The rules of multicast
The first rule of multicast is you don't talk about multicast
Most networks don't do multicast routing, which means most network guys don't
have much experience with it. Sure they know that it exists, and it's probably
used on their layer 2, but they don't do multicast routing. These "rules" list
some things that you should know when configuring or troubleshooting multicast.
The second rules of multicast is you do not forward packets coming
from a non-RPF interface
If a multicast packet is received, and it's not the multicast RPF interface,
it's dropped. The table can be viewed with "show ip mroute", and it will clearly
show what interface is acceptable. Here's an example of group 239.0.0.1:
R1#show ip mroute 239.0.0.1
[...]
(*, 239.0.0.1), 00:00:08/stopped, RP 1.0.0.1, flags: S
Incoming interface: Null, RPF nbr 0.0.0.0
Outgoing interface list:
FastEthernet1/0, Forward/S...
