Subject: a question when switch (rp,g) to (sp,g)

From: liaohua <[email-masked]>
Date: Mon, 25 Jan 1999 16:58:07 +0800

HI:
    I am a new bee .
    When I reading PIM-SM  rfc 2362, I have a question :

   When a source begin to send message to a group g, router will switch
(rp,g) to a (sp,g) of the source . My question is if  the (sp,g) is
really the SHORTEST tree . Maybe in the process of building a (rp,g)
tree, the shortest interface(s)  is pruned for (rp , g). While switching
, is the (sp,g)  tree use the path of (rp,g) tree  or use the shortest
path ?

    see below (with the cost beside ):

    RP_2__________2_ROUTER A----------Source
    |2                                  |1
    |                                    |1
    |_____________2_ROUTER B----------group member

Of course , the shortest path from router A  to  router B is not the
path in the (rt,g) tree . Does  router A rebuild a tree after switch
for group G.  or When build (rp,g) tree  the router A also keep the
interface to router B as a oif ? when keep as oif , I think router A's
oif to B will receive the packets from router B's oif to A. An assert
will coming , suppose A's oif is pruned . then when a source began to
send  message , how can A do ?

    thanks for your help ^_^

liaohua