The multicast that you are probably familiar with (PIM sparse and dense mode) using IGMPv2 are also known as ASM (Any Source Multicast). This means that the receivers really don’t care what source they receive multicast traffic from, all sources are accepted.
Using sparse mode our receivers require the RP (Rendezvous Point) to discover new sources in the network. SSM (Source Specific Multicast) requires IGMPv3 and lets us join multicast groups from specified source addresses.
Besides just joining any group, the receiver is able to receive the group from a specified source. When we use SSM / IGMPv3 there are no shared trees. We only build SPTs (Shortest Path Trees) towards our sources. This also means that we don’t use any RPs …you won’t need Auto-RP or Bootstrap anymore!
Configuring source specific multicast is pretty easy, let’s use the following topology to take a look at the configuration:
We have a very small network with only 3 routers. Let’s prepare them by enabling OSPF and multicast routing:
R1,R2 and R3:
ip multicast-routing
router ospf 1
network 0.0.0.0 255.255.255.255 area 0Let’s 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 0/1
R2(config-if)#ip pim sparse-modeR3(config)#interface fastEthernet 0/0
R3(config-if)#ip pim sparse-modeNow we can dive int the configuration of SSM. First we’ll enable SSM on all routers. If you do not specify a group then the 232.0.0.0/8 range will be used. If you do try a (*,G) join for any group in the SSM range then it will be dropped.
R1,R2 and R3:
ip pim ssm defaultNext step is to enable IGMPv3 on the source and receiver:
R1(config)#interface fastEthernet 0/0
R1(config-if)#ip igmp version 3R3(config)#interface fastEthernet 0/0
R3(config-if)#ip igmp version 3Now we’ll configure R3 to join a multicast group address in the 232.0.0.0/8 range and we’ll specify R1 as the source:
Hi Rene,
I am a beginner and interested in learning IGMP and PIM protocols . Can you please point me the links to start these protocols?
Thanks,
Srini
Rene,
Can we enable SSM with Dense-mode?
Regards,
Ulrich
Hello DJAN
When configuring SSM, you can use either sparse mode or sparse-dense mode. More information on configuring SSM can be found here:
https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipmulti_pim/configuration/xe-16/imc-pim-xe-16-book/imc-ssm.html
I hope this has been helpful!
Laz
Hi Laz,
This pdf link is no longer valid. Do you have a copy of the document elsewhere or can you summarize the purpose of the pdf?
Thank you,
Jay Karson
Hello Sean
If you use the command
... Continue reading in our forumip ssm default, as in the lesson, then the 232.0.0.0/8 range is used by default. Alternatively, you can use theip ssm range ACL_NAMEwhere the the ACL_NAME is the name of the access list that defines the multicast address range you want to define. This way, you can use a range different from 232.0.0.0/8. In both cases, if you try to join a group outside of the specified range (either default, or otherwise specified) it will be dropped.