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Cisco ONS 15454 Reference Manual, R7.0
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Chapter 13 Management Network Connectivity
13.7 13.7.4 OSI Routing
In OSI networking, discovery is based on announcements. An ES uses the ES-IS protocol end system
hello (ESH) message to announce its presence to ISs and ESs connected to the same network. Any ES
or IS that is listening for ESHs gets a copy. ISs store the NSAP address and the corresponding
subnetwork address pair in routing tables. ESs might store the address, or they might wait to be informed
by ISs when they need such information.
An IS composes intermediate system hello (ISH) messages to announce its configuration information to
ISs and ESs that are connected to the same broadcast subnetwork. Like the ESHs, the ISH contains the
addressing information for the IS (the NET and the subnetwork point-of-attachment address [SNPA])
and a holding time. ISHs might also communicate a suggested ES configuration time recommending a
configuration timer to ESs.
The exchange of ISHs is called neighbor greeting or initialization. Each router learns about the other
routers with which they share direct connectivity. After the initialization, each router constructs a
link-state packet (LSP). The LSP contains a list of the names of the IS’s neighbors and the cost to reach
each of the neighbors. Routers then distribute the LSPs to all of the other routers. When all LSPs are
propagated to all routers, each router has a complete map of the network topology (in the form of LSPs).
Routers use the LSPs and the SPF algorithm to compute routes to every destination in the network.
OSI networks are divided into areas and domains. An area is a group of contiguous networks and
attached hosts that is designated as an area by a network administrator. A domain is a collection of
connected areas. Routing domains provide full connectivity to all ESs within them. Routing within the
same area is known as Level 1 routing. Routing between two areas is known as Level 2 routing. LSPs
that are exchanged within a Level 1 area are called L1 LSPs. LSPs that are exchanged across Level 2
areas are called L2 LSPs. Figure 13-22 shows an example of Level 1 and Level 2 routing.
Figure 13-22 Level 1 and Level 2 OSI Routing
When you provision an ONS 15454 for a network with NEs that use both the TCP/IP and OSI protocol
stacks, you will provision it as one of the following:
• End System—The ONS 15454 performs OSI ES functions and relies upon an IS for communication
with nodes that reside within its OSI area.
• Intermediate System Level 1—The ONS 15454 performs OSI IS functions. It communicates with IS
and ES nodes that reside within its OSI area. It depends upon an IS L1/L2 node to communicate with
IS and ES nodes that reside outside its OSI area.
Level 2
routing
Area 1
IS IS
IS IS
Area 2
Domain
Level 1
routing
Level 1
routing
ES
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ES
ES
ES