Multicast confusion !

Answered Question
Nov 16th, 2009
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Hi all, i am talking about just pim dense mode. I have 1 router with source and receiver directly connected. I know i have to enable multicast routing and pim dense mode on receiver attached interface. But if i dont enable pim dense mode on source attached interface why wont it work ? when i dont enable pim on source related interface, multicast traffic is shown as unroutable on router. For pim to calculate the incoming source, i dont see anywhere there is a requirement that pim should be running on that incoming interface as well. I have seen this in williamson beau book as well. In my case when router knows that traffic is being sourced from on of its directly connected interface then why doesnt it calculates RPF accordingly ?


Kindly guide me

Correct Answer by Peter Paluch about 7 years 6 months ago

Hello,


Having the PIM enabled on an interface actually designates that interface eligible to transport multicast - in both ways, that is, both inbound and outbound traffic. In addition, it also activates the PIM protocol on that interface.


You should be better asking - 'why do I need to activate PIM on the destination interface?' Activating PIM on the source interface is kind of logical, as you should not be having any assumptions about where the multicast source is - it might be right on that interface, but it also might be several routers away behind the source interface. In such case, you would absolutely need to activate the PIM, otherwise you would not be able to send Joins/Prunes to the RPF neighbors to make them send that traffic to you (of course, the DM has its exceptions here).


The fact is that activating PIM on an interface activates the entire multicast support on that interface, including IGMP, the PIM itself and the ability of that interface to both send and receive multicast traffic and participate in multicast routing table.


Best regards,

Peter


Correct Answer by Mohamed Sobair about 7 years 6 months ago


Hi,


The simplest answer would be that PIM is the routing protocol for Multicast, The RPF in multicast is calculated using the unicast routing table, but how? its determined through PIM protocol and not the unicast routing table it self. Thats why is the need for PIM. and since the router it self doesnt forward broadcast, each interface is a broadcast domain, PIM multicast routing is needed.


The second Point, is that routers dont route Multicast traffic directly , I mean the Unicast routing table doesnt have the capability to forward multicast traffic.



HTH

Mohamed

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Correct Answer
Mohamed Sobair Mon, 11/16/2009 - 01:30
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Hi,


The simplest answer would be that PIM is the routing protocol for Multicast, The RPF in multicast is calculated using the unicast routing table, but how? its determined through PIM protocol and not the unicast routing table it self. Thats why is the need for PIM. and since the router it self doesnt forward broadcast, each interface is a broadcast domain, PIM multicast routing is needed.


The second Point, is that routers dont route Multicast traffic directly , I mean the Unicast routing table doesnt have the capability to forward multicast traffic.



HTH

Mohamed

Peter Paluch Mon, 11/16/2009 - 02:17
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  • Cisco Employee,

Hello Mohamed,


You wrote: The RPF in multicast is calculated using the unicast routing table, but how? its determined through PIM protocol and not the unicast routing table it self


I do not agree with that. The PIM is more of a signalling protocol than a routing protocol by itself. The PIM does not discover RPF neighbors, on the contrary, it relies on them being already identifiable in the unicast routing table.


Best regards,

Peter


Correct Answer
Peter Paluch Mon, 11/16/2009 - 02:11
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  • Cisco Employee,

Hello,


Having the PIM enabled on an interface actually designates that interface eligible to transport multicast - in both ways, that is, both inbound and outbound traffic. In addition, it also activates the PIM protocol on that interface.


You should be better asking - 'why do I need to activate PIM on the destination interface?' Activating PIM on the source interface is kind of logical, as you should not be having any assumptions about where the multicast source is - it might be right on that interface, but it also might be several routers away behind the source interface. In such case, you would absolutely need to activate the PIM, otherwise you would not be able to send Joins/Prunes to the RPF neighbors to make them send that traffic to you (of course, the DM has its exceptions here).


The fact is that activating PIM on an interface activates the entire multicast support on that interface, including IGMP, the PIM itself and the ability of that interface to both send and receive multicast traffic and participate in multicast routing table.


Best regards,

Peter


Mohamed Sobair Mon, 11/16/2009 - 02:27
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Hi Peter,


Sentence Mistake, you are correct.


I meant PIM and RPF both relies on the unicast routing table for thier calculation.



HTH

Mohame


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