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Multicast questions


I have a couple of multicast questions.

1. Does the first hop to the mc source have to be the RP?

2. Can you have multiple sources send to the same mc group

6 Replies 6

Rising star
Rising star


1. Does the first hop to the mc source have to be the RP?

No it doesn't. The RP can be anywhere in the network, although, somewhere in the center is prefereble. The first hop router however needs to know the address of the RP, either via static config, or by auto-rp/bootstrap-rp

2. Can you have multiple sources send to the same mc group

Yes, no problem with that.



What @mlund is correct, but some additional information. . .

#1 As Mikael notes, often the RP is somewhere near/at the center of your topology, but first, your MC topology might not strictly align with your unicast topology, and if it doesn't, your RP would perhaps be better near/at the center of your MC topology.  Second, since an RP, at least initially, creates a shared MC distribution tree, your might have other reason for moving your RP location toward the near/at the center of your heavy MC usage.  (BTW, by default [or at least it used to be the default], any one MC flow can switch over from the shared RP distribution tree to a shortest path tree from source to client[s].)

#2 At Mikael also correctly notes, yes you can, but keep in mind all clients of a MC group will then receive all the traffic from all the sources sending to that group.  Depending on your applications, this might generate a lot of needless traffic between source(s) and client(s).  (BTW, as multicast group will be the same, clients that don't want a particular source's traffic will have their NIC physically accept the unwanted multicast traffic, and then pass it up to the application for the application to discard.)

(Also BTW, even when IP MC groups don't overlap, Ethernet MC can overlap such traffic.  In that case, again a client's NIC will physically accept the packet, but the IP layer, on the client, will discard the packet.  [Moral of this story, try to avoid needless overlapping usage of IP MC IPs and/or different MC IPs that map to the same Ethernet MC MAC.])

Thank you very much for your reply.  So if I had 3 mc sources each connected to 3x different ToR switches,  which were connected via L3 links to a core switch the core switch could be the RP, and each server on each different ToR switch could send to the same group?

As previously answered in our (Mikael's and my) prior posts, the answer is "yes", i.e. multiple MC servers can transmit to the same MC group, concurrently.

understood, I was simply looking for clarification that the sources did not need to be behind the same L3 device

Ok, for clarification - they don't need to be behind same L3 device.

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