15.5.2 Redundant routes (route switching due to failure, etc.)

The following explains the point to keep in mind for the Switch when multicast paths are redundant.

<Structure of this section>

(1) Using PIM-SM

Keep in mind that for PIM-SM, it might take some time for multicast communication to restart for the following route switching. In the following time indications, the switching time for sender network information (unicast routing information) is represented as U.

The time shown here indicates how long it takes the Switch to perform switching. Because of this, join notification time (from the connection request from the Switch to the upstream router to the arrival of the multicast data from the upstream) is required to actually restart multicast forwarding.

When the priority route is switched, the time required to resume communication is U seconds.

However, note the following exceptions: The default values are shown in parentheses.
- If it takes more than five seconds to switch from a priority route to a redundant route due to a line failure, it may take the following time to resume communication.
  
  U+0 to Join/Prune send cycle (60 sec)
- When switching from a redundant route to a priority route due to line recovery, the following time is required to switch to the priority route:
  
  U+0 to Send period of PIM-Hello messagetoward sender (30 sec)
When a rendezvous point and BSR are switched to the Switch (the Switch becomes the rendezvous point and BSR, such as due to failure or configuration), the following time is required until communication restarts.

The time until communication restarts differs depending on the rendezvous point or BSR. The default values are shown in parentheses.
- For rendezvous point switching: 285 seconds
```
RP-Holdtime(150 sec) +Query-interval(125 sec) +Query Response Interval(10 sec)
```
- For BSR switching: At most 348 seconds
```
Bootstrap-Timeout(130 sec) +BS_Rand_Override(5~23 sec) +Bootstrap-Period(60 sec)
 +Query-interval(125 sec) +Query Response Interval(10 sec)
```
When a DR is switched to the Switch, the following time is required until communication restarts. The default values are shown in parentheses.
- For DR switching: 240 seconds
```
Hello-Holdtime(105 sec) +Query-interval(125 sec) +Query Response Interval(10 sec)
```

Multicast communication might stop at these times not only for redundant route switching due to failure, but also when path switching is performed explicitly due to a configuration change. Plan ahead when changing the system configuration.

(2) Using PIM-SSM

Keep in mind that for PIM-SSM, it might take some time for multicast communication to restart for the following path switching. In the following time indications, the switching time for sender network information (unicast routing information) is represented as U.

When the priority route is switched, the time required to resume communication is U seconds.

However, note the following exceptions: The default values are shown in parentheses.
- If it takes more than five seconds to switch from a priority route to a redundant route due to a line failure, it may take the following time to resume communication.
  
  U+0 to Query Interval(125 seconds) + Query Response Interval(10 seconds)
- When switching from a redundant route to a priority route due to line recovery, the following time is required to switch to the priority route:
  
  U+0 to Send period of PIM-Hello messagetoward sender (30 sec)
When a DR is switched to the Switch, the following time is required until communication restarts. The default values are shown in parentheses.
- For DR switching: 240 seconds
```
Hello-Holdtime(105 sec) +Query-interval(125 sec) +Query Response Interval(10 sec)
```