This lab is set up for Source Specific Multicast (SSM) for this to work we need to enable our PIM interfaces for IGMPv3 as this supports source filtering which is fundamental for SSM. The IANA have reserved the 232.0.0.0/8 for SSM. With SSM we don’t require a RP.
SSM is enabled on your router with the command “ip pim ssm default” you can specify a range but if you choose default then the 232.0.0.0/8 range will apply. Then on each interface participating in PIM we use the command under interface configuration “ip igmp version 3”
SSM Topology
Attached are the config’s for each router (Router Configs). As I’m using GNS3 I had to enter commands “no ip route-cache” and “no ip mroute-cache” on each PIM interface. Also add “no ip cef”. You won’t need this for physical routers.
We can verify the IGMP groups from the DR to do this we use the command “show ip igmp membership x.x.x.x” on R3, in our case we can see SSM entries for 232.1.1.1
Channel/Group Reporter Uptime Exp. Flags Interface
/*,232.1.1.1 10.10.10.4 03:16:22 stop 3MA Fa0/0
15.15.15.2,232.1.1.1 03:16:22 02:44 RA Fa0/0
56.56.56.1,232.1.1.1 03:16:22 02:44 RA Fa0/0
To verify our SSM is working correctly let’s generate some traffic from the specific source in the above output. I will source traffic from 15.15.15.2 to 232.1.1.1 and we should see some replies.
R2#ping 232.1.1.1 source 15.15.15.2 rep 3
Type escape sequence to abort.
Sending 3, 100-byte ICMP Echos to 232.1.1.1, timeout is 2 seconds:
Packet sent with a source address of 15.15.15.2
Reply to request 0 from 10.10.10.4, 32 ms
Reply to request 1 from 10.10.10.4, 80 ms
Reply to request 2 from 10.10.10.4, 68 ms
Now let’s source the traffic from the same router but a different interface source address and see what happens………………… no replies.
R2#ping 232.1.1.1 source 56.56.56.2 rep 3
Type escape sequence to abort.
Sending 3, 100-byte ICMP Echos to 232.1.1.1, timeout is 2 seconds:
Packet sent with a source address of 56.56.56.2
…
Running a packet capture on the DR router shows the IGMPv3 membership report / Join Group from R4. This is sent to the multicast address of 224.0.0.22. This lists the group record for our multicast address and the sources from which we want to receive.
PCAP IGMPv3
Some other points to mention, when we use source specific multicast we can re-use an existing address an example would be as above the group 232.1.1.1 is used twice as it’s from a different source this is sometimes referred to as channels. For example (15.15.15.2 , 232.1.1.1) is a different SSM “channel” from (56.56.56.1 , 232.1.1.1).
With SSM the last hop router will continue to send (S , G) join messages if they still exist on the receivers side, therefore the shortest path tree (SPT) from host to source will be maintained even if the source is no longer sending traffic.
RH
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