Multicast Dense Mode Prune Message Behavior on Shared Media
In Multicast Dense Mode, when an upstream router receives a prune message on a shared medium interface (like Ethernet), it must respond with another prune message. This behavior is defined in the Upstream(S,G) State Machine, detailed in RFC 3973.
The process involves:
- The upstream router detecting a Prune(S,G) message on its shared medium interface.
- Setting an Override Timer (OT(S,G)) to t_override seconds.
- When OT(S,G) expires, the router sends a Join(S,G) to override the previous prune.
- If the outgoing interface list becomes empty (olist(S,G) -> NULL), the router:
- Transitions to the Pruned (P) state
- Sends a Prune(S,G) to RPF'(S)
- Sets the Prune Limit Timer (PLT(S,G)) to t_limit seconds
This seemingly redundant process ensures robustness and reliability in multicast communication on shared media. While the exact reasoning behind this design isn't explicitly stated, it likely helps maintain consistent state information across all routers on the shared segment.
Links:
https://forum.networklessons.com/t/multicast-pim-prune-override/1332/6?u=lagapidis
https://networklessons.com/multicast/multicast-pim-prune-override/