Overview

In this chapter we look at Tag Switching. We begin with an overview of the Tag Switching design goals and how it provides a variety of different functions. These include destination-based routing, hierarchy of routing knowledge, multicast, and explicit routes. Following this, we describe possible alternatives for carrying tag information in packets. Unlike IP Switching, Tag Switching is not restricted to running only on ATM hardware. In fact, there are some special procedures required to cover the case of ATM. We conclude this chapter with a brief overview of the main new protocol that Tag Switching requires, the Tag Distribution Protocol (TDP).

Like IP Switching, Tag Switching has its own terminology. A router that supports Tag Switching is called a Tag Switching Router (TSR). Labels are called tags (which is why the scheme is called Tag Switching ). Instead of incoming label we ll use incoming tag, and instead of outgoing label we ll use outgoing tag. A label switching forwarding table is called a Tag Forwarding Information Base (TFIB).

The material in this chapter is largely based on the publicly available documents on Tag Switching. More details are provided in Section 4.8, Further Reading, at the end of this chapter.

4.1 Tag Switching Overview

In Chapter 1 we saw that there were many motivations behind the development of the various label switching approaches, such as additional routing functionality, improved scalability of the routing system, better forwarding performance, and more flexibility of the routing system. However, the one...