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PIM Triggered Joins
The PIM Triggered Joins feature is a high availability (HA) multicast enhancement that improves the reconvergence of multicast routes (mroutes) after an RP switchover.
Finding Feature Information
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Restrictions for PIM Triggered Joins
All PIM neighbors must be compliant with RFC 4601 and be able to process GenID differences in PIM hello messages.
Information About PIM Triggered Joins
Functional Overview of PIM Triggered Joins
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.
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.
Generation IDs
A Generation ID (GenID) is a randomly generated 32-bit value that is regenerated each time PIM forwarding is started or restarted on an interface. In order to process the GenID value in PIM hello messages, PIM neighbors must be running software with an implementation of PIM that is compliant with RFC 4601.
Note | PIM neighbors that are not compliant with RFC 4601 and are unable to process GenID differences in PIM hello messages will ignore the GenIDs. |
PIM Triggered Joins and Multicast Traffic Flow
Multicast traffic flow on the neighbors is not affected if it detects a PIM triggered join or PIM hello message from a node with the failing RP within the default PIM hello hold-time interval. Multicast traffic flow on a failing RP is not affected if it is NSF capable.
Additional References
Related Documents
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Related Topic |
Document Title |
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Cisco IOS commands |
Cisco IOS Master Commands List, All Releases |
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IP multicast commands |
Cisco IOS IP Multicast Command Reference |
Standards and RFCs
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Standard/RFC |
Title |
|---|---|
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RFC 4601 |
Protocol Independent Multicast - Sparse Mode (PIM-SM): Protocol Specification (Revised) |
MIBs
Technical Assistance
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Description |
Link |
|---|---|
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Feature Information for PIM Triggered Joins
The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
| Table 1 | Feature Information for PIM Triggered Joins |
|
Feature Name |
Releases |
Feature Information |
|---|---|---|
|
PIM Triggered Joins |
12.2(33)SXH Cisco IOS XE Release 2.6 15.0(1)M 12.2(33)SRE 15.1(1)SG Cisco IOS XE Release 3.3SG |
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 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. |
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