Hi,

Here I have one question, As I read that for multicast packet
forwarding, router uses its unicast spanning tree of router by looking
into its routing table. So while maintaining info about multicast
groups, does router send query to neighbour routers also??

Like as we know, for querying about multicast group subscription,
router will send packet to 224.0.0.1(means to all host in the subnet).
Dose this "all host" includes its adjacent or neighbour routers(which
are directly connected with it) also. So that based on their response
it could maintain routing table.

So if each router(say A) could receive the neighbour router's(say B)
multicast group query, then it could inform that router about its
multicast group subscriptions. So that later, whenever B gets any
packet to forward, it will fwded to A, only if A has
any subscription for that group, otherwise not. This will avoid
unnecessary flooding of multicast packets. Thats why I wondered, if B
sends a multicast query packet at 224.0.0.1, then will it be received
and processed by its neighbour router A also??

As well as I am not clear with TTL significance in multicast routing.
Somewhere I read, "For multicast messages(i.e. expect query/report) the
value of TTL should be greater than one". Dose it mean, by putting any
TTL value greater than one(say 2) will assure the delivery of this
packet to all of its group subscribers?? However I don't think so, in
that case how we decide about TTL value for a muticast packet on a
originating source.

Thanks in advance.


Albert Manfredi wrote:
> <myself_rajat@yahoo.com> wrote:
>
> > ya I did check with IGMP version(using sniffer), its V3. So host is
> > sending multicast reports at 224.0.0.22. As I found this special class
> > D address is ment for IGMP, does this mean that its specifying the
> > address of local IGMP enabled router.
>
> The address 224.0.0.22 goes to all multicast-enabled (IGMPv3) routers
> within one hop, meaning in the same IP subnet of the host. But only one
> of these routers becomes the "querier," as the RFCs explain. So IGMP
> never worries about the specific address of any one router. The routers,
> if multiple exist in the subnet, negotiate among themselves who gets to
> be in control, and the hosts can remain as dumb as possible. Always do
> the same thing, no matter what the router situation is.
>
> > And what will happen if the local subnet it having no multicast
> > enabled
> > router but simple unicast-router, then how that router will behave
> > after getting multicast report destined to 224.0.0.22??
>
> You would get no IGMP queries from any router, and IGMP reports would be
> dropped. You will note, for example, that Windows Media Player does not
> depend on IP Multicast. It can also use unicast, TCP, HTTP, etc.
>
> Buy the way, all of this discussion has been about edge multicast router
> to multicast hosts. There is a whole other discussion about building
> multicast trees between routers. There have been many schemes developed
> to achieve that, but it looks like the ones in favor now are under the
> Protocol Independent Multicast (PIM) banner.
>
> Bert