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Configuring PIM Sparse Mode Graceful Restart

You can configure PIM sparse mode to continue to forward existing multicast packet streams during a routing process failure and restart. Only PIM sparse mode can be configured this way. The routing platform does not forward multicast packets for protocols other than PIM during graceful restart, because all other multicast protocols must restart after a routing process failure. If you configure PIM sparse-dense mode, only sparse multicast groups benefit from a graceful restart.

The routing platform does not forward new streams until after the restart is complete. After restart, the routing platform refreshes the forwarding state with any updates that were received from neighbors during the restart period. For example, the routing platform relearns the join and prune states of neighbors during the restart, but it does not apply the changes to the forwarding table until after the restart.

When PIM sparse mode is enabled, the routing platform generates a unique 32-bit random number called a generation identifier. Generation identifiers are included by default in PIM hello messages, as specified in the Internet draft draft-ietf-pim-sm-v2-new-10.txt. When a routing platform receives PIM hello messages containing generation identifiers on a point-to-point interface, the Junos OS activates an algorithm that optimizes graceful restart.

Before PIM sparse mode graceful restart occurs, each routing platform creates a generation identifier and sends it to its multicast neighbors. If a routing platform with PIM sparse mode restarts, it creates a new generation identifier and sends it to neighbors. When a neighbor receives the new identifier, it resends multicast updates to the restarting router to allow it to exit graceful restart efficiently. The restart phase is complete when the restart duration timer expires.

Multicast forwarding can be interrupted in two ways. First, if the underlying routing protocol is unstable, multicast RPF checks can fail and cause an interruption. Second, because the forwarding table is not updated during the graceful restart period, new multicast streams are not forwarded until graceful restart is complete.

1. Enable graceful restart.
   [edit protocols pim]user@host# set graceful-restart
2. (Optional) Configure the amount of time the routing device waits (in seconds) to complete PIM sparse mode graceful restart. By default, the router allows 60 seconds. The range is from 30 through 300 seconds. After this restart time, the Routing Engine resumes normal multicast operation.
   [edit protocols pim graceful-restart]user@host# set restart-duration 120
3. Monitor the operation of PIM graceful restart by running the show pim neighbors command. In the command output, look for the G flag in the Option field. The G flag stands for generation identifier. Also run the show task replication command to verify the status of GRES and NSR.