Multicast Series – Part I (ASM)

Today, we are going to discuss in depth about the Native Multicast flow.

Below mentioend are the details, which is covered in this section.

1) What will happen when Receiver is active.
2) What will happen when Source is active.
3) What will happen when Register receive at RP.
4) How (S,G) formed. Till FHR.
5) Which path it takes for the first Multicast Stream.
6) What will happen when two stream receive at the LHR.
7) How STP formed over Shared tree. Exactly what happen. Why the switchover takes place.

Reference Cisco Link: Native Multicast Flow – Any-Source Multicast Model

STEP 1 : When Receiver is active. It sends IGMP Query Message 


Step 1 :  On receiving the receiver’s expression of interest, the DR then sends a PIM Join message towards the RP for that multicast group.

This Join message is known as a (*,G) Join because it joins group G for all sources to that group.

The (*,G) Join travels hop-by-hop towards the RP for the group, and in each router it passes through, multicast tree state for group G is instantiated.

LHR#sh ip igmp groups
IGMP Connected Group Membership
Group Address    Interface                Uptime    Expires   Last Reporter   Group Accounted        GigabitEthernet1/0       00:37:30  00:02:02       FastEthernet0/0          01:21:01  00:02:43

LHR#sh ip mroute

(*,, 00:00:29/00:02:30, RP, flags: SJCL
Incoming interface: GigabitEthernet0/0/1, RPF nbr
Outgoing interface list:
GigabitEthernet0/0/0, Forward/Sparse

C flag in the (*,G) entry which means that is has directly connected receiver.

RP #sh ip mroute
(*,, 00:10:39/00:02:39, RP, flags: S
Incoming interface: Null, RPF nbr
Outgoing interface list:
FastEthernet0/0, Forward/Sparse
S Flag Sparse mode created.
(*,, 01:56:40/00:02:58, RP, flags: SJPCL
Incoming interface: FastEthernet0/0, RPF nbr
Outgoing interface list: Null(*,, 01:56:40/00:02:58, RP, flags: SJPCL
Incoming interface: FastEthernet0/0, RPF nbr
Outgoing interface list: Null

The value of “” means self, and it appears in the output if the router is the RP itself

There is a single (*,G) entry for the group which is Auto-RP Discovery group address.

NOTE : To prevent a stale PIM-SM forwarding state from getting stuck in the routers, it is given a finite lifetime (3 minutes), after which it is deleted.

Routers refresh shared trees by periodically (once a minute) sending (*, G) Joins to the upstream neighbor in the direction of the RP.


STEP 2 : When Source is active


If no active receiver present at RP, then RP sends REGISTER STOP DR will be silent for default 60 seconds may result in the so-called “join latency” where a newly Joined listener may have to way for almost a minute before it can discover a multicast source. This is why in many practical deployments with dynamic listeners you see PIM SSM being used in favor of complicated PIM SM mechanics.


RP #

(,, 00:00:43/00:02:16, flags: P
Incoming interface: FastEthernet0/0, RPF nbr
Outgoing interface list: Null

Prune Flag (P) is set as no active receiver (*,G) entry present in RP.


STEP 3 : Forming Shared Tree.



STEP 4 : (S,G) packet reaching towards the FHR.




STEP 5 : First Stream of Multicast Packet, reach to Receiver via Shared Tree.






STEP 6 : After receipt of two stream of Multicast Traffic, the LHR starts receving the traffic from SPT tree and sends prune message towards the Shared tree.




Categories: Multicast

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