multicast and unicast comparison

[zillah2004]

Cisco says:

"The reason for turning on multicast on every interface is that IP multicast routing is upside-down routing that forwards (or does not forward) multicast packets based on where the packets came from (source IP address) as opposed to where they are going (multicast group address). Consequently, the control mechanism for multicast traffic paths is not quite the same as the control mechanism for unicast routing".

I am looking to understand the concept of :

"upside-down routing that forwards (or does not forward) multicast packets based on where the packets came from (source IP address) as opposed to where they are going (multicast group address):

[bhedlund]

This concept is also known as Reverse Path Forwarding.

Basically, the router will forward the multicast away from the source ... out of all interfaces except from the one in which the packet arrived.

Attached is good document to help grasp the various concepts in multicasting.

HTH

Please rate the post.

Thanks,

Brad