I thought i would write about the multicast 'shared tree' and how it is built. Once understood, I feel this has certainly helped me with multicast and troubleshooting along the way.
This is a two stage process: the server 'registers' to the RP and the client 'joins' the RP. These 2 processes are independent of each other and is the same regardless of the underlying PIM RP selection protocol e.g. Auto rp, static rp, or BSR.
In this example i use multicast servers attached to R6 and clients attached to R4.
The RP is 150.1.5.5 on R5
First the registration process.
A server at 204.12.1.254 starts to send to a multicast address multicast address 224.4.4.4. The PIM interface on the local LAN segment receives the multicast packet and sends a PIM 'register' to the RP, 150.1.5.5 in this example. This message is actually encapsulated as a unicast message to 150.1.5.5.
When the RP receives this register message it acknowledges receipt
The output of the debug ip pim on R6 (local PIM interface) and R5 (the RP) shows this...
R6#
*May 21 06:35:16.647: PIM(0): Check RP 150.1.5.5 into the (*, 224.4.4.4) entry
*May 21 06:35:16.655: PIM(0): Send v2 Register to 150.1.5.5 for 204.12.1.254, gr
oup 224.4.4.4
*May 21 06:35:17.143: PIM(0): Received v2 Register-Stop on FastEthernet0/0 from
150.1.5.5
*May 21 06:35:17.147: PIM(0): for source 204.12.1.254, group 224.4.4.4
*May 21 06:35:17.147: PIM(0): Clear Registering flag to 150.1.5.5 for (204.12.1.
254/32, 224.4.4.4)
R5#
*May 21 06:35:16.483: PIM(0): Received v2 Register on Serial2/0 from 192.10.1.6
*May 21 06:35:16.487: for 204.12.1.254, group 224.4.4.4
*May 21 06:35:16.491: PIM(0): Check RP 150.1.5.5 into the (*, 224.4.4.4) entry
*May 21 06:35:16.495: PIM(0): Send v2 Register-Stop to 192.10.1.6 for 204.12.1.2
54, group 224.4.4.4
Following this registration process entries are placed in the mroute table on R6 and R5, but not any intervening routers in the unicast path between R6 and R5.
On R6 2 mroute entries are created: the (*,G) entry and the (S,G) entry. Both entries at this stage have a null output interface as no client has yet registered for this multicast feed. The (S,G) entry denotes that a server is sending to the multicast group.
R6#s ip mroute 224.4.4.4
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
(*, 224.4.4.4), 00:11:12/stopped, RP 150.1.5.5, flags: SPF
Incoming interface: FastEthernet0/0, RPF nbr 192.10.1.1
Outgoing interface list: Null
(204.12.1.254, 224.4.4.4), 00:11:12/00:02:51, flags: PFT
Incoming interface: FastEthernet1/0, RPF nbr 0.0.0.0
Outgoing interface list: Null
On R5, the RP, a similar 2 entries are created.
R5#s ip mroute 224.4.4.4
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
(*, 224.4.4.4), 00:10:16/stopped, RP 150.1.5.5, flags: SP
Incoming interface: Null, RPF nbr 0.0.0.0
Outgoing interface list: Null
(204.12.1.254, 224.4.4.4), 00:10:16/00:02:47, flags: P
Incoming interface: Tunnel53, RPF nbr 154.1.0.3, Mroute
Outgoing interface list: Null
This completes the register process.
Second the client 'join' process.
A mcast client is connected to R4 and sends an IGMP join for the multicast group 224.4.5.6. Upon receipt of the IGMP join R4 sends a PIM join message towards the RP (R5).
Debug output on R5 shows receipt of the join message.
R5#
*May 21 06:59:48.519: PIM(0): Received v2 Join/Prune on Tunnel53 from 154.1.0.3,
to us
*May 21 06:59:48.523: PIM(0): Join-list: (*, 224.4.5.6), RPT-bit set, WC-bit set
, S-bit set
*May 21 06:59:48.527: PIM(0): Add Tunnel53/154.1.0.3 to (*, 224.4.5.6), Forward
state, by PIM *G Join
On the RP (R5) a (*,G) entry is created in the mroute table
R5#s ip mroute 224.4.5.6
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
(*, 224.4.5.6), 00:02:47/00:02:58, RP 150.1.5.5, flags: SJC
Incoming interface: Null, RPF nbr 0.0.0.0
Outgoing interface list:
Tunnel53, Forward/Sparse-Dense, 00:01:48/00:02:40
FastEthernet1/0, Forward/Sparse-Dense, 00:02:47/00:02:58
Notice this time the entry has a populated outgoing interface. With the join process all PIM enabled routers in the path to the RP also build such an entry in their mroute table.
This completes the join process.
Tying 'register' and 'join' together
The RP ties the join and register processes together. I initiate a server multicast feed to the multicast address 224.4.5.6.
I start a ping to 224.4.5.6
ping 224.4.5.6 repeat 10000
Reply to request 23 from 154.1.0.4, 552 ms
Reply to request 24 from 154.1.0.4, 676 ms
Reply to request 25 from 154.1.0.4, 788 ms
Reply to request 26 from 154.1.0.4, 964 ms
Reply to request 27 from 154.1.0.4, 824 ms
This will again initiate a new register from the local PIM interface to the RP for this multicast group.
I examine the mroute table on the RP with this multicast ping in process.
R5#s ip mroute 224.4.5.6
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
(*, 224.4.5.6), 00:03:55/00:03:29, RP 150.1.5.5, flags: SJC
Incoming interface: Null, RPF nbr 0.0.0.0
Outgoing interface list:
Tunnel53, Forward/Sparse-Dense, 00:02:56/00:03:29
FastEthernet1/0, Forward/Sparse-Dense, 00:03:55/00:02:50
(204.12.1.254, 224.4.5.6), 00:00:22/00:02:59, flags: T
Incoming interface: Tunnel53, RPF nbr 154.1.0.3, Mroute
Outgoing interface list:
FastEthernet1/0, Forward/Sparse-Dense, 00:00:22/00:02:50
As before the RP contains both the (*,G) and (S,G) entries. Since a client has registered for this feed both entries also contain an interface in the outgoing interface list (OIL).
I examine the mroute table on R6 (the PIM router connected to the multicast server). Similarly there are 2 entries for the multicats group 224.4.5.6. Note the (S,G) entry has an OIL entry, however the (*,G) entry does not.
Only those routers in the path between the multicast client an the RP will have a populated OIL for this entry.
R6#s ip mroute 224.4.5.6
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
(*, 224.4.5.6), 00:03:13/stopped, RP 150.1.5.5, flags: SPF
Incoming interface: FastEthernet0/0, RPF nbr 192.10.1.1
Outgoing interface list: Null
(204.12.1.254, 224.4.5.6), 00:03:13/00:03:25, flags: FT
Incoming interface: FastEthernet1/0, RPF nbr 0.0.0.0
Outgoing interface list:
FastEthernet0/0, Forward/Sparse-Dense, 00:03:12/00:03:16
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