Can DRs on a PIM SM network change dynamically


DRs can change dynamically on a PIM SM network. Even though a DR is functioning properly, it may be replaced by a router with a higher priority.

Other related questions:
what are diffirence between PIM-SM and PIM-DM
PIM-DM: Small-scale LANs where multicast group members are distributed densely. Using the flood-prune mechanism, PIM-DM creates and maintains a unidirectional and loop-free SPT connecting a multicast source and group members. PIM-SM(ASM model): Large-scale network where multicast group members are distributed sparsely. An MDT is set up when receivers join a multicast group. PIM-SM needs to maintain an RP, set up an RPT, and register a multicast source. PIM-SM(SSM model): Scenarios where user hosts know the exact positions of multicast sources in advance and can specify the sources from which they want to receive data before they join multicast groups. PIM-SSM does not need to maintain an RP, set up an RPT, or register a multicast source.

Can a PIM domain run PIM-DM and PIM-SM simultaneously
A PIM Domain cannot run PIM-DM and PIM-SM simultaneously. Since PIM Hello packets do not carry messages in PIM mode, a PIM switch cannot know the mode of the PIM protocol that its PIM neighbor runs. If the RPF interface and the RPF neighbor run PIM protocols of different modes, two neighboring devices cannot learn PIM routing information from each other. As a result, related multicast forwarding entries cannot be correctly created.

How to perform the RPF check for establishing an RPT in a PIM-SM domain
A receiver's DR first performs the RPF check: It searches for a unicast route to the RP. The outbound interface of the searched route acts as an upstream interface and the next hop of the searched route acts as an RPF neighbor. Then, the receiver's DR sends a Join message to the RPF neighbor. After receiving the Join message, the RPF neighbor performs the RPF check and forwards the Join message upstream at the same time. The Join message is sent upstream hop by hop until it reaches the RP. In the preceding process, if there is one device that has no route to the RP, the RPT cannot be established.

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