6.1. Introduction

Neighbor discovery refers to procedures by which an optical Network Element (NE) automatically determines its connectivity to different types of adjacent elements (called neighbors). Neighbor discovery is perhaps the most useful control plane function. Its utility in a network is independent of whether other distributed control plane functions like routing and signaling are implemented. The connectivity information determined using neighbor discovery procedures is essential for connection routing and provisioning. Without neighbor discovery, the interconnection information must be manually configured in provisioning systems. In a network with many NEs, and with each NE having numerous ports, manual configuration of this information could be error-prone and inconvenient. Furthermore, keeping track of changes in the connectivity between NEs manually could become difficult. As we shall see in this chapter, neighbor discovery procedures also aid in detecting errors in physical wiring or connectivity between adjacent NEs.

It should be noted that while neighbor discovery procedures allow an NE to determine its local connectivity, how this information is disseminated and used depends on the nature of the provisioning system. With a centralized provisioning system, the local connectivity information would be collected by the central system from each NE for use in route computation. With a distributed control plane, the local connectivity information would be propagated by the routing protocol to form the topology map at each NE (see Chapter 10). In addition to its use in connection provisioning, the neighbor information could be used to verify that link parameters are configured in a consistent manner at both ends of a link. This is called link property correlation. With a centralized provisioning system, link property correlation can be done at the central system. With a distributed control plane, a protocol is required for neighboring nodes to exchange and compare configuration information pertaining to discovered links.

In the rest of this chapter, we examine neighbor discovery and link property correlation in detail. In the next section, we describe the different types of adjacencies and define the neighbor discovery function. In section 6.3, we look at the protocol requirements for neighbor discovery and link property correlation. The Link Management Protocol (LMP) developed by the IETF is discussed in section 6.4. This protocol aims to support neighbor discovery in both networks with OEO and OOO switching elements, and hence it is somewhat more complex than a protocol targeted solely for OEO elements. Finally, conclusions are discussed in section 6.5.