The third question was:
* If the host is sending data for group 239.0.1.23 (and sparse-mode is being used), which router (A or B) is going to forward the stream to the RP?
Answer: If you take a look back at the answer to Multicast Question #1, the host will send an IGMP membership report using the destination address of the group (in this case, 239.0.1.23). If both router A and B are listening to these IGMP messages, they will both receive the same membership report. It is then the job of the router (A or B) to send the PIM Register message to the RP. Assuming that there are hosts out there somewhere who want to receive the stream, the RP will accept the PIM Register message and the stream will head toward the RP. Of course, the question was, WHICH router?! The answer is the same as it was in Question #1. The PIM DR for the segment will be the forwarder.
After firing up GNS3, we run a test:
R1#show ip mroute
IP Multicast Routing Table
Flags: D - Dense, S - Sparse, B - Bidir Group, s - SSM Group, C - Connected,
L - Local, P - Pruned, R - RP-bit set, F - Register flag,
T - SPT-bit set, J - Join SPT, M - MSDP created entry,
X - Proxy Join Timer Running, A - Candidate for MSDP Advertisement,
U - URD, I - Received Source Specific Host Report,
Z - Multicast Tunnel, z - MDT-data group sender,
Y - Joined MDT-data group, y - Sending to MDT-data group
Outgoing interface flags: H - Hardware switched, A - Assert winner
Timers: Uptime/Expires
Interface state: Interface, Next-Hop or VCD, State/Mode
(*, 239.0.1.23), 00:03:23/stopped, RP 192.168.0.1, flags: SJPCF
Incoming interface: FastEthernet0/1, RPF nbr 192.168.0.1
Outgoing interface list: Null
(10.0.1.110, 239.0.1.23), 00:03:23/00:02:57, flags: FT
Incoming interface: FastEthernet0/0, RPF nbr 0.0.0.0
Outgoing interface list:
FastEthernet0/1, Forward/Sparse, 00:00:07/00:02:52
Notice the flags that are set on the (*,G) entry. The ‘C’ flag is set which indicates that there is a device connected to the router which belongs to the group. In the (S,G) entry the RPF nbr of 0.0.0.0 means that it is connected to the sender. It looks like our PIM DR theory proved true!