Exam 350-501 topic 1 question 303 discussion

Actual exam question from Cisco's 350-501

Question #: 303
Topic #: 1

Refer to the exhibit. A network engineer is implementing multicast Source-A to send a multicast stream for Receiver-A, and multicast Source-B to send a multicast stream for Receiver-B. Router RC forwards the IGMP host a report and leaves messages to IP address 192.168.10.1. How must the multicast features be implemented to prevent RB from receiving multicast flooding from Source-A?

A. Change the helper-address value to 192.168.10.2 on RC.
B. Enable ip pim neighbor-filter on RC interface Ge0/0.
C. Configure PIM-SSM on RB and RC interface Ge 0/0.
D. Enable ip pim passive on RB interface Ge0/0.

Show Suggested Answer

Suggested Answer: D 🗳️

by thejag at March 9, 2023, 9:06 p.m.

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kirrim

6 months ago

Selected Answer: A

B and D are incorrect because this will just prevent RC and RB from becoming PIM neighbors, but traffic still gets dense-mode flooded from RC to RB. That's how multicast stub routing behaves. And now RC has no way to prune it back via normal PIM mechanisms. C: It is true that PIM-SSM would stop the flooding since multicast stub routing requires dense mode flooding to work. But SSM is not configured on an interface, it's enabled globally (and any modified SSM range ACL would also have to be configured globally, if the multicast group ranges fell outside the default SSM group range) By process of elimination, A is all that's left. And it would work, although it's clumsy as hell. Ideally you'd just delete the IGMP helper-address altogether, you don't want to point it to the wrong router IP to stop it from forwarding traffic to RB.

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17c8777

7 months ago

As I understand it, ip igmp helper-address x.x.x.x is just a way to forward the IGMP messages to the configured IP. So for vlan20, with config excrept, RC will forward all IGMP messages to 192.168.10.1 (interface Ge0/0 of RB). What if we just change the address to 192.168.10.2 ? Then RB should not receive multicast flooding from Source-A in my opinion. So I would choose A

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17c8777

7 months ago

Hmm I'm confused ... IGMP = to join trees (ip pim passive doesn't change anything to IGMP), while PIM is used for the mutlicast distribution trees / to deliver the actual multicast traffic. With ip pim passive, I guess it does the job. Maybe answer D is not that bad

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chst

2 years, 3 months ago

why not is C?

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thejag

2 years, 2 months ago

From the doc: If the ip pim passive command is configured on an interface enabled for IP multicast, the device will operate this interface in PIM passive mode, which means that the device will not send PIM messages on the interface nor will it accept PIM messages from other devices across this interface. The device will instead consider that it is the only PIM device on the network and thus act as the DR and also as the DF for all bidir-PIM group ranges. IGMP operations are unaffected by this command.

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Mike_Rowehard

1 year, 3 months ago

Your statement is correct, but it does not appear to have anything to do with the question being asked. This will not prevent RB from receiving multicast streams from Sender A as the question asks for.

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thejag

2 years, 3 months ago

ip pim passive https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipmulti_pim/configuration/15-s/imc-pim-15-s-book/imc_stub_routing.html

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