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PIM Questions

Question 1

Which statement about the RPF interface in a BIDIR-PIM network is true?

A. In a BIDIR-PIM network, the RPF interface is always the interface that is used to reach the PIM rendezvous point.
B. In a BIDIR-PIM network, the RPF interface can be the interface that is used to reach the PIM rendezvous point or the interface that is used to reach the source.
C. In a BIDIR-PIM network, the RPF interface is always the interface that is used to reach the source.
D. There is no RPF interface concept in BIDIR-PIM networks.


Answer: A

Question 2

Refer to the exhibit.


Which three statements about the output are true? (Choose three)

A. An mrouter port can be learned by receiving a PIM hello packet from a multicast router.
B. This switch is configured as a multicast router.
C. Gi2/0/1 is a trunk link that connects to a multicast router.
D. An mrouter port is learned when a multicast data stream is received on that port from a multicast router.
E. This switch is not configured as a multicast router. It is configured only for IGMP snooping.
F. IGMP reports are received only on Gi2/0/1 and are never transmitted out Gi2/0/1 for VLANs 10 and 20.


Answer: A B C

Question 3

Which mechanism can be used on Layer 2 switches so that only multicast packets with downstream receivers are sent on the multicast router-connected ports?

A. IGMP snooping
B. Router Guard
C. PIM snooping
D. multicast filtering


Answer: C

Question 4

Refer to the exhibit.

Router#show ip pim tunnel
Type : PIM Encap
RP :*
Type : PIM Decap
RP :*
Source: –

What is the role of this multicast router?

A. a first-hop PIM router
B. a last-hop PIM router
C. a PIM rendezvous point
D. a PIM inter-AS router


Answer: C

Comments (9) Comments
  1. B
    November 9th, 2014

    I believe the answer to question 1 is wrong. In bi-directional PIM, there is (D) no concept of the RPF interface as RPF is no longer needed due to the Designated Forwarder election on the segment. In Bi-Directional PIM, one router on the segment will be elected to be the Bi-Directional forwarder to the RP, so there are no possibilities of loops, making the RPF check invalid.

  2. Hamid
    December 26th, 2014

    Q1 – agree to B
    PIM bidirectional has no RPF check. There is a different solution to prevent loops, we will use a DF (Designated Forwarder). This designated forwarder is the only router on the segment that is allowed to send multicast traffic towards the RP. When there is only 1 router per segment that forwards multicast traffic there will be no loops.
    RSTUT Please explain

  3. CCIE101
    March 2nd, 2015

    Any comment for Q3, PIM snooping please.

  4. ABIN
    March 20th, 2015

    Q3. i think pim snooping is right, as igmp snooping and pim snooping do the same thing, but igmp is generated by routers and pim snooping is done by l2 devices ie switches

  5. me
    March 29th, 2015

    Different multicast routing protocols determine their RPF interfaces in different ways, as follows:
    Distance Vector Multicast Routing Protocol (DVMRP) maintains a separate multicast routing
    table and uses it for the RPF check.
    Protocol Independent Multicast (PIM) and Core-Based Tree (CBT) generally use the unicast
    routing table for the RPF check, as shown in Figure 8-3.
    PIM and CBT can also use the DVMRP route table, the Multiprotocol Border Gateway
    Protocol (MP-BGP) route table, or statically configured multicast route(s) for the RPF check.
    Multicast OSPF does not use the RPF check, because it computes both forward and reverse
    shortest-path source-rooted trees by using the Dijkstra algorithm

    it is used, made sh ip mroute and you will se RPF in the command A is valid

  6. me
    March 29th, 2015

    Q1 A correct

  7. me
    March 30th, 2015

    Q3 correct
    Understanding PIM Snooping
    In networks where a Layer 2 switch interconnects several routers, such as an Internet exchange point (IXP), the switch floods IP multicast packets on all multicast router ports by default, even if there are no multicast receivers downstream. With PIM snooping enabled, the switch restricts multicast packets for each IP multicast group to only those multicast router ports that have downstream receivers joined to that group. When you enable PIM snooping, the switch learns which multicast router ports need to receive the multicast traffic within a specific VLAN by listening to the PIM hello messages, PIM join and prune messages, and bidirectional PIM designated forwarder-election messages. With Release 12.2(33)SXJ2 and later releases, PIM snooping also constrains multicast traffic to VPLS interfaces.

    Note To use PIM snooping, you must enable IGMP snooping on the switch. IGMP snooping restricts multicast traffic that exits through the LAN ports to which hosts are connected. IGMP snooping does not restrict traffic that exits through the LAN ports to which one or more multicast routers are connected.


  8. sh01
    May 1st, 2015

    where is the exhibit of the question .They are mentioning that refer exhibit.

  9. facepalm
    May 19th, 2015

    I think q2 is incorrect – I see no evidence that this switch is a layer3 switch with multicast routing turned on, igmp snooping definitely.

    detecting mrouter ports does not require multicast routing

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