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Introduction to Multiprotocol Label Switching

Multiprotocol Label Switching (MPLS) is a method of switching IP packets through a network by applying simple labels to packets. This allows devices in the network core to switch packets according to these labels with minimal lookup activity. Besides the obvious advantage of faster network transit, MPLS also provides the privacy and quality of service (QoS) advantages of connection-oriented services such as ATM without the complexity of manually creating fully-meshed PVCs.

MPLS integrates the performance and traffic-management capabilities of the data link layer (Layer 2) with the scalability and flexibility of network layer (Layer 3) routing. MPLS is applicable to networks using any Layer 2 switching, but it has particular advantages when applied to ATM networks. It integrates IP routing with ATM switching to offer scalable IP-over-ATM networks.

In contrast to label switching, conventional Layer 3 IP routing is based on the exchange of network reachability information. As a packet traverses the network, each router extracts all the information relevant to forwarding from the Layer 3 header. This information is then used as an index for a routing table lookup to determine the packet's next hop. This is repeated at each router across the network. At each hop in the network, the packet's optimal forwarding must again be determined.

The information in IP packets, such as information on IP Precedence and Virtual Private Network (VPN) membership, is usually not considered when packets are forwarded. To get maximum forwarding performance, typically only the destination address is considered. However, because other fields could be relevant, a complex header analysis must be done at each router the packet meets.

The main concept of MPLS is to include a label on each packet. Packets or cells are assigned short, fixed-length labels. Switching entities perform table lookups based on these simple labels to determine where data should be forwarded.

The label summarizes essential information about routing the packet:

  • Destination

  • Precedence

  • VPN membership

  • QoS information from Resource Reservation Protocol (RSVP)

  • The packet's route, as chosen by traffic engineering (TE)

With label switching, the complete analysis of the Layer 3 header is performed only once: at the edge label switch router (LSR), which is located at each edge of the network. At this location, the Layer 3 header is mapped to a fixed-length label.

At each router across the network, only the label need be examined in the incoming cell or packet in order to send the cell or packet on its way across the network. At the other end of the network, an edge LSR swaps the label out for the appropriate header data linked to that label.

A key result of this arrangement is that forwarding decisions based on some or all of these different sources of information can be achieved by means of a single table lookup from a fixed-length label. For this reason, label switching makes it feasible for routers and switches to make forwarding decisions based on multiple destination addresses.

Label switching integrates switching and routing functions, combining the reachability information provided by the router function with the traffic engineering benefits achieved by the optimizing capabilities of switches.

ATM MPLS Technical and Business Benefits

MPLS, in conjunction with other standard technologies, offers many features that are critical for service providers:

  • MPLS, in combination with the standard IP routing protocols OSPF and IS-IS, provides full, highly scalable support of IP routing within an ATM infrastructure.

  • MPLS, in combination with Border Gateway Protocol (BGP), provides support for highly scalable IP VPN services. IP VPN services are an invaluable development in provider networks, giving enterprise customers a service that meets their needs for private, connectionless delivery of IP services.

  • Service-Level Agreements (SLAs) can be provided in a form suitable for connectionless traffic. Cisco networks assist the process of providing SLAs by supporting MPLS in combination with forthcoming standards. Along with supporting VPNs, the ability to offer SLAs suitable for IP traffic is a critical requirement to meet new demands for IP services.

  • Cisco's implementation of MPLS allows support for harder QoS where required using full ATM switch capabilities.

Cisco IP+ATM networks fully support all relevant IP routing protocols and MPLS while fully supporting traditional ATM services. MPLS and IP routing can readily be introduced into traditional ATM networks by using permanent virtual path (PVP) or PVC tunnels since MPLS-capable switches are continuously being introduced.

Cisco IP+ATM switches allow carriers to continue meeting their existing demands for virtual circuit services while adding optimized support for critically important new services: IP and IP VPNs. Furthermore, Cisco supports all the standards relevant to carrier-class IP services: MPLS, Multiprotocol BGP, other standard routing protocols, and MPLS traffic engineering.

Label Switching Advantages

MPLS offers many advantages over traditional IP over ATM.

