352-001 · Question #44
Which statement accurately describes how PIM sparse mode operates?
The correct answer is C. If an RP fails and there is no backup RP, new sources that begin sending will not be discovered by. In PIM sparse mode, the RP is required only for initial source registration and shared-tree joins - existing shortest-path-tree flows survive an RP failure, but new sources cannot be discovered by receivers.
Question
Which statement accurately describes how PIM sparse mode operates?
Options
- ARPs can become bottlenecks, since multicast traffic must always flow through the RP and down the
- BRPs switch each traffic flow to the shortest path tree when more than one receiver is present.
- CIf an RP fails and there is no backup RP, new sources that begin sending will not be discovered by
- DIf an RP fails and there is no backup RP, multicast traffic will cease flowing in the network.
How the community answered
(32 responses)- A3% (1)
- B3% (1)
- C94% (30)
Why each option
In PIM sparse mode, the RP is required only for initial source registration and shared-tree joins - existing shortest-path-tree flows survive an RP failure, but new sources cannot be discovered by receivers.
After SPT switchover, PIM sparse mode forwards traffic directly from source to last-hop router without traversing the RP, so the RP is not a persistent bottleneck for established multicast flows.
It is the last-hop router - not the RP - that detects traffic rate and sends a PIM Join directly toward the source to trigger the SPT switchover; the RP does not initiate this process.
Once a multicast flow has transitioned from the shared tree to the shortest path tree (SPT), the RP is no longer in the data-plane forwarding path, so those flows continue unaffected by an RP failure. However, a new source must send PIM Register messages to the RP so that receivers can learn about it; if the RP is unavailable, Register messages are dropped and receivers have no mechanism to discover or join the new source's traffic.
Existing multicast flows that have already completed SPT switchover continue flowing even after an RP failure because the RP is no longer in the forwarding path for those established sessions.
Concept tested: PIM sparse mode RP failure and new source discovery
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-mode.html
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