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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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and to see a list of the releases in which each feature is supported, see the feature information table at the end of this
module.
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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.
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.
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.
Caution
The default PIM hello hold-time interval is 3.5 x the PIM hello period. Multicast HA operations may not behave properly if
the PIM hello interval is configured with a value lower than the default value of 30 seconds.
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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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