MPLS: Technology and Applications
This guide sets the groundwork for and gives explanations of MPLS, RFCs, QoS, VPNs, and more.
TABLE OF CONTENTS MPLS: Technology and Applications
Chapter 7: Constraint-Based Routing
In this chapter we describe how MPLS can be used to support constraint-based routing. We begin by defining what constraint-based routing is. Following this, we describe the components of constraint-based routing. We then look at several possible applications of MPLS constraint-based routing: traffic engineering, fast reroute, and support for QoS.
7.1 What Is Constraint-Based Routing?
To understand the concept of constraint-based routing, we first look at the routing system that is conventionally used in IP networks, such as the Internet. A network is modeled as a collection of Autonomous Systems (ASs), where routes within an AS are determined by intradomain routing and routes that span multiple ASs are determined by interdomain routing. Examples of intradomain routing protocols are RIP, OSPF, and IS-IS. The interdomain routing protocol that is used today in IP networks is BGP. For the rest of this chapter we focus on intradomain routing.
Path computation for any of the intradomain routing protocols we mentioned above is based on an algorithm that optimizes (minimizes) a particular scalar metric. In the case of RIP, this metric is the number of hops. That is, given the choice of multiple paths to a given destination, RIP uses the Bellman-Ford algorithm to compute a path with the minimum number of hops. In the case of OSPF or IS-IS, this metric is the administrative metric of a path. That is, with OSPF (or IS-IS), a network administrator assigns to each link in the network an administrative metric. [1] Given the choice of...
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