Clarification on Source Specific Multicast functionality

Answered Question
Sep 12th, 2009

Hi, I use IGMPv3 SSM in my environment and my understanding is that IGMPv3 has no concept of RP - OK. That said, when I do 'show ip mroute', I see (S,G) information.


If I see (S,G) I identify this is as a source-tree, correct? So it is right to say that on SSM we will always see a SPT(Shortest Path Tree) or Source Tree, never a Shared Tree (*,G), right?


It is a little bit confusing to me that althought SSM is Sparse Mode, we will use a Source Tree and never Shared Tree...


RFC 3569

"...

C) Handling of well-known sources: SSM requires only

source-based forwarding trees; this eliminates the need for a

shared tree infrastructure. This implies that neither the

RP-based shared tree infrastructure of PIM-SM nor the MSDP

protocol is required. Thus the complexity of the multicast

routing infrastructure for SSM is low, making it viable for

immediate deployment. Note that there is no difference in how

MBGP is used for ASM and SSM."


Correct Answer by Edison Ortiz about 7 years 5 months ago

If I see (S,G) I identify this is as a source-tree, correct?


Correct.


So it is right to say that on SSM we will always see a SPT(Shortest Path Tree) or Source Tree, never a Shared Tree (*,G), right?


If you have non-IGMPv3 multicast clients, you may find *,G entries in your mroute table.


It is a little bit confusing to me that althought SSM is Sparse Mode, we will use a Source Tree and never Shared Tree...


You need to enable PIM for multicast on a router interface and the option are either Sparse or Dense while we've come to understand Sparse as a Shared Tree mechanism, SSM uses PIM-SM for multicast dissemination in the network. PIM-SM will also be used for clients without v3 support.


I believe this document will be helpful for you:


http://www.cisco.com/en/US/docs/ios/ipmulti/configuration/guide/imc_cfg_ssm_ps6350_TSD_Products_Configuration_Guide_Chapter.html


ISM service is described in RFC 1112. This service consists of the delivery of IP datagrams from any source to a group of receivers called the multicast host group. The datagram traffic for the multicast host group consists of datagrams with an arbitrary IP unicast source address S and the multicast group address G as the IP destination address. Systems will receive this traffic by becoming members of the host group. Membership to a host group simply requires signalling the host group through IGMP Version 1, 2, or 3.


In SSM, delivery of datagrams is based on (S, G) channels. Traffic for one (S, G) channel consists of datagrams with an IP unicast source address S and the multicast group address G as the IP destination address. Systems will receive this traffic by becoming members of the (S, G) channel. In both SSM and ISM, no signalling is required to become a source. However, in SSM, receivers must subscribe or unsubscribe to (S, G) channels to receive or not receive traffic from specific sources. In other words, receivers can receive traffic only from (S, G) channels to which they are subscribed, whereas in ISM, receivers need not know the IP addresses of sources from which they receive their traffic. The proposed standard approach for channel subscription signalling utilizes IGMP INCLUDE mode membership reports, which are supported only in IGMP Version 3.


HTH,


__


Edison.


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Correct Answer
Edison Ortiz Sat, 09/12/2009 - 13:21

If I see (S,G) I identify this is as a source-tree, correct?


Correct.


So it is right to say that on SSM we will always see a SPT(Shortest Path Tree) or Source Tree, never a Shared Tree (*,G), right?


If you have non-IGMPv3 multicast clients, you may find *,G entries in your mroute table.


It is a little bit confusing to me that althought SSM is Sparse Mode, we will use a Source Tree and never Shared Tree...


You need to enable PIM for multicast on a router interface and the option are either Sparse or Dense while we've come to understand Sparse as a Shared Tree mechanism, SSM uses PIM-SM for multicast dissemination in the network. PIM-SM will also be used for clients without v3 support.


I believe this document will be helpful for you:


http://www.cisco.com/en/US/docs/ios/ipmulti/configuration/guide/imc_cfg_ssm_ps6350_TSD_Products_Configuration_Guide_Chapter.html


ISM service is described in RFC 1112. This service consists of the delivery of IP datagrams from any source to a group of receivers called the multicast host group. The datagram traffic for the multicast host group consists of datagrams with an arbitrary IP unicast source address S and the multicast group address G as the IP destination address. Systems will receive this traffic by becoming members of the host group. Membership to a host group simply requires signalling the host group through IGMP Version 1, 2, or 3.


In SSM, delivery of datagrams is based on (S, G) channels. Traffic for one (S, G) channel consists of datagrams with an IP unicast source address S and the multicast group address G as the IP destination address. Systems will receive this traffic by becoming members of the (S, G) channel. In both SSM and ISM, no signalling is required to become a source. However, in SSM, receivers must subscribe or unsubscribe to (S, G) channels to receive or not receive traffic from specific sources. In other words, receivers can receive traffic only from (S, G) channels to which they are subscribed, whereas in ISM, receivers need not know the IP addresses of sources from which they receive their traffic. The proposed standard approach for channel subscription signalling utilizes IGMP INCLUDE mode membership reports, which are supported only in IGMP Version 3.


HTH,


__


Edison.


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