What are the functions of PIM silent on a PIM interface


On the access layer, if the interface directly connected to a host is enabled with the PIM protocol, PIM neighbors can be established on this interface to process various PIM protocol packets. Such configuration, however, may bring security problems. For example, when malicious hosts send a large number of pseudo PIM Hello packets, it may lead to the collapse of the device.

To avoid the preceding problem, you can run the pim silent command on the interface to set the interface to work in PIM silent state. After the interface enters the PIM silent state, it is forbidden to receive or forward any PIM protocol packet. All PIM neighbors and the PIM state machine on this interface are deleted and the interface automatically becomes a DR. Meanwhile, the PIM silent function does not affect the IGMP function on the interface.

The PIM silent function is applicable only to the interface that is directly connected to the network segment of user hosts, and only one PIM device can be connected to this network segment.

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.

On which interfaces does PIM need to be enabled
When receiving a multicast packet, a switch performs Reverse Path Forwarding (RPF) check on the packet and searches for the optimal route to the destination. The outbound interface of the route is the RPF interface and the next hop of the route is the RPF neighbor. A multicast forwarding entry can be created only after PIM is enabled on the RPF interface and the RPF neighbor. In this way, a complete multicast distribution tree can be set up along the forwarding path of the packet. It is recommended that you enable PIM on all non-edge interfaces in the PIM domain because all these interfaces have a chance to become an RPF interface or RPF neighbor. On the switch directly connecting to a multicast source, the RPF interface for packets sent from the multicast source is the interface connected to the multicast source. Therefore, PIM must be enabled on this interface.

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