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

The following explains what to keep in mind for the Switch when IPv6 multicast paths are redundant.

<Structure of this section>

(1) Using IPv6 PIM-SM

Keep in mind that for IPv6 PIM-SM, it might take some time for IPv6 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 a prioritized route is switched, the following time might be required until communication restarts:
```
U +20 sec
```
When a prioritized route is switched to a redundant route due to line failure, the following time is required until communication restarts:
```
When U is less than 5 :5~10 sec
When U is 5 or more :U +0~60 sec
```
When a redundant route is switched to a prioritized route due to line restoration, the following time might be required until the prioritized route is used in communication:
```
0 sec
```
The following time is necessary to perform switching back.
```
U + ([Transmission cycle time of PIM-Hello message in the sender direction] +20)  sec   (U[by default] +30 +20 = U +50 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)
```
- BSR switching: 385 seconds max.
```
Bootstrap-Timeout(130 sec) +BS_Rand_Override(0~60 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)
```

IPv6 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.

Especially when switching the device that is running the rendezvous point and BSR, make sure that you set a higher priority value on the new device in configuration mode than the priority value of the current device.

(2) Using IPv6 PIM-SSM

Keep in mind that for IPv6 PIM-SSM, it might take some time for IPv6 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 a prioritized route is switched, the following time might be required until communication restarts:
```
U +20 sec
```
When a prioritized route is switched to a redundant route due to line failure, the following time is required until communication restarts:
```
When U is less than 5 :5~10 sec
UWhen U is 5 or more :U +0~135 sec
```
When a redundant route is switched to a prioritized route due to line restoration, the following time might be required until the prioritized route is used in communication:
```
0 sec
```
The following time is necessary to perform switching back.
```
U + ([Transmission cycle time of PIM-Hello message in the sender direction] +20)  sec   (U[by default] +30 +20 = U +50 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)
```