Introduction

Purpose

If network sizes of more than 250 OSI nodes are required, hierarchical routing makes it possible to increase the number of nodes in an OSI management domain, (for example, NEs with OSI LAN and/or DCC connectivity for operations communications among the NEs). Increasing the number OS nodes is achieved through the use of IS-IS Level 2 Routing.

IS-IS Level 2 Routing involves assigning NEs to multiple areas of 250 nodes or less. Level 2 routers support OSI communications between the NEs in different areas. Both the assignment of NEs to areas and the enabling of NEs as Level 2 routers is accomplished by provisioning. For additional information, see Area address assignment and Level 2 router assignment, respectively.

OSI node vs. NE terminology

Each Alcatel-Lucent 1665 DMX is an OSI node, but some other NEs support multiple OSI nodes within a single NE, so the term "OSI node", or simply "node", is generally used in this context instead of "NE".

Advantages

The primary advantage of IS-IS Level 2 Routing is to limit the OSI routing information that needs to be maintained by each node. Each node within an area maintains routing tables to route OSI LAN and DCC messages to the other nodes in the same area. If a node needs to route a message to a node in a different area, the node relies on a Level 2 router in its own area to route the message to a Level 2 router in the target area and from that Level 2 router to the target node within that area.

The Level 2 routers route messages between nodes in different areas. These messages include all of the NE-NE (and OS-NE) OSI communications required for the supported OI applications, (for example, TL1 login messages from TL1–GNEs to TL1-RNEs.)

In addition to maintaining a routing table to route messages to other nodes in the same area, Level 2 routers need to maintain routing information for the Level 2 routers in all other areas (but not for all nodes within each of those other areas).

Symbols

The graphical examples is this section use the symbols that are defined in the following figure. Figure 6-25, Network with Level 2 routers illustrates an example network with nodes assigned to four different areas connected by Level 2 routers.

Figure 6-25: Network with Level 2 routers