Hi guys, Pls help me to explain a multicast problem: I have a topology like this: source(126.96.36.199)--R1---R2(RP)---R4---R5---R6---Receiver
I use GNS3 lab, PIM-SM, R2 is RP. So I think between R2--R4--R5--R6 just see the shared tree, but when i show ip mroute on R4, i see (S,G) !!! pls tell me why ! I have config ip pim spt-threshold infinity, but (S,G) is still here !!!!
The mroute output: R4#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, V - RD & Vector, v - Vector Outgoing interface flags: H - Hardware switched, A - Assert winner Timers: Uptime/Expires Interface state: Interface, Next-Hop or VCD, State/Mode
(*, 188.8.131.52), 00:00:58/stopped, RP 184.108.40.206, flags: S Incoming interface: GigabitEthernet2/0, RPF nbr 10.0.24.2 Outgoing interface list: GigabitEthernet1/0, Forward/Sparse, 00:00:51/00:02:38
*Sep 13 16:28:42.727: PIM(0): Building Periodic (*,G) Join / (S,G,RP-bit) Prune message for 220.127.116.11
*Sep 13 16:28:42.731: PIM(0): Insert (*,18.104.22.168) join in nbr 10.0.56.5's queue
*Sep 13 16:28:42.739: PIM(0): Building Join/Prune packet for nbr 10.0.56.5
*Sep 13 16:28:42.739: PIM(0): Adding v2 (22.214.171.124/32, 126.96.36.199), WC-bit, RPT-bit, S-bit Join
*Sep 13 16:28:42.739: PIM(0): Send v2 join/prune to 10.0.56.5 (GigabitEthernet2/0)
You can see? the (S,G) is add from R5 !!!, R5 is not the edge router, it's just the normal transit router. Do u know why it do that? how it know about the source. From my knowledge, R5 just know about RP.
You should see a S,G entry because it indicates the presence of multicast in the forwarding plane i.e. if everything is on the shared tree you will still see S,G because it is indicating that traffic is actually flowing from a multicast sender. If there are no receivers you will still see S,G from the source to the RP but not beyond. You will also see S,G on routers between the receiver router and the RP because it indicates that traffic is flowing. You need to understand that S,G results from traffic flowing and NOT from someone doing a join. A join results in a *,G entry being created or an existing one modified (e.g. an interface added to the OIL)