This chapter covers the following objectives:
- Concepts: This section discusses the general concepts of a link state protocol. An overview of the different stages in a link state protocol is given.
- IS-IS architecture details: This section discusses the high level IS-IS architecture details.
- TRILL and FabricPath relevant changes: This section covers in detail the changes specific to TRILL and FabricPath.
Introduction to IS-IS
This chapter provides a brief explanation of Intermediate Systems – Intermediate Systems (IS-IS1) Intra Domain Routing Protocol. Both TRILL and FabricPath use IS-IS as their control protocol. Consequently, you must be familiar with the inner workings of IS-IS. This chapter, by no means, provides comprehensive details on IS-IS. Many good resources and books are dedicated to IS-IS. Some of them are listed in the “References” section at the end of the chapter. The purpose of this chapter is to provide the necessary details on IS-IS so that you can get started. This chapter also explains in detail the extensions that were done to IS-IS to support TRILL. The extensions to IS-IS for TRILL and FabricPath are also referenced in the respective chapters. Although there is some redundancy, this has been done for added convenience to address both class of readers, those who want to understand the IS-IS specific details and those who may be interested in knowing only enough about IS-IS that is necessary to understand TRILL and FabricPath.
- IS-IS is an ISO standard and like Open Shortest Path First (OSPF)2 is a link state protocol. IS-IS was originally designed as a dynamic routing protocol for ISO’s Connectionless Mode Network Protocol (CLNP) in 1987. It was later extended by IETF to carry IP addresses in 1990. IS-IS operates over the link layer. Though it was originally developed for the OSI protocol stack, all its messages are easily extendable (unlike OSPF, which is primarily written for IPV4). IS-IS uses a generic Type Length Value (TLV) format for its message exchange. Supporting any new protocol can be done elegantly in IS-IS by adding new extensions without modifying the base infrastructure. For this reason, TRILL and FabricPath chose IS-IS with some extensions as their control protocol.