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[ns] question about tree computation and RP resetting in Centralized multicast



Hi, I really appreciate your previous replies.
 
I got two more questions on centralized multicast.

1. about multicast tree construction

    what is the tree construction method from RP to receivers? Is it Reverse Path       Forwading ? 

    and If I will construct tree from RP through intermediate several nodes to Receivers as I want to, How and what can I modify ? 

2. about RP reset 
I modify tcl example.
After setting up a RP as below, I want to change the RP to another one.
How can I do that in same tcl script?

---------------------------------------
if {$mrthandle != ""} {
$ns at 0.5 "$mrthandle set_c_rp [list $n1]"
}
----------------------------------------
 
When I inserted something as below. ns starts RP from $n6, not $n1.
I want to simulate first RP $n1 and second RP $n2.
I saw the nam siwtch the simulation from RPT to SPT by using switch-tree procedure in example file. 
How can I do that?
-----------------------------------------
if {$mrthandle != ""} {
$ns at 0.90 "$mrthandle set_c_rp [list $n6]"
$ns at 0.90 "$mrthandle compute-mroutes"
}
------------------------------------------
I am looking forward to your reply.
Thanks a lot.
 
from ns beginner.....