— Exhibit –
— Exhibit —
Click the Exhibit button.
Referring to the exhibit, the RPT from R3 towards R2 is established.
What happens if the multicast source connected to R1 starts sending multicast traffic towards R1?
A.
R1 encapsulates the multicast packets into a PIM register multicast packet.
B.
R1 encapsulates the multicast packets into PIM join unicast messages.
C.
R1 forwards the multicast packets on the (S,G) tree towards the RP.
D.
R1 tunnels the multicast packets in PIM register messages towards the RP.
answer is C
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Can you explain why is the correct would be C?
A – The correct can be A, but I am unsure about “PIM register multicast packet.” On reference, RP sends an unicast packet, not multicast.
B – Is wrong because “PIM join” is send from Receiver not Sender.
C – Is wrong too, because R1 encapsulate the packet before send to RP.
D – The correct can be D. According to reference the packet is encapsulated. The word tunnels is “confusing” the answer.
“The source sends multicast packets out on the LAN, and the source DR encapsulates the packets in a PIM register message and forwards the message toward the RP router by means of unicast.”
Reference: http://www.juniper.net/documentation/en_US/junos14.1/topics/concept/multicast-pim-sparse-characteristics.html
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Hi Eddie. I think C is correct because if the RPT is already formed then the encapsulated register message has already been sent from R1 to the RP. After the RPT is formed multicast traffic is sent via this path to the source without encapsulation. After that when the receiver’s DR gets to know the source IP the SPT will be created. Correct me if I’m wrong. Cheers
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I was mistaken… this step is before anybody joins the multicast group. C is not correct for sure because as you said when a source starts to send multicast traffic then the source DR must send an encapsulated unicast PIM register packet to the RP. D is correct for me considering other possible choices. Sorry for my english. I need more practice lol
Cheers
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I would say D is correct. Here is an excerpt for those interested in some details.
“PIM Register Messages
PIM-SM requires that source DRs encapsulate multicast traffic inside register messages, which are
then sent as unicast datagrams to the group’s RP. Upon receipt, the RP removes the register
encapsulation and forwards the traffic down the shared tree as native multicast, if the tree exists.
Register encapsulation is necessary because initially no state in the network exists to allow native
multicast to flow from the sender to the RP. Put differently, the RP functions to allow receivers to
locate active senders without the prior knowledge that senders actually exist. By allowing a sender to
initially contact the RP using unicast forwarding, we eliminate the problem of the RP having no way to
learn of active senders.
On a Junos OS router, the source DR and RP must have tunnel services enabled to encapsulate and
decapsulate register messages. To provide tunnel services some Junos OS routers require an
Adaptive Services or Tunnel Services PIC.”
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Koraa is right..D is the answer for me!
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Besides, part of that new 80Q JN0-647 dumps for your reference:
https://drive.google.com/open?id=0B-ob6L_QjGLpejBJUUI1X2NuU3M
Best Regards!
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