Expand Configuration and Management Manual (G06.24+)

Subsystem Description
Expand Configuration and Management Manual—523347-008
18-28
Routing Algorithms
Routing Algorithms
Routing algorithms determine what and how much routing information $NCP will share
with the $NCPs at its neighbor nodes. You can select from two different routing
algorithms by setting the ALGORITHM modifier: modified split horizon (MSH) and split
horizon (SH). MSH is the default algorithm.
Modified Split Horizon (MSH)
When the modified split horizon (MSH) algorithm is used, $NCP tells its neighbor
$NCP the best-path route to a destination node. If that route leads through the
neighbor being updated, $NCP tells its neighbor $NCP that no route exists to the
destination.
Figure 18-9 shows the routing information known by node \D when the MSH algorithm
has been selected. Each entry indicates the TF and HC to each node from the
perspective of node \D. For example, the best-path route from node \D to node \A by
means of node \C is 2(2) (TF 2 and HC 2). Node \C reports to node \D that no path
exists from itself to node \B because its best-path route leads through node \D.
The advantage of the MSH algorithm is its efficiency: it requires less processing time
than the SH algorithm and avoids loop routing. (Loop routing is a disadvantage of the
SH algorithm; it is explained in Split Horizon (SH) on page 18-30.)
The disadvantage of the MSH algorithm is that the network may experience temporary
discontinuity, which occurs because $NCPs are not immediately aware of alternate
paths that may exist to a destination node.
For example, suppose that the path fails between node \D and node \B. Node \D is not
aware of an alternate path, although one exists through node \C. Before node \D can
reroute traffic through node \C, the following events must occur:
•
Node \D must inform node \C of the failed path.
•
Node \C must update its best-path route.
•
Node \C must inform node \D of its new best-path route information.
Node \D may complete requests with an error 250 (all paths to the system are down)
before it receives alternate path information.
You can offset this disadvantage of the MSH algorithm by specifying the
ABORTTIMER modifier, which enables you to ensure that $NCP has an opportunity to
obtain alternate routing information before requests are completed with an error 250.
The opportunity interval is the number of minutes you have defined as the
ABORTTIMER modifier value.
Note. ALGORITHM 0 specifies MSH, and ALGORITHM 1 specifies SH. The ALGORITHM
modifier is explained in Section 6, Configuring the Network Control Process.
Note. The ABORTTIMER modifier is explained in more detail in Section 6, Configuring the
Network Control Process.