Chapter 11. Extensions for MPLS Traffic Engineering


As mentioned in Chapter 9, one trend in modern router design is to assign the two fundamental functions of route processing and packet forwarding to separate physical modules. Another way to view this trend is as a separation of intelligencewhich is processor intensive and hence time-consumingfrom basic packet forwarding, improving the performance of both. This same trend of separating intelligence from packet forwarding can be seen in large networks as a whole with the increasingly widespread adoption of Multiprotocol Label Switching (MPLS). The fundamental idea behind MPLS is to push intelligence to the edge of the network, leaving the core free to do little more than forward packets.

When the predecessors of MPLS, such as tag switching, began appearing in the mid and late 1990s, the motivation of separating intelligence from forwarding was to make the forwarding speeds of routers comparable to those of ATM switches. This was accomplished by adding to packets a fixed-length address, called a tag or label, that was independent of the data link layer but below the network layer. Routers then switched the packets from incoming interface to outgoing interface based on these "layer 2.5" addresses, the same way ATM or Frame Relay switches on VCI or DLCI labels. Because the labels are a fixed length, and small enough to be able to look them up by indexing into a table, no time-consuming lookups of variable-length IP addresses were necessary. And like ATM or Frame Relay, switching between interfaces is based on a predetermined forwarding table, eliminating the need for complex and time-consuming route processing.

But during the same years that MPLS was evolving, router technology was also evolving. More efficient IP address lookup algorithms were developed, and packet-forwarding functions that had been performed in software began to be implemented in Application-Specific Integrated Circuits (ASICs) and other high-speed hardware components. As a result, shortly before the turn of the century high-end routers routinely had packet-forwarding speeds not just comparable to ATM but exceeding them. High-speed switching ceased to be a motivation for MPLS.

MPLS did not die, however, because the technology offered another similarity to ATM and Frame Relay: The series of forwarding table entries that switched a given packet across a path from an ingress point to an egress point constitutes a virtual circuit (VC). And if you can build VCs over your routed network, you can offer services that normally require ATM or Frame Relay without the expense of building and managing an ATM or Frame Relay infrastructure in addition to your routed IP network. The key services currently offered over MPLS networks by service providers are layer 2 and layer 3 Virtual Private Networks (VPNs).

Another advantage of being able to create VCs over your router-based network is traffic engineering (TE), which provides you with great flexibility in distributing the VCsand hence your trafficacross your network to bypass trouble spots and better utilize all of your available bandwidth.

Both OSPF and IS-IS have been extended to support TE. To understand those extensions and why they are necessary, it is first necessary to understand the basics of both MPLS and TE.




OSPF and IS-IS(c) Choosing an IGP for Large-Scale Networks
OSPF and IS-IS: Choosing an IGP for Large-Scale Networks: Choosing an IGP for Large-Scale Networks
ISBN: 0321168798
EAN: 2147483647
Year: 2006
Pages: 111
Authors: Jeff Doyle

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