We can verify it as well on R5 as below that it has joined the group 188.8.131.52 as below:
R5#sh ip igmp groups
IGMP Connected Group Membership Group Address Interface Uptim ExpiresLast Reporter Group Accounted
184.108.40.206 Loopback0 00:14:43 00:02:31 220.127.116.11
18.104.22.168 Loopback0 00:14:39 00:02:25 22.214.171.124
We see router has also joined group 126.96.36.199 which is a default multicast group that each router joins to listen to RP discovery messages advertised by mapping agent.Mapping agent on other hand receives the announcement for RP-candidates on group address 188.8.131.52.
Configure the RP as below using Cisco Proprietary Auto-RP which enables the RP to be discovered dynamically.
(Config)# ip pim send-rp-announce Loopback0 scope 32 >>>>>>To announce itself as RP candidate>
(Config)# ip pim send-rp-discovery Loopback0 scope 32 >>>>>>>Configures the router to be an RP mapping agent.>
As soon as Router R5 joins group 184.108.40.206 it sends IGMPV2 join message.
*Dec 23 01:54:28.923: IGMP(0): Send v2 Report for 220.127.116.11 on Loopback0*
Dec 23 01:54:28.923: IGMP(0): Received v2 Report on Loopback0 from 18.104.22.168 for 22.214.171.124
This JOIN message is then forwarded towards the RP by each router and a (*,G) entry gets created on each router.
R1#sh 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, E – Extranet,
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,
G – Received BGP C-Mroute, g – Sent BGP C-Mroute,
Q – Received BGP S-A Route, q – Sent BGP S-A Route,
V – RD & Vector, v – Vector
Outgoing interface flags: H – Hardware switched, A – Assert winner
Interface state: Interface, Next-Hop or VCD, State/Mode
(*, 126.96.36.199), 00:00:36/00:03:19, RP 188.8.131.52, flags: S
Once the Source information is received on the RP in REGISTER MESSAGE from first hop router, the RP sends a JOIN message towards the Source now and builds the Shortest path tree and then RP sends a REGISTER STOP message towards the source to complete registration process.
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