Introduction to Networking for NonStop S-Series Servers

Expand Network
Introduction to Networking for HP NonStop S-Series Servers
—520670-005
5-8
Distributed Control
Distributed Control
The control function of the Expand subsystem is distributed throughout the network.
Unlike a hierarchical network, in which a central computer, or host, controls the
communications environment, nodes in an Expand network communicate with each
other as peers. Distributed networks have the following additional advantages:
•
Distributed applications. Applications can be distributed so that multiple nodes
share the processing load.
•
Flexible network topologies. The network topology can be designed without regard
to host or controlling processors.
•
Network reliability. Failure of one node does not necessarily affect the operation of
other nodes in the network.
Automatic Message Routing
The Expand subsystem’s routing facilities ensure that a message sent from any node
in the network will arrive at its destination as long as there is at least one active
communications path available. The Expand subsystem’s routing capabilities also
include the following:
•
Passthrough
•
Best-path
•
Priority
Passthrough Routing
The Expand subsystem uses a sophisticated routing scheme that permits intermediate
nodes to route, or pass through, data packets to the destination node. This scheme
reduces the number of lines required between nodes because nodes do not have to be
directly connected in order to exchange data.
Best-Path Routing
When a message is sent over an Expand network, the Expand subsystem determines
the best-path route to the destination node by calculating time factors (TFs) and the
number of intermediate nodes (or hops) to the destination node. A TF is calculated for
a line, path, or route. The best-path route is the route with the lowest TF and hop count
(HC).
The Expand subsystem dynamically revises its best-path route determination if a node
or path status changes when nodes or paths become operational or nonoperational.
TF calculation and best-path route selection are discussed in the Expand Configuration
and Management Manual.