The answer given seems a little thin. Here's how I understand
this to work. When a router wants to join (*, G), it either
needs to know the IP of the rendezvous point (RP) or a source
generating the mulitcast traffic. Until the tree to the RP is
built, the router only knows about the RP IP address. Thus, when
the router wants to join (*, G), it does a routing loopkup for
the RP IP address. The interface that would be used to reach the
RP IP address is marked as the *incoming* interface, and then the
router sends a JOIN (*, G) out this interface. This recursively
happens, one router at a time, until the JOIN reaches the RP.
If there's s source generating traffic, and once the tree to the
RP is built, the router will start to receive multicast traffic.
The router will therefore know a source for the multicast traffic,
and could then, if so configured, build a tree to the source.
The difference is now the router uses the source IP in its
lookup into the routing table. The interface returned is now
the incoming interface for the source and the interface over
which the (S, G) JOIN is sent. Thus, using the source IP,
the tree grows back to the source forming the SPT.
The routing table referenced, when using PIM, can be the unicast
routing table. However, if there's a need to apply policy
unique to the multicast traffic (ie which paths to use to get
to the RP or source IP), a second routing table can be referenced.
This is the mroute table. Information typically inserted
into this routing table is via static routes or MBGP (multicast
BGP).
In order to build the tree to the RP, the router must know the
RP IP address. There are a number of ways for the router to
become aware of the RP IP address. The first is for this
information to be statically configured. Other approaches
include the use of PIM's bootstrap message mechanism. A third
approach is cisco's auto-RP approach.
Antero Vasconcelos wrote:
question
In a PIMv2 Sparse Mode network, the incoming interface for a
(*, G) mroute
entry is calculated using:
answer
The address of a directory connected member of group G.
anybody can explaim this answer.
I4ve looked in the Books and still don4t undestand the question
neither the
answer!!
thx in adv
Antero
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