So now that our PIM sparse mode is set up,
it's enabled on our interfaces, we have our RP configured,
and we've taken a look at IGMP, now
let's actually jump in and see how
multicast operates by starting up a source and a receiver.
So we'll start in this video on the receiver side,
have it try to join a group, and we'll
see what happens with the star comma G or the shared tree.
So let's bring up our topology, just so we
can see what's going to happen.
And then, of course, we'll jump onto the command line
and verify that everything's working as it should
in our multicast environment.
So we are still on multicast topology 1.
And in this video, we're going to be configuring our receiver
over here, router 8, to join the group to 224.1.1.1.
This is going to cause it to send an IGMP report
message over to router 7.
Then router 7 will attempt to locate
this group for the receiver, and it
will send a start comma G join towards the rendezvous point.
So this will go towards router 3.
Router 3 will then send it over to router 4.
So ultimately, we should see the star comma G
tree build from 7, to 3, to 4.
We'll jump in and examine that, and we'll also
watch the IGMP messages between router 8 and router 7.
So let's go ahead and jump in.
And we'll start, of course, by setting up some debugs.
And we'll also set up Wireshark, so we can see this happening
as it goes.
So we'll begin on router 7 by turning on some debugs.
Let's do a debug IP IGMP, so we can see the IGMP messages.
And we'll also do a debug IP PIM.
We also have a Wireshark packet capture
running on the gigabit 1 interface of router 8
as well to capture that IGMP traffic.
So now, let's go over to router 8 and have it join our group.
We'll go into our gigabit 1 interface,
and we say IP IGMP, join group 224.1.1.1.
Now that we've joined the group, let's
go take a look at our debugs on router 7.
So first, let's turn off the debugs.
And then let's take a look back over these
and see what actually happened.
So a lot of these messages are going
to have to do with that default 224.0.1.40 group.
So we need to find the ones related to our group.
So we see the first one here, we have an ICMP
received a version 2 report on the gigabit 3 interface
from router 8 for the group 224.1.1.1.
Then we see IGMP actually adding the interface gigabit 3
to the outbound list for that group.
It inserts that group into the interface,
and it updates the timers and everything to make sure
that that is a new entry.
The next thing we have happening here
is we check the RP, in other words,
we look at the RP map for the 10.4.4.4
for the group 224.1.1.1.
So now that we know that the 10.4.4.4, notice the next thing
we do is we're now going to build a triggered star comma G,
either join or prune message.
This is a generic entry.
It doesn't tell us whether it's a join or prune right now.
But it's for the group 224.1.1.1.
We also see it adding these entries
into the multicast routing table.
And notice here.
It's actually sending that version
to join or prune to 50.1.37.3, which is its upstream RPF.
So we can see the triggering of the star comma G tree
here on router 7.
We can also see this if we do a show IP mroute.
Notice that we have a star comma G entry for the 224.1.1.1.
The RP is 10.4.4.4, as it should be.
The incoming interface is gigabit 2
with the RPF neighbor of router 3.
And the outgoing interface is gigabit 3 towards router 8.
So we can see that this star comma G entry has been built
properly here on router 7.
Now we'll go ahead and look at router 3 and 4
in just a moment.
But before we do that, let's go look at the IGMP report message
that we captured in Wireshark.
So we are filtering this output by IGMP.
And what we see is that packet 66 is an IGMP version 2 report
message for the group 224.1.1.1.
Also notice that for the source and destination,
the source is of course router 8,
and the destination is the group itself, 224.1.1.1.
If we open up the detail on this packet,
we see under Internet Group Management Protocol
that again, it's IGMP version 2.
It is a report message.
And it is for the multicast group 224.1.1.1.
So this is what the IGMP report message looks like on the wire.
Now let's go take a look at the rest of our tree.
Let's jump over to router 3.
So here on router 3, let's do a show PIM topology.
Notice that we have our entry here for the star comma
G for 224.1.1.1.
It is in sparse mode.
It is up.
The is 10.4.4.4.
We see the RPF interface and the RPF neighbor,
as well as the outgoing interface is gigabit 0003,
towards router 7, as it should be.
Let's go ahead and verify this on router 4.
Remember that since router 4 is the rendezvous point,
this should be the root of the tree.
Notice on the star comma G here on the rendezvous point,
the RP is listed as itself.
It has an asterisk, indicating that that is the local router.
It's never sent a join to anybody
because it doesn't need to.
It's the root of the tree.
Notice that the RPF check is the de-encapsulation tunnel
0 for 10.4.4.4.
This would be the receiving end of the PIM register message
tunnel.
Then once it gets the information through
the encapsulated tunnel, the outgoing interface is gigabit
000, which is going over towards router 3.
So what we see here is that we have a star comma G
tree starting at the rendezvous point here,
router 4, outgoing interface to router 3.
Router 3 has an outgoing interface to router 7.
So our star comma G tree has built correctly.
Now routers outside of that path,
say for example, router 1, at this point,
should know nothing about that group.
If we do show PIM topology, router 1
knows about that default group for auto RP, the 224.0.1.40,
but it knows nothing about the group
that we created because it's not on the star comma G tree.
So in this video, we started up a receiver,
and that sent an IGMP message, specifically IGMP version 2,
to the last hop router, which caused
it to generate a PIM join message for the star comma G
tree for that group towards the rendezvous point.
Then we went in and explored that tree
to see how it was built properly from the rendezvous point, all
the way to that last hop router.
Well, none of this does us any good
unless we actually have a source.
So what we're going to do next is fire up a source
and see how this whole thing works together.
Until then, I hope this has been informative for you,
and I'd like to thank you for viewing.