When integrated with ATM switches, label switching uses switch hardware optimized to take advantage of the fixed length of ATM cells and to switch the cells at high speeds. For multiservice networks, label switching allows the Cisco WAN switch to provide ATM, Frame Relay, and IP Internet service all on a single platform in a highly scalable way. Support of all these services on a common platform provides operational cost savings and simplifies provisioning for multiservice providers.

For ISPs using ATM switches at the core of their networks, label switching allows the Cisco BPX 8600 series, the 8540 Multiservice Switch Router, MGX 8850-PXM45 (discussed in a moment), and other Cisco ATM switches to provide a more scalable and manageable networking solution than overlaying IP over an ATM network. Label switching avoids the scalability problem of too many router peers and provides support for a hierarchical structure within an ISP's network.

These MPLS benefits are analyzed in greater detail in the following list:

  • Integration—When applied to ATM, MPLS integrates IP and ATM functionality rather than overlaying IP on ATM. This makes the ATM infrastructure visible to IP routing and removes the need for approximate mappings between IP and ATM features. MPLS does not need ATM addressing and routing techniques such as PNNI, although these can be used in parallel if required.

  • Greater reliability—In wide-area networks (WANs) with ATM infrastructures, MPLS is an easy solution for integrating routed protocols with ATM. Traditional IP over ATM involves setting up a mesh of PVCs between routers around an ATM cloud, and the Next-Hop Resolution Protocol (NHRP) achieves a similar result with switched virtual circuits (SVCs). But a number of problems exist with this approach, all arising from the fact that the PVC links between routers are overlaid on the ATM network. This makes the ATM network structure invisible to the routers. A single ATM link failure could make several router-to-router links fail, creating problems with large amounts of routing update traffic and subsequent processing. (See the next section for details.)

  • Better efficiency—Without extensive tuning of routing weights, all PVCs are seen by IP routing as single-hop paths with the same cost. This might lead to inefficient routing in the ATM network.

  • Direct CoS implementation—When used with ATM hardware, MPLS makes use of the ATM queueing and buffering capabilities to provide different classes of service. This allows direct support of IP precedence and CoS on ATM switches without complex translations to the ATM Forum service classes.

  • More elegant support of multicast and RSVP—In contrast to MPLS, overlaying IP on ATM has other disadvantages, particularly in support of advanced IP services such as IP multicast and RSVP. Support of these services entails much time and work in the standards bodies and implementation; the resulting mapping between IP features and ATM features is often approximate.

  • VPN scalability and manageability—MPLS can make IP VPN services highly scalable and very easy to manage. VPN services are an important way to provide enterprises with private IP networks within their infrastructures. When an ISP offers a VPN service, the carrier supports many individual VPNs on a single infrastructure. With an MPLS backbone, VPN information can be processed only at the ingress and exit points, with MPLS labels carrying packets across a shared backbone to their correct exit point. In addition to MPLS, Multiprotocol (BGP) is used to deal with information about the VPNs. The combination of MPLS and Multiprotocol BGP makes MPLS-based VPN services easier to manage, with straightforward operations to manage VPN sites and VPN membership. It also makes MPLS-based VPN services extremely scalable, with one network able to support hundreds of thousands of VPNs.

  • Reduces the load on network cores and is more robust—VPN services demonstrate how MPLS supports a hierarchy of routing knowledge. Additionally, you can isolate Internet routing tables from service provider network cores. Similar to VPN data, MPLS allows access to the Internet routing table only at the ingress and egress points of a service provider network. With MPLS, transit traffic entering at the edge of the provider's autonomous system can be given labels that are associated with specific exit points. As a result, internal transit routers and switches need only process the connectivity with the provider's edge routers, shielding the core devices from the overwhelming routing volume exchanged on the Internet. This separation of interior routes from full Internet routes also provides better fault isolation and improved stability.

  • Traffic engineering capabilities—Other benefits of MPLS include traffic engineering capabilities needed for the efficient use of network resources. Traffic engineering lets you shift the traffic load from overutilized portions of the network to underutilized portions, according to traffic destination, traffic type, traffic load, time of day, and so on.

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