Mpls Wan Connectivity

With the history lesson done, the conversation now moves to MPLS. Simply put, MPLS extends Layer 3 natively across the distance between central, branch, and SOHO sites. The MPLS network, though owned by the service provider, is an extension of the enterprise network. Picture the entire WAN, which was previously a Layer 2 obstacle, as a single router with multiple interfaces. It contains a routing table with all of the route entries of the enterprise network. The WAN provides any-to-any connectivity between sites without the hassle of administering a large number of circuits. Like any routed network with diverse paths, the MPLS network converges dynamically, supports multiple routing protocols, and honors QoS traffic tags and policies.

Figure 8-5 illustrates the basic concept of the MPLS network.

Figure 8-5 MPLS WAN Concept

Figure 8-5 illustrates the basic concept of the MPLS network.

Each site requires only one connection to the service provider network. This connection will most likely be Frame Relay or a similar technology at the local loop; however, that is where the similarity stops with traditional WAN technologies.

MPLS Terminology

To fully appreciate and understand the technology behind MPLS, it is necessary to have a grasp on associated terminology. These terms are addressed throughout this chapter and are merely offered here for reference. Some of the common MPLS terms defined in RFC 3031 are as follows:

■ Label—A short, fixed-length, physically contiguous identifier used to identify a group of networks sharing a common destination, usually of local significance.

■ Label stack—An ordered set of labels attached to a packet header. Each label in the stack is independent of the others.

■ Label swap—The basic forwarding operation, which consists of looking up an incoming label to determine the outgoing label, encapsulation, port, and other data-handling information.

■ Label-switched hop (LSH)—The hop between two MPLS nodes, on which forwarding is done using labels.

■ Label-switched path (LSP)—The path through one or more LSRs at one level of the hierarchy followed by a packet in a particular FEC.

■ Label switching router (LSR)—An MPLS node that is capable of forwarding labeled packets.

■ MPLS domain—A contiguous set of nodes performing MPLS routing and forwarding. These are typically in one routing or administrative domain.

■ MPLS edge node—An MPLS node that connects to a neighboring node outside of its MPLS domain.

■ MPLS egress node—An MPLS edge node that handles traffic leaving an MPLS domain.

■ MPLS ingress node—An MPLS edge node that handles traffic entering an MPLS domain.

■ MPLS label—A label that is carried in a packet header and represents the packet's FEC.

■ MPLS node—A node running MPLS. An MPLS node is aware of MPLS control protocols, operates one or more Layer 3 routing protocols, and is capable of forwarding packets based on labels. Optionally, an MPLS node can also forward native Layer 3 packets.

176 Chapter 8: The MPLS Conceptual Model MPLS Features

As the name denotes, MPLS is a switching mechanism. The process of switching MPLS packets includes the analysis of a label. This label contains the forwarding information needed to perform a path switch of the packet inside the LSR. It is possible that forwarding is performed by devices that are capable of doing label lookup and replacement but incapable either of analyzing network layer headers or of analyzing them at adequate speed. In other words, LSRs need not be capable of performing native Layer 3 routing.

Labels usually correspond, in some manner, to destination networks similar to traditional routing protocol operations. However, they can correspond to other variables such as the Layer 3 VPN destination, Layer 2 virtual circuit, egress interface, QoS, or a source address. These options are configurable on a per-device basis. The reason for this is that MPLS was not necessarily designed to forward only IP packets. Certainly, IP is at the forefront, as is IPv6, of the architectural vision.

As packets traverse the network from router to router, the role of each router is simply to make a forwarding decision, perform a path switch, and dispatch the packets to the next-hop router. Essentially, this process amounts to a high-speed and high-tech game of "pass the buck." This game is played based on information contained in the label imposed on the packet, whatever the Layer 3 protocol might be.

The architects of MPLS as a technology hold to the simple idea that the Layer 3 header contains significantly more information than is necessary to perform the forwarding functions. An idea behind MPLS is to build a Layer 3 routing protocol that functions in the absence of unnecessary information and without dependence on individual Layer 3 routed protocols. The basic principals of routing apply to MPLS just as do to any other routing protocol.

