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352-001 · Question #222

In a PIM sparse mode network, how is the flow of multicast traffic restored when a link fails along a branch of a multicast distribution tree?

The correct answer is D. The router downstream of the break recalculates the RPF interface when it notices a change in. In PIM sparse mode, multicast traffic is restored after a link failure when the downstream router detects a unicast routing table change and recalculates its RPF interface.

Layer 3 Control Plane

Question

In a PIM sparse mode network, how is the flow of multicast traffic restored when a link fails along a branch of a multicast distribution tree?

Options

  • AThe downstream receivers notice the loss of the multicast flow and resend PIM join messages to
  • BThe downstream receivers notice the loss of the multicast flow and resend IGMP Membership
  • CThe traffic reverts to the shared tree while the downstream router recalculates the RPF interface
  • DThe router downstream of the break recalculates the RPF interface when it notices a change in

How the community answered

(23 responses)
  • A
    4% (1)
  • B
    13% (3)
  • C
    4% (1)
  • D
    78% (18)

Why each option

In PIM sparse mode, multicast traffic is restored after a link failure when the downstream router detects a unicast routing table change and recalculates its RPF interface.

AThe downstream receivers notice the loss of the multicast flow and resend PIM join messages to

PIM Join messages are sent by routers, not by end receivers - receivers use IGMP to communicate group membership to their local router.

BThe downstream receivers notice the loss of the multicast flow and resend IGMP Membership

IGMP Membership Reports are used by hosts to join groups on their local segment and do not influence the multicast distribution tree reconstruction in the provider core.

CThe traffic reverts to the shared tree while the downstream router recalculates the RPF interface

PIM-SM does not automatically revert to the shared tree on link failure - recovery is driven by the downstream router recalculating the RPF interface based on unicast routing changes, not a tree switchover.

DThe router downstream of the break recalculates the RPF interface when it notices a change inCorrect

In PIM-SM, the downstream router monitors the unicast routing table via IGP updates (e.g., OSPF). When a link failure triggers a route change, the router recalculates its RPF interface - the interface pointing toward the source or RP - and sends new PIM Join messages along the updated path to restore multicast traffic flow.

Concept tested: PIM sparse mode RPF recalculation on link failure

Source: https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipmulti_pim/configuration/xe-16/imc-pim-xe-16-book/imc-pim-sparse.html

Topics

#PIM sparse mode#RPF interface#multicast tree recovery#link failure

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