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.

Other related questions:
Principles of RPF check on the AR router
In unicast routing and forwarding, unicast packets are transmitted along a P2P path. The router only needs to consider the destination address, so it can learn the outbound interface through which a packet can be forwarded to the destination. Multicast routing and forwarding are different. The destination address of multicast packets is the multicast address that just identifies a group of receivers. That is, the receiver's position cannot be found based on the destination address. However, the source address of multicast packet is determined. Therefore, the correctness of forwarding path of multicast packets depend on the source address. When a router receives a multicast packet, it searches the unicast routing table for the route to the source address of the packet. After finding the route, the router checks whether the outbound interface of the route is the same as the inbound interface of the multicast packet. If they are the same, the router considers that the multicast packet is received from a correct interface. This ensures correctness and uniqueness of the entire forwarding path. This process is called RPF check. The correct interface is called an RPF interface.

How to implement PIM multi-domain on the AR router
MSDP is designed to connect multiple PIM-SM domains. MSDP peer relationships are set up when multicast data needs to be transmitted between ASs.

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.

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.

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