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)Step 2: Enable PIM sparse-mode on the interfaces:
Step 3: Enable SSM on all routers
Step 4: Enable IGMPV3 on source and receiverStep 5: Configure R3 to join a multicast group address in the 18.104.22.168/8 range and we’ll specify R1 as the source:
Now let us check the multicast routing table at R2
R2#sh ip mroute
In the output above you see how R2 has built a SPT from source to receiver.
R3#sh ip igmp groups 22.214.171.124 detail