Exam 350-501 topic 1 question 171 discussion

I can aid in the discussion with the following premises: Based on my research - BIDIR-PIM is an extension to PIM-SM (requires PIM-SM to be pre-configured). Cisco recognizes PIM-DM and PIM-SM as the initial set of protocols. I'm voting for "A" as the answer

SSM will skip the RP. PIM-DM will send to the RP inconsequentially. PIM-SM will initially send to RP then swap to SPT. PIM-BIDIR will always keep the RP in the path. Both answers are correct. This question sucks.

Bidir-PIM shares many of its shortest path tree (SPT) operations with PIM-SM. Bidir-PIM also has unconditional forwarding of source traffic toward the RP upstream on the shared tree,

Based on wording of question and answers, I will also agree with B. PIM-BIDI does not create S,G entries and thus will never have a path directly to source. PIM-SM, we can make the spt-threshold to inifinty to force traffic to always take shared tree path to RP... however, this is not default behavior. Thus B makes the most sense.

I agree with @kolinsteroy

BIDIR-PIM - Seems more in line with what the question is asking, rather poorly.

B - BIDIR-PIM

I will choose B.

Bit of a tricky one, but... The default SPT switchover threshold in PIM-SM for all groups is 0. This means that the switchover will happen immediately after the first multicast packet arrives. Of course you could set the SPT switchover to infinity, but that is NOT the default configuration nor is that mentioned anywhere here. Whereas with BIDIR-PIM only builds (*,G) to the RP to accommodate many-to-many multicast applications (lowering memory and CPU utilization associated with (S,G)). I'm going with B!

Correct answer is B (BIDIR-PIM) Answer is A is wrong, RC - RA - RB - RD - RF isn't an optimal path based on default behaviour of PIM-SM protocol. Therefore, PIM-SM protocol selects RC - RD - RF optimal path using Shortest Path Tree Switch over. So in those cases, it is desirable for the multicast traffic from source flows to receiver on optimal path. This is done by SPT switchover and this is done when the multicast group traffic crosses the SPT threshold configured. In Cisco the default SPT threshold is Zero kbps. So when the last hop router receives first multicast packet via shared tree, it does SPT switchover. This is done by sending a SPT Join towards the source thus bypassing the RP. Now the traffic from source prefers the most optimal path to reach the receiver. https://community.cisco.com/t5/switching/can-someone-explain-shortest-path-switch-over-in-multicast/td-p/1444174

B is correct, coz In section 15.7 e-learning Cisco, we have: PIM-SM is unidirectional in its native form. The traffic from sources to the RP initially flows encapsulated in register messages. This activity presents a significant burden because of the encapsulation and de-encapsulation mechanisms. Additionally, an SPT is built between the RP and the source, which results in (S,G) entries being created between the RP and the source. BIDIR-PIM (Bidirectional PIM) is designed to be used for many-to-many applications. Multicast groups in bidirectional mode can scale to an arbitrary number of sources without incurring overhead due to the number of sources. BIDIR-PIM eliminates the registration and encapsulation process and the (S,G) state. Packets are natively forwarded from a source to the RP using the (*,G) state only. This capability ensures that only (*,G) entries will appear in multicast forwarding tables

As there is 1 sender and 1 receiver, BIDIR-PIM goes out of the question. I think A is the only correct answer here.

such a stupid question. PIM-SM does auto send to, but PIM-BIDIR does auto to and from.... since the question just says sending, I would go with PIM-SM https://www.cisco.com/c/en/us/td/docs/ios/solutions_docs/ip_multicast/White_papers/rps.html https://networklessons.com/multicast/multicast-bidirectional-pim

B.. PIM-SM will use source tree between sender and receiver later but BIDIR-PIM will always use shared tree meaning traffic always flow through RP.

A https://www.cisco.com/c/en/us/td/docs/ios/solutions_docs/ip_multicast/White_papers/rps.html

Answer is B. BIDIR-PIM establishes what is called a DF, which forwards all upstream traffic from the sources and sends them towards the RP.