Expand Configuration and Management Manual (G06.24+)

Subsystem Description
Expand Configuration and Management Manual—523347-008
18-31
Multi-CPU Paths
The disadvantage of the SH algorithm is that it increases the occurrence of loop
routing, which results in excessively long routes. Loop routing most often occurs in
large, multi-ringed networks. For example, in Figure 18-10, suppose the path fails
between node \D and node \E. If a message is sent from node \A to node \E, the
Expand subsystem will attempt to reroute traffic in the following sequence:
•
Through nodes \B, \A, \C, and \D. This route is not usable because it uses the
failed path between nodes \D and \E.
•
Through nodes \C, \A, \B, and \D. This route is also not usable because it uses the
failed path between nodes \D and \E.
After these two failed rerouting attempts, the Expand subsystem will determine that the
path between nodes \D and \E has failed.
You can offset this disadvantage of the SH algorithm by specifying the
NETWORKDIAMETER modifier, which defines the maximum HC that is acceptable
between two nodes. If a route is calculated that exceeds this limit, packets are
discarded. By specifying the NETWORKDIAMETER modifier, you increase the speed
with which unreachable nodes are discovered.
Multi-CPU Paths
To guarantee message order when a multi-CPU path is used, one Expand line-handler
process at each source node and one Expand line-handler process at each destination
node are paired; all messages between that source and destination node are sent
through this Expand line-handler pair.
Whether the source and destination nodes are regarded as a group of processors or
as a single system, and when the Expand line-handler pair is formed, depends on
whether the source and the destination nodes are neighbors.
Non-Neighbor Nodes
For non-neighbor nodes, the Expand line-handler pairs are similar to those used by
single-line and multi-line paths—they apply to each source and destination system
combination. The NRTs in all processors in the entire system point to the same path,
so global NRT updates are used. The Expand line-handler pair is established by $NCP
when the connection is first made to the remote node. The pairing is symmetrical;
messages traveling in either direction use the same Expand line-handler pair.
When $NCP initiates a connection to a non-neighbor node and the best-path route to
the node is a multi-CPU path, $NCP selects one Expand line-handler process in the
multi-CPU path and starts the connection over that Expand line-handler process. The
neighbor node directs traffic from all its processors to the Expand line-handler process
from which the connection initiation was received using information maintained in the
reverse pairing table (RPT).
Note. The NETWORKDIAMETER modifier is explained in more detail in Section 6,
Configuring the Network Control Process.