Document how to configure/use isolated networks

doc/html/topology.shtml

@@ -132,6 +132,16 @@ SwitchName=s3 Nodes=tux[12-15]
 SwitchName=s4 Switches=s[0-3]
 </pre>
 
+<p>Note that compute nodes on switches that lack a common parent switch can
+be used, but no job will span leaf switches without a common parent.
+For example, it is legal to remove the line "SwitchName=s4 Switches=s[0-3]"
+from the above topology.conf file.
+In that case, no job will span more than four compute nodes on any single leaf
+switch.
+This configuration can be useful if one wants to schedule multiple phyisical
+clusters as a single logical cluster under the control of a single slurmctld
+daemon.</p>
+
 <h2>User Options</h2>
 
 <p>For use with the topology/tree plugin, user can also specify the maximum
@@ -161,6 +171,6 @@ The value will be set component types listed in SLURM_TOPOLOGY_ADDR.
 Each component will be identified as either "switch" or "node".
 A period is used to separate each hardware component type.</p>
 
-<p style="text-align:center;">Last modified 13 January 2014</p>
+<p style="text-align:center;">Last modified 5 February 2014</p>
 
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