as it has inherent self-healing ability by automatically re-routing traffic and also ensures Quality of Service including high throughput with low latency and jitter. This is possible because unlike the public Internet, the routers are interconnected over fully dedicated long distance backbone transmission network of the service provider. The traffic forwarding is based on label swapping across label switched paths.

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