5.1. INTRODUCTION
In the Traffic Engineering chapter (Chapter 2), we have seen how to compute and signal traffic-engineered paths that comply with a set of user-defined constraints. A key step in this process is acquiring the information regarding the constraints for all the links in the network. This information is distributed by a link-state IGP and is therefore confined within the same boundaries as the link-state advertisements. Because the visibility of the topology and of the constraints is limited to a single IGP area, TE LSPs dynamically computed by the head end are also limited in the same way. This becomes a problem in large networks that deploy several IGP areas for scalability or in the case of services spanning across several service providers.
In this chapter we will see how RSVP-signaled TE LSPs can extend across IGP areas and across AS boundaries. These solutions are known as interarea TE and inter-AS TE respectively and are referred to collectively as interdomain TE. They apply equally to classic TE and to Diff-Serv Aware TE (described in the DiffServ-TE chapter, Chapter 4). In this chapter the term 'domain' is used to denote either an IGP area or an AS.
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