Detail Look at MPLS (Multiprotocol Label Switching)

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 increased considerably, but MPLS still has many benefits.

MPLS enables service providers to offer additional services to its enterprise customers, including virtual private networks (VPNs), improved traffic engineering, quality of service (QoS), Layer 2 tunneling, and multiprotocol support.

You can deploy MPLS as a multiservice-based network, providing an Internet Protocol (IP) alternative to Frame Relay, ATM, and leased line. This feature presents a cost savings to service providers. Rather than build out separate networks for IP, Frame Relay, and ATM users, the provider can build a single MPLS network and support them all.

An MPLS network must differentiate all the packets from each other. It does so by labeling each packet. To add a label to a packet, the network must first determine all the normal information that a typical router does. In other words, the first router a packet encounters must fully analyze the header, from which the label is made.

After the packet has a label, the rest of the routers in the network must have a way to act upon the information contained in the label.

MPLS Equipment
The three primary equipment types in an MPLS networks are the following:

• Customer premise equipment (CPE)�'Equipment on the customer site. All traffic leaving the local site is routed through this point. Often referred to as customer equipment (CE).
• Edge label switch routers (ELSR)�'Located at the ingress point of the service-provider network, this equipment assigns (and removes) labels. ELSR can either be routers or high-end stitches. Often referred to as provider equipment (PE).
• Label switch routers (LSR)�'Located in the core of the service-provider network, LSRs forward packets or cells based on their labels.

MPLS Labels
The forwarding mechanism in MPLS uses a label to make decisions where and how to send packets or cells through the network. The label is applied at the ingress to the service-provider network and removed at the network egress point. The router responsible for adding the label is the only network router that needs to process the entire packet header. The network uses the information contained in the header along with the preconfigured instructions, to generate the label. Routers can base labels on IP destinations (this is what traditional routing uses) and other parameters, such as IP sources, QoS, VPN membership, or specific routes for traffic-engineering purposes. MPLS also supports forwarding mechanisms from other protocols.

MPLS Security
An additional benefit of MPLS is its inherent security. As illustrated in the next diagram, after the packet or cell from a company enters the service-provider network, the label essentially keeps that packet segregated from all other customers’ packets and cells. Because there is no place where one customer can view another customer’s packet and cells, there is no danger of someone outside the network snooping for packets. Obviously, this limitation does 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.
Note: MPLS is no more or less secure than Frame Relay or ATM. Also, there is a common misconception that MPLS is encrypted. Although it is possible to encrypt MPLS, it is not encrypted by default.

MPLS Architecture
MPLS has two layers, or planes, each with an area of specific function in the network. The layers are the control plane and the data plane. The control plane is responsible for the exchange of routing information (including labels) between adjacent devices. The data plane handles forwarding operations.

The routers in an MPLS network forward packets based on labels, but the router must know the relationship between a label and a path through the network. This relationship is established and communicated throughout the network using forwarding equivalence classes (FECs). A FEC is a specific path through the network of LSRs; it 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 process is simpler, fastei and more flexible than traditional IP routing.

Other MPLS Features
Traffic engineering is the ability to dynamically define routes based on known demand or alternate available routes. Traffic engineering can also optimize network usage.

Intelligent rerouting refers to MPLS’s ability to reroute based on network congestion. Rather than change the route on a packet-by-packet 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 Internet.

MPLS Layer 2 tunneling, also known as Any Transport over MPLS (AToM), allows a service provider to transport Frame Relay and ATM over an MPLS-based network. This feature increases the range of services that the service provider can offer.