Expand Configuration and Management Manual (G06.24+)

Subsystem Description
Expand Configuration and Management Manual—523347-008
18-25
Network Routing Table (NRT) and Multiple Path
Table (MPT)
•
If two or more routes have the same TF, the route that has the lowest hop count
(HC)—the fewest intervening nodes—is selected. Each path between two nodes is
one hop. For example, a route that includes one passthrough node has a HC of 2;
a route that includes two passthrough nodes has an HC of 3, and so on.
•
If two or more routes have the same TF and HC, the first path that is operational
after the node is started is selected.
If $NCP determines that the best-path route to a destination node is a multi-CPU path,
the NRT lookup routine selects a path within the multi-CPU path that spreads the load
over all the paths in the multi-CPU path. Path selection is performed using the path’s
effective time factor (ETF). The ETF is an extension of the path TF that represents
both the speed of the path and the resources available on the path to accommodate
more traffic.
The ETF indicates the inverse proportion of traffic that should be sent over a path
compared to an unloaded path that has a TF of 1. For example, a path that has a TF of
6 reports an ETF of 12 when it is half loaded. The ratio between a path’s ETF and its
base TF is called the load factor (LF) of the path.
To compute a path’s ETF, aggregate line utilization must be determined in both
directions on the path. Information obtained by the congestion control feature, along
with information about local memory usage, is used to compute the ETF. If there has
been no recent traffic on a path, a separate extended packet called a PING message
is sent to measure the round-trip time to the neighbor node.
Network Routing Table (NRT) and Multiple Path Table (MPT)
The network routing table (NRT) resides in each processor in each node in the
network. The NRT associates each destination node with the logical device (LDEV)
number of the Expand line-handler process that is chosen to use to send messages to
that node (the best-path route). The NonStop Kernel operating system uses the NRT to
select the appropriate line LDEV for message transmission to other nodes.
The additional routing information required for multi-CPU paths is maintained in the
multiple path table (MPT). The NRT contains an entry that points to the MPT. Like the
NRT, the MPT resides in each processor in each node in the network. MPT entries are
assigned to specific multi-CPU paths by $NCP. The MPT also includes an entry called
the reverse pairing table (RPT), which contains information about Expand line-
handler pairs. For more information on pairing, refer to Multi-CPU Paths on
page 18-31.
The $NCP at each node depends on the routing information it receives from neighbor
$NCPs to keep the routing information in the NRT and MPT up to date. Each node
updates its NRT and MPT as it becomes aware of changes in network status, thus
allowing message traffic to be routed correctly.