The PIM Triggered Joins feature is an HA multicast enhancement that improves the reconvergence of mroutes after an RP switchover. In the event of an RP switchover, this feature utilizes the Protocol Indepedent Multicast sparse mode (PIM-SM) GenID value as a mechanism to trigger adjacent PIM neighbors on an interface to send PIM join messages for all (*, G) and (S, G) mroutes that use that interface as an RPF interface, immediately reestablishing those states on the newly active RP. A GenID is a randomly generated 32-bit value regenerated each time PIM forwarding is started or restarted on an interface.

Note	In order to process the GenID value in PIM hello messages, PIM neighbors must be running Cisco IOS software with an implementation of PIM that is compliant with RFC 4601. For more information about RFC 4601, see the Additional References section.

After an RP switchover, all instances of PIM running on the newly active RP will modify the value of the GenID that is included in PIM hello messages sent to adjacent PIM neighbors. When an adjacent PIM neighbor receives a PIM hello message on an interface with a new GenID, the PIM neighbor will process the modified GenID as an indication that the PIM neighbor has gone down. A modified GenID, thus, is a mechanism to alert all adjacent PIM neighbors that PIM forwarding on that interface has been lost, which then triggers adjacent PIM neighbors to send PIM joins for all (*, G) and (S, G) mroute states that use that interface as an RPF interface.

The figure illustrates the operations that occur in association with the PIM Triggered Joins feature during an RP switchover.

Figure 1

Operation of PIM Triggered Joins During a Switchover

The mechanics of the PIM Triggered Joins feature are as follows:

• In steady state, PIM neighbors exchange periodic PIM hello messages.
• An active RP receives PIM joins to periodically refresh mroute states.
• When an active RP fails, the standby RP takes over to become the new active RP.
• The new active RP then modifies the GenID value and sends the new GenID in PIM hello messages to adjacent PIM neighbors.
• Adjacent PIM neighbors that receive PIM hello messages on an interface with a new GenID then send PIM triggered joins for all (*, G) and (S, G) mroutes that use that interfaces as an RPF interface.
• Those mroute states are then immediately reestablished on the newly active RP.

Prior to the introduction of the PIM Triggered Joins feature, in the event of an RP switchover, all hardware entries would have been suspended until the control plane was reestablished. States for mroutes on the new active RP, thus, would not have been rebuilt until the periodic joins sent by adjacent PIM neighbors were received by PIM neighbors on the new active RP. The PIM Triggered Joins feature, thus, improves the recovergence time of mroutes after an RP switchover, enabling mroute states to be rebuilt promptly after an RP switchover.

Note	If a PIM neighbor is unable to process the GenID value, the PIM neighbor will ignore the GenID value and behave in the manner that it would prior to the introduction of the PIM Triggered Joins feature.