Chapter 4. Path Calculation and Setup

Chapter 3, "Information Distribution," asked these questions: How does MPLS Traffic Engineering really work? What are the underlying protocols, and how do they all tie together? And, most importantly, what do you need to type on your router to make all this wonderful stuff come to life?

Chapter 3 broke MPLS Traffic Engineering into three paths:

• Information distribution

• Path calculation and setup

• Forwarding traffic down a tunnel

This chapter, as you can tell from the title, is about the second part—path calculation and setup.

Path calculation and setup are two different things. Functionally, the code that decides what path a tunnel takes across your network is different from the code that actually sets up this tunnel; however, the two technologies are so closely related that it makes sense to discuss them in the same chapter.

First, this chapter discusses path calculation. This discussion is broken into three pieces:

• The basic Shortest Path First (SPF) calculation that OSPF and IS-IS use to build their routing tables in an IP network

• MPLS Traffic Engineering's Constrained SPF (CSPF) and how it differs from the traditional SPF performed by IP routing protocols

• Mechanisms you can use to influence CSPF's path calculation

After path calculation, this chapter covers MPLS Traffic Engineering Label-Switched Path (LSP) setup. The actual setup is done using a protocol called Resource Reservation Protocol (RSVP). RSVP plays a part not only in path setup, but also in error signalling and path teardown; that too is covered.

An alternative to RSVP for MPLS TE is Constrained Routing with LDP (CR-LDP). Cisco does not implement CR-LDP, so it is not covered further in this book.