multicast split-horizon??

Can someone explain this, please.

         R5
          |
          |
 R1 ------R3------R2

From what i understood reading multicast documentation, the multicast traffic is never sent back out the interface it was received on, similarly to unicast protocols.
But in the example above, let's say that R1 is source of traffic, R2 has ip igmp join-group a.b.c.d configured on loopback, and R5's loopback is configured as static RP for group.
The traffic will flow up to R5, but R5 will send it back down to R2, even if you disable R2 to switchover to SPT.
To add to the confusion, this wouldn't work on NMBA aka Frame relay multipoint interface - where i need to configure ip pim nmba-mode to allow R5 to send it down the same interface.

SO does that mean that split-horizon is only for multipoint nmba interface or what??

Thanks in advance

Find more posts tagged with

Save $250 on 2025 certification boot camps from Infosec!

Book now with code EOY2025

Button

Comments

networker050184

The multicast from the source to the RP won't actually be sent as multicast packets. They will be encapsulated into unicast register packets and the RP will decapsulate and forward on to the receiver forming the shared tree from there. The receiver will see the first packet that was sent from the RP and read the source address and form its source tree.

So, technically, the actual multicast stream does not flow in one interface and out the same.

acidsatyr

yes, but, what if the destination never wants to switch to shortest path tree? The traffic must flow up to r5 and come down.

networker050184

acidsatyr wrote: »

yes, but, what if the destination never wants to switch to shortest path tree? The traffic must flow up to r5 and come down.

The traffic will continue to flow to the RP in unicast packets and decapsulated and sent on its way. So no matter if the tree switches or not the multicast stream won't come in and go out the same interface. Another thing to keep in mind is that the tree from the RP to the source and the shared tree from the RP out will be two different trees and perform different RPF checks.

It starts to get confusing, and I am no multicast expert, so feel free to double check and post back if I'm wrong. The best way I've seen it explained is from Routing TCP/IP Vol II.