MPLS Overview

ATM was once envisioned as the best technology for multiservice-based core and edge networks. Many large enterprise customers (LECs) thought ATM would extend from the desktop through the core of the network and terminate at another desktop. This speculation was based on the fact that ATM already had the capability for different classes of service, such as constant bit rate (CBR), variable bit rate (VBR), available bit rate (ABR), and unspecified bit rate (UBR).

Today, these features have been built into an IP-based network and IETF standards included as well. However, economics will play an important role in determining the adoption rate of these next-generation IP networks. Upgrading entire networks by swapping out hundreds or even thousands of pieces of equipment is not cost-effective. Therefore, many service providers will continue to maintain ATM in their networks for the foreseeable future. Less risk is also involved in incrementally designing an IP + ATM multiservice switching network because ATM QoS is proven on a large scale in live deployments.

Consequently, next-generation networks will be built using new technologies that leverage existing technologies such as IP + ATM that are already deployed in existing networks.

A new paradigm is also presented in this book for service providers and large enterprise networks intending to provide existing services such as point-to-point ATM and Frame Relay virtual circuits (VCs), ATM switched virtual circuits (SVCs), and circuit emulation services (CES) over cell-based networks while adding IP services to their portfolio so that those services don't interfere with each other and maintain inherent ATM QoS. For incremental service management and network investment protection, multiservice switching networks will play a very important role in driving next-generation services. Many of these access technologies can be IP-enabled so that customers can choose between a pure Layer 2 or an enhanced Layer 3 service with Layer 2 guarantees.

Finally, the proven reliability, availability, and serviceability (RAS) characteristics inherent in ATM networks can be leveraged to provide similar availability in IP networks.

Label-switching technology is the result of the desire to combine the benefits of switching technologies that live in the core of the network with the benefits of IP routing technologies that live at the edge of the network, getting the best of both worlds. A blended network using both of these technologies creates a problem best described as "how to make IP and ATM interoperate." The IETF and the ATM Forum initially took on this challenge and defined standards such as Classical IP over ATM (RFC 1577 and RFC 2225) and multiprotocol over ATM (MPOA), which allow IP to work over an ATM network. However, MPLS is tackling a different problem best described as "how to integrate the best components of traditional Layer 2 and Layer 3 technologies." As previously mentioned, multiprotocol label switching seeks to combine the best features of Layer 2 switching technologies as they exist in the ATM and Frame Relay world with the best features of Layer 3 routing technologies as they exist in the IP world. This is Layer 3 plus Layer 2 as opposed to Layer 3 over Layer 2.

MPLS, as the standards-based approach to label switching, specifically defines a set of protocols and procedures that allow the fast-switching capabilities of ATM and Frame Relay to be used by IP networks. The key concept in MPLS is identifying and marking IP packets with labels at the edge and forwarding them to a modified switch or router, which then uses the labels to switch the packets through the network. This new equipment with router and switch characteristics is a label switch router (LSR). The labels are created and assigned to IP packets based on the information gathered from existing IP routing protocols. This classification happens in the edge in edge Label Switch Routers (eLSRs), also called Label Edge Routers (LERs).

The upcoming sections cover the following topics:

• The role of routing Gathering information necessary to get to an IP prefix destination

• The role of switching Forwarding packets in a simple and efficient way

• The role of MPLS Separating routing and forwarding, gathering the best of Layer 2 and Layer 3 to then bring them together

The Role of Routing

Internet Protocol (IP) packets contain a header with sufficient information that lets them be forwarded through a network. Packet forwarding has traditionally been based on packet routing. The packet-routing technique used in IP networks is a destination-based routing paradigm. This means that an IP packet is routed through the network based on the destination address contained in the packet. An IP packet is shown in Figure 4-1.

Figure 4-1. IP Packet

A note on the Type-of-Service (ToS) field shown in Figure 4-1 is included in the upcoming section "Quality of Service and Multi-vc" as well as Figure 4-17.

Figure 4-17. Type of Service IPv4 Header Octet

The forwarding mechanism used by IP networks is hop-by-hop routing, which means that every packet entering a router is examined, and a decision is made as to where to send the packet (in other words, what the packet's next hop is). In this manner, a packet is routed through a network from its source to its destination. Because the packets are individually routed through a network and don't follow a predetermined path, the network is said to be connectionless. Routers exchange information by establishing an adjacency (in other words, a conversation) with every directly connected peer.

To properly route a packet, a router must be able to determine the packet's next hop. Routing protocols, such as Open Shortest Path First (OSPF), allow each router to learn the network's topology. Using the information provided by the routing protocols, the routers build forwarding tables that identify the appropriate next hop for all known IP destination addresses.

NOTE

Routers typically store IP prefixes rather than complete IP addresses in their forwarding tables; however, the details of IP routing are outside the scope of this book.

The Role of Switching

Switching technologies based on ATM and Frame Relay use a much different forwarding algorithm than monolithic routers, which essentially use a label-swapping algorithm. Because this forwarding algorithm is so simplistic, it is typically done in hardware, yielding a better price/performance advantage than traditional IP routing that takes place in a software path.

ATM and Frame Relay are connection-oriented technologies, meaning that traffic is sent between two endpoints only after a connection (in other words, a predetermined path) has been established. Because traffic between any two points in the network flows along a predetermined path, connection-oriented technologies make the network more predictable and manageable. The combination of these attributes helps explain why large networks have been built with a switching fabric in the network's core.

The Role of MPLS: Bringing Routing and Switching Together

MPLS is designed to meet all the mandatory characteristics and requirements of large-scale carrier-class networks. It is evolutionary in the sense that it uses existing Layer 3 routing protocols as well as all the widely available Layer 2 transport mechanisms and protocols, such as ATM, Frame Relay, PPP over leased lines, and Ethernet. For large-scale public networks, Frame Relay and particularly ATM are of great interest, primarily because they support the underlying concepts of QoS and CoS.

MPLS solves the problem of how to integrate the best features of traditional Layer 2 and Layer 3 technologies. It does this by defining a new operating methodology for the network. The key component within an MPLS network, the LSR, can understand and participate in both IP routing and Layer 2 switching. By combining these technologies into a single operating methodology, MPLS avoids the problems associated with methods that define a way for Layer 2 and Layer 3 to interoperate while maintaining two distinct operating paradigms.

Even though MPLS requires LSRs to participate in IP routing, MPLS's forwarding aspect differs significantly from hop-by-hop routing. The LSRs participate in IP routing to understand the IP reachability and network topology from a Layer 3 perspective. This routing knowledge is then used to assign labels to packets. Labels are analogous to the VPI/VCI used in ATM and the DLCI used in Frame Relay. When viewed on an end-to-end basis, the labels combine to define the path between endpoints. These paths, called label-switched paths (LSPs), are intentionally very similar to the VCs used by switching technologies because they provide benefits such as predictability and manageability. In addition, the connection or LSP lets a Layer 2 forwarding mechanism be used. As described earlier, a label-swapping mechanism is typically very fast and can be implemented very easily in hardware.