irwan piesessa

One of my favourite topic when i preparing for CCIE Routing and Switching Lab is IP Multicast, especially in Auto-RP section.

There is one scenario about IP Multicast in NBMA environment that used PIM (Protocol Independent Multicast) with Auto-RP mechanism to propagate RP (Rendezvous Point) information when every interfaces that running Multicast use sparse-mode. If we put the RP Router and Mapping Agent on the spoke, the other spoke router that participated in the Multicast Group will not get the RP or Mapping Router information.

This is because there is a rule that every multicast packet that came from one physical interface, will not send back again to that interface. (Like IP Split Horizon rule in RIP). so, every packet from Mapping Agent (Router C) send to 224.0.0.40 and from RP-Candidate (Router D) send to 224.0.0.39 will not reach other spoke router(s), but can reach to Router D (forwarded from Hub Router A).

To solve this problem, Cisco give a solution, that is to break the rule above. Simply just put one command in the Multicast interface at the Hub (Router A) that connected to the spokes, ” ip pim nbma-mode “, so every packet from Mapping Agent Router or Candidate-RP Router send to Multicast Address 224.0.0.40 and 224.0.0.39 can reach the other spoke router(s).

There is another solution for this scenario, that is make a tunnel between Router B and Router C, so the multicast traffic from those routers can reach each other without need router A to forward. But to create tunnel need more effort than previous solution.

CMIIW….