Table Of Contents
PIM Triggered Joins
First Published: August 21, 2007Last Updated: November 20, 2009
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
Your software release may not support all the features documented in this module. For the latest feature information and caveats, see the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the "Feature Information for PIM Triggered Joins" section.
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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
To understand the PIM Triggered Joins feature, you should be familiar with the following concept:
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.
Figure 1 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.
The following sections provide references related to the PIM Triggered Joins feature.
Related Topic Document Title
Multicast configuration tasks on Catalyst 6500 series switches
Multicast concepts and tasks
Cisco IOS IP Multicast Configuration Guide, Release 12.4T
IP multicast commands: complete command syntax, command mode, command history, defaults, usage guidelines, and examples
Cisco IOS IP Multicast Command Reference, Release 12.2SX
No new or modified standards are supported by this feature, and support for existing standards has not been modified by this feature.
Feature Information for PIM Triggered Joins
Table 1 lists the release history for this feature.
For information on a feature in this technology that is not documented here, see the "IP Multicast Features Roadmap."
Not all commands may be available in your Cisco IOS software release. For release information about a specific command, see the command reference documentation.
Use Cisco Feature Navigator to find information about platform support and software image support. Cisco Feature Navigator enables you to determine which Cisco IOS and Catalyst OS software images support a specific software release, feature set, or platform. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.
Note Table 1 lists only the Cisco IOS software release that introduced support for a given feature in a given Cisco IOS software release train. Unless noted otherwise, subsequent releases of that Cisco IOS software release train also support that feature.
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