Expand Configuration and Management Manual (G06.24+)

Expand Overview

Expand Configuration and Management Manual—523347-008

2-8

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 Section 18, Subsystem

Description.

Priority Routing

You can assign different priorities to messages sent over an Expand network. Priority

routing allows an important message to reach its destination even when the network is

congested.

Fault-Tolerant Operation

Using careful configuration and network-topology design, you can configure an Expand

network to be continuously available.

You can configure as many as eight lines between the same two nodes using the

Expand subsystem’s multi-line path feature. The Expand subsystem can

simultaneously transmit data over all of the lines, thus increasing overall bandwidth,

and will automatically reroute data over remaining lines if one or more lines fail.

You can configure as many as 16 paths between the same two nodes using the

Expand multi-CPU feature. The Expand multi-CPU feature enables you to spread the

communications load over multiple processors by connecting multiple Expand

line-handler processes on separate processors at one node to Expand line-handler

processes on separate processors on another node. The Expand subsystem transmits

data between neighbor nodes over all of the paths in a multi-CPU path, and will

automatically reroute data over remaining paths if one or more paths fail. The Expand

subsystem also uses a rebalancing algorithm to ensure that the average

communications load of all the paths in a multi-CPU path is close to equal.

You can also configure lines to be controlled by different communications hardware

devices to ensure that a single hardware failure will not disable a connection between

two servers.