[pim] DR priority adjustment automatically according to the status of the tracked interfaces

[Xu Xiaohu <xuxh at huawei.com>]

Hi all,

In the following PIM-SM (or SSM) scenario, R!, R2 and Client are located
with the same LAN. R1 is elected as the DR for that LAN. Now R1's uplink to
the PIM-SM Network  is down, as long as R1 still acts as the DR, the
multicast stream will be broken soon or later. 

             |
             |   +-----+    +------------------+
             +---+ R1  +----+                  |
             |   +-----+    |                  |
 +-------+   |              |                  |  +-------+
 |Client +---+              | PIM-SM Network   +--+Source |
 +-------+   |              |                  |  +-------+
             |   +-----+    |                  |
             +---+ R2  +----+                  |
             |   +-----+    +------------------+
             |

I wonder whether it is suitable for the DR (e.g., R1 in the above figure) to
track the upstream link, and once the tracked link is down, its DR priority
is reduced automatically so as to cease the DR role. Of couse, this
mechanism is also suitable for the scenario where the Multicast Souce is
connected to more than one PIM routers via LAN.

Any comment? Or whether some vendor has already implement this feature?

Xiaohu