Source Specific Multicast

Rashmi Bhardwaj | Blog,Routing & Switching

The multicast we usually hear is of 2 types, “Any Source Multicast” and “Source Specific Multicast”. Any source multicast has 2 flavors namely PIM Sparse mode and PIM Dense mode about which we have already discussed in out multicast basics post. Both the flavors of ASM use the IGMPV2. In ASM the receivers don’t know about the source initially and require a rendezvous point in the network to get to know about the active source.
Source Specific Multicast (SSM) uses IGMPV3 and in this the receivers don’t need a Rendezvous point as they know about the source specifically. In SSM there are no shared trees and only shortest path trees are built towards the source.

Let us take a simple topology below to configure the SSM:

R1 acts as the source and R3 acts as the receiver –


Step 1: Enabling the Multicast routing on all routers along with an IGP (OSPF)

ip multicast-routing
router ospf 1
network area 0
Step 2: Enable PIM sparse-mode on the interfaces:

R1(config)#interface fastEthernet 0/0
R1(config-if)#ip pim sparse-modeR2(config)#interface fastEthernet 0/0
R2(config-if)#ip pim sparse-mode
R2(config-if)#interface fastEthernet 1/0
R2(config-if)#ip pim sparse-mode

R3(config)#interface fastEthernet 0/0
R3(config-if)#ip pim sparse-mode

Step 3: Enable SSM on all routers

R1(config)#ip pim ssm default
R2(config)#ip pim ssm default
R3(config)#ip pim ssm default

Step 4: Enable IGMPV3 on source and receiver

R1(config)#interface fastEthernet 0/0
R1(config-if)#ip igmp version 3

R3(config)#interface fastEthernet 0/0
R3(config-if)#ip igmp version 3

Step 5: Configure R3 to join a multicast group address in the range and we’ll specify R1 as the source:
R3(config)#interface fastEthernet 0/0
R3(config-if)#ip igmp join-group source

Now let us check the multicast routing table at R2

R2#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
Timers: Uptime/Expires
Interface state: Interface, Next-Hop or VCD, State/Mode

(,, 00:00:06/00:03:24, flags: sT
Incoming interface: FastEthernet0/0, RPF nbr
Outgoing interface list:
FastEthernet1/0, Forward/Sparse, 00:00:06/00:03:24

(*,, 00:02:47/00:02:58, RP, flags: DCL
Incoming interface: Null, RPF nbr
Outgoing interface list:
FastEthernet1/0, Forward/Sparse, 00:02:30/00:00:29
FastEthernet0/0, Forward/Sparse, 00:02:44/00:02:58

In the output above you see how R2 has built a SPT from source to receiver.

R3#sh ip igmp groups detail

Flags: L – Local, U – User, SG – Static Group, VG – Virtual Group,
SS – Static Source, VS – Virtual Source,
Ac – Group accounted towards access control limit

Interface: FastEthernet0/0
Flags: L SSM
Uptime: 00:03:29
Group mode: INCLUDE
Last reporter:
Group source list: (C – Cisco Src Report, U – URD, R – Remote, S – Static,
V – Virtual, M – SSM Mapping, L – Local,
Ac – Channel accounted towards access control limit)
Source Address  Uptime     v3 Exp     CSR         Exp  Fwd Flags                00:01:48 00:02:39 stopped    Yes   RL        00:03:29 00:02:39 stopped    Yes  RL

Related- Multicast Address


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