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Group meaning in ASM and SSM multicast.


Just can't understand rule by what Multicast group (Multicast adress) is created. For example, there are many sources in network, spreading multicast traffic, and there are several clients what want to accept it.  Some of these sources are in different places of network, some of them is on one switch. How many multicast group should be in that case? What it depends of? May be it has to be just one group for all network? 

6 Replies 6


ASM = Any Source Mc (classic pim sparse mode) means join this group to any source  (*.G) required Rendezvous Point (RP), Shared tree
SSM = Specfic Source Mc - Means join this group with this specific source (S,G)  Shortest Path Tree (SPT), no need for RP



@Chingiz  wrote:

How many multicast group should be in that case? What it depends of? May be it has to be just one group for all network? 

They can be as many as you require, the private mc address is quite large, and through the implementation of access-lists you can specify what host/host ranges can access what MC groups

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"Just can't understand rule by what Multicast group (Multicast adress) is created."

Either by host just sending to a multicast address and/or by "registering" it wants to receive packets addressed to one or more multicast addresses.

As to your questions concerning getting multicast packets from sender(s) to receiver(s), that can be very simple on a shared network and media, especially w/o something like IGMP snooping switches, to rather complex when multicast traffic needs to cross networks.  The latter uses various forms or multicast routing and possibly, other "efficiency" techniques like the already mentioned IGMP snooping, or Cisco's PIM snooping.

On the subject of "efficiency", as the most inefficient multicast might be treated like broadcast but which might be sent to all connected networks.  Conversely, the most efficient multicast tries to work much like unicast, i.e. sending traffic from sending host to only destination hosts that want it, yet without, where possible, any additional duplication of multicast packets or sending multicast packets where not needed and often using the shortest path between sender and receiver too.

As to number of multicast groups, as @paul driver mentions, the multicast address space is fairly large, so often you might have one address/group for every unique multicast stream.  This though, can create a very large multicast table to support.

You can also "share" multicast addresses across different unique streams, then the receiving hosts have to determine if the received multicast packet is one they want (much like dealing with broadcast packets).  Sharing multicast address may also send multicast streams where they are not actually desired by receiving hosts.

Since multicast can go further than desired, you can also "scope" limit multicast addresses, so they have physical distribution limits (this also allows reuse of the multicast addresses [somewhat like defining your own unicast private IPs]).

Unsure the forgoing helps answer your questions, but understand multicast is as, or possibly more, complicated than unicast, but often less well understood as, historically, it's not nearly as widely used as unicast.

Ok, got it. But how does client send request to join to certain source in multicast group in ASM? He sends (*,G) type Join to FHR, and how does router can define certain source? There is some technology named SSM Mapping, but it is something like static routes written on router. But how does he understand what source wants client, what send Join type (*,G), what not contents source IP?

Sorry, not FHP, LHP

ASM sources don't care or map to clients.

Such clients make their desire known to their local multicast router.  It's the multicast routers that "knit" together a multicast source and multicast client(s).  How that's done depends on how multicast has been configured on the multicast routers.

An "inelegant" (but it works) example is multicast routing that uses the flood-and-prune technique.  Multicast router with local connection to any transmitting multicast source relays multicast stream to any other connected multicast routers.  They in turn do likewise.  This continues to multicast routers which also relay multicast stream on all multicast enabled connections, including those without another multicast router.  This is the "flood".

Routers with multicast connections without other multicast routers use send out queries basically asking if any device wants the multicast stream.  If not, it stops the stream on that connection.  This is the "prune".

Routers that have no active requestors, or "downstream" router which has not indicated to stop stream, notifies "upstream" multicast router to stop stream to it.  This is also the "prune".

In short, this technique floods any multicast stream, at first, everywhere, and then stops it unless desired.  If no other part of the network wants that multicast stream, the local source's multicast router, eventually, doesn't relay multicast stream from an active source.

When you get into stuff like (*,G), you getting into the workings of multicast router, which don't really concern a source or receiver beyond they "trigger" such on the multicast routers to allow multicast between the source and receiver.

Yes, it how ASM Dense mode works. I want understand how works SSM Mapping, when router already knows all sources, but client doesn't. How does it explain what source traffic it wants? There are many sources in one group, and LHP doesn't know which of them client wants. If LHP router, what running SSM Mapping , accepted (*, G) Join from client, it simply spread it to all next routers? So where is different between SSM Mapping and ASM? What is advantage of Mapping?

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