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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
• Intermediate System Level 1/Level 2—The ONS 15454 performs IS functions. It communicates
with IS and ES nodes that reside within its OSI area. It also communicates with IS L1/L2 nodes that
reside in other OSI areas. This option should not be provisioned unless the node is connected to
another IS L1/L2 node that resides in a different OSI area. The node must also be connected to all
nodes within its area that are provisioned as IS L1/L2.
13.7.4.1 End System-to-Intermediate System Protocol
ES-IS is an OSI protocol that defines how ESs (hosts) and ISs (routers) learn about each other. ES-IS
configuration information is transmitted at regular intervals through the ES and IS hello messages. The
hello messages contain the subnetwork and network layer addresses of the systems that generate them.
The ES-IS configuration protocol communicates both OSI network layer addresses and OSI subnetwork
addresses. OSI network layer addresses identify either the NSAP, which is the interface between OSI
Layer 3 and Layer 4, or the NET, which is the network layer entity in an OSI IS. OSI SNPAs are the
points at which an ES or IS is physically attached to a subnetwork. The SNPA address uniquely identifies
each system attached to the subnetwork. In an Ethernet network, for example, the SNPA is the 48-bit
MAC address. Part of the configuration information transmitted by ES-IS is the NSAP-to-SNPA or
NET-to-SNPA mapping.
13.7.4.2 Intermediate System-to-Intermediate System Protocol
IS-IS is an OSI link-state hierarchical routing protocol that floods the network with link-state
information to build a complete, consistent picture of a network topology. IS-IS distinguishes between
Level 1 and Level 2 ISs. Level 1 ISs communicate with other Level 1 ISs in the same area. Level 2 ISs
route between Level 1 areas and form an intradomain routing backbone. Level 1 ISs need to know only
how to get to the nearest Level 2 IS. The backbone routing protocol can change without impacting the
intra-area routing protocol.
OSI routing begins when the ESs discover the nearest IS by listeni ng to I SH pack ets. W hen an ES want s
to send a packet to another ES, it sends the packet to one of the ISs on its directly attached network. The
router then looks up the destination address and forwards the packet along the best route. If the
destination ES is on the same subnetwork, the local IS knows this from listening to ESHs and forwards
the packet appropriately. The IS also might provide a redirect (RD) message back to the source to tell it
that a more direct route is available. If the destination address is an ES on another subnetwork in the
same area, the IS knows the correct route and forwards the packet appropriately. If the destination
address is an ES in another area, the Level 1 IS sends the packet to the nearest Level 2 IS. Forwarding
through Level 2 ISs continues until the packet reaches a Level 2 IS in the destination area. Within the
destination area, the ISs forward the packet along the best path until the destination ES is reached.
Link-state update messages help ISs learn about the network topology. Each IS generates an update
specifying the ESs and ISs to which it is connected, as well as the associated metrics. The update is then
sent to all neighboring ISs, which forward (flood) it to their neighbors, and so on. (Sequence numbers
terminate the flood and distinguish old updates from new ones.) Using these updates, each IS can build
a complete topology of the network. When the topology changes, new updates are sent.
IS-IS uses a single required default metric with a maximum path value of 1024. The metric is arbitrary
and typically is assigned by a network administrator. Any single link can have a maximum value of 64,
and path links are calculated by summing link values. Maximum metric values were set at these levels
to provide the granularity to support various link types while at the same time ensuring that the
shortest-path algorithm used for route computation is reasonably efficient. Three optional IS-IS metrics
(costs)—delay, expense, and error—are not supported by the ONS 15454. IS-IS maintains a mapping of
the metrics to the quality of service (QoS) option in the CLNP packet header. IS-IS uses the mappings
to compute routes through the internetwork.