Essentially, the choice of a next-hop device, regardless of the nature of the underlying routing process, is one that can be broken into two basic functions:

■ Sort entire sets of possible packets into classes based on the destination address of each known as forwarding equivalence classes (FEC).

■ Map each FEC to a next-hop address.

It should be noted that packets assigned to the same FEC are indistinguishable when it comes to forwarding decisions. All packets in a particular FEC will follow the same pathway as the path is associated with the FEC, not the individual packets.

In traditional IP routing, a router considers two packets to belong to the same FEC if they contain a destination address matching the same "longest match" prefix entry in the routing table. This could be a prefix of any length. Obviously, an 8-bit prefix has the potential to match a very large number of packets, whereas a 32-bit prefix would match comparatively fewer packets. As packets are forwarded on to next-hop devices in the pathway, each is re-examined and assigned to an FEC based on that individual router's view of the network. So, it is entirely conceivable that packets sorted into the same FEC at one router will be sorted into separate FECs at another router down the line.

In MPLS, there is only one examination of the packet and only one assignment to an FEC. This is done at the MPLS ingress node. The FEC is encoded as a short, fixed-length value known as a label. When a packet is sent to a next-hop device, the FEC is sent with it. In other words, packets are labeled prior to being forwarded. At subsequent hops, only the FEC or label is examined. There is no routing table lookup. The ingress label is used as an index to allow the choice of an egress label identifying the next-hop device. The ingress label is then discarded by the device and replaced with an appropriate new label that will get it to the next-hop. The packet is then forwarded on to the next-hop device, where the process is repeated.

More simply put, in MPLS networks, only the edge LSRs perform the routing table lookup, in the process-switching sense. All non-edge LSRs perform their forwarding processes based on the label only, not on the Layer 3 header information. This allows for decreased latency through the network path (that is, faster packet forwarding).

Service providers use MPLS technologies to allow each customer's routing information to be isolated from every other customer's routing information within the provider cloud. For this reason, MPLS networks are called MPLS VPNs. The addition of the VPN designation denotes a secure and reliable transport. This is the case with an MPLS VPN. The routes advertised within an enterprise network are advertised to the MPLS network, which are then redistributed into what amounts to a customer-specific instance of BGP configured throughout the provider network. Routes are tagged with a specific Route Descriptor (RD) that keeps them unique and separate from another company's routes inside the provider cloud.

MPLS Concepts

The concept of switching should not be foreign to anyone contemplating taking the ISCW exam by any means. MPLS is simply another methodology for switching paths of traffic. Rather than looking into Layer 3 headers, the MPLS devices need only look at labels. This gives MPLS Layer 3 protocol independence. The label on an inbound packet is examined and compared to a label database. Based on the information therein, a new label is attached and the packet is transmitted out the appropriate interface. Figure 8-6 illustrates this concept.

Figure 8-6 MPLS Label Switching

Figure 8-6 MPLS Label Switching

Figure 8-6 shows a pair of core routers labeled A and B. Two additional routers exist on the edges of the MPLS cloud. The traffic flow is sourced from the host on the far left and destined for the host on the far right. Each router builds a label database that ties destination subnets to a label tag. There is an inbound and an outbound label entry in the table associated with each destination. For this reason, they are called Label Switching Routers (LSRs).

As Figure 8-6 shows, the core routers do not participate in the routing table lookup. The initial edge router performs the routing lookup and attaches the egress label. Once the packet is dispatched, it travels from device to device where a forwarding decision is made solely on the basis of the label. The LSRs in the core see only the ingress label and replace it with an appropriate egress label prior to forwarding the packet to the next-hop device. The final edge router "pops" (removes) the label from the packet and performs a new routing table lookup prior to forwarding the packet on to its destination.

At times, an LSR immediately prior to the destination edge router will pop the label before sending the packet to the final edge LSR or node. This is known as a penultimate hop pop of the label. This is advantageous at times, because the final edge device does not need to perform both a label lookup and a network layer routing lookup once it figures out that it is the last hop prior to the destination.

Continue reading here: CEF Switching

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