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[AUDIO LOGO]

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So let's jump in
and start talking

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about multicast operation.

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What we're really going
to be focusing on here

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is how packets are forwarded
through the network

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incoming interfaces, outgoing
interfaces, S comma G entries

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in our multicast routing table.

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What is an S comma G entry?

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And how do we get them?

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Now, we're not going
to be focusing here

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on say PIM sparse mode
versus PIM dense mode.

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We do have other
videos coming up,

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in this skill, that
will cover that.

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But for now, we're just going
to discuss generic IP forwarding

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for multicast operations.

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So let's go ahead and jump in.

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We'll take a look
at a diagram, here,

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so we can discuss
what happens as we're

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learning information
and passing packets

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through our multicast
environment.

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So the first step in
our multicast routing

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is going to be to enable PIM,
on the necessary interfaces,

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on the necessary devices.

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So let's start with
the obvious interfaces.

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All of the interfaces
on the routers that

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are facing other routers are
going to have to run PIM.

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So that takes care of
all of those interfaces.

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However, what might
not be so clear is also

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the interface facing the source,
it also has to be PIM-enabled.

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And the interfaces
facing any clients also

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needs to be PIM-enabled.

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The simple fact is here.

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Cisco routers will not
forward routed multicast

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unless an interface
is PIM-enabled.

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So essentially, we
need to enable it

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on all the interfaces,
on all the routers

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unless we have some reason
that a particular interface

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or circuit should
not carry multicast.

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Generally speaking, we're going
to want it enabled everywhere.

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The next step is
our multicast source

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needs to start sending multicast
packets onto the network.

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Now remember, there
is no protocol

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or anything running
between the source

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and what's called the first
hop router, which in this case

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would be router 1.

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This is our first hop router.

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The multicast packets
simply need to show up

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on this PIM-enabled interface.

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As soon as that
happens, router 1

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is going to make what's called
an S comma G entry or a source

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comma group.

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Let's take a look at
what that would be.

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Let's say, for example,
that this source

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has an address of 10.1.1.1.

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And it is sending to the
multicast destination of--

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we'll make it real easy.

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We'll just make it 224.1.1.1.

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And it doesn't have to be fancy.

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What the S comma G
entry on router 1

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is now going to look like in
the multicast routing table

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is it's going to have 10.1.1.1.

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That's the source, comma,
the group 224.1.1.1.

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This is what we call
an S comma G entry

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in the multicast routing table.

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It's how we know what sources
are available from which

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groups.

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So this is the fundamental
of multicast routing.

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All of these routers
are going to ultimately

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need an S comma G entry in
order to forward the traffic.

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Now they're also going to need
an oil, which is, of course,

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the outbound interface list.

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And we'll keep track of
that just on router 1

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here, just as an example.

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But we'll keep track of our oil.

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Now in order to do
this, we are going

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to have to label
our interfaces here.

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I don't care so much about
the incoming interface.

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But we'll just say that,
perhaps, this is gigabit 1.

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We'll say that this interface
up here is gigabit 2.

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And this one is gigabit
3 just to make it simple.

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So now we have this multicast
traffic showing up on router 1.

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Since router 1 has
seen the traffic,

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it now knows the S comma G. An
important thing to note here,

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the S comma G entry
is traffic-driven.

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And we'll see this as we move
through PIM, both sparse mode

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and dense mode.

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There's no magic signaling or
anything that happens here.

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The way the routers learn
the S comma G entries

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is simply by seeing the traffic.

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In the IP headers of
the multicast traffic

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is the source and the
destination information.

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We simply read that
information out of the packet

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headers and make
an S comma G entry.

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So one of the
fundamental principles

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for multicast routing is to
get the multicast traffic

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to the appropriate routers.

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So that they can make
an S comma G entry.

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Now how that's done,
that's a big difference

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between sparse mode
and dense mode.

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How do the routers get
the S comma G entry?

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That is our fundamental focus
and what we need to examine.

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Now like I said, in this
video, that's not our focus.

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Although I will
point out that as we

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go through this
discussion, this is

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going to be closer to
how dense mode works.

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But let's just go ahead,
and move forward, and talk

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about the moving parts here.

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Now before router 1 can
begin forwarding traffic,

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we have to know what
interfaces are on the oil.

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And as we've mentioned,
this is, of course,

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going to be different between
sparse and dense mode.

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This is a hybrid.

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Again, we're just having
a general discussion here.

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But the first thing
that needs to happen

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is our clients over here
need to tell us that they

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want this particular group.

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So let's talk about that.

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What's going to
happen is the client

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will signal what's called
the last hop router.

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In this case, the
last hop routers

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are router 2, router
5, and router 7.

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But the clients signaled their
interest to the last hop router

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with something called IGMP
or Internet Group Management

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Protocol.

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Now IPv6 6 does this a
little bit differently.

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It uses something called MLD or
Multicast Listener Discovery.

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But we're focusing
right now on IPv4.

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So we'll be talking about IGMP.

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IGMP comes in three versions.

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Version 1 is a bit old.

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It's missing something
called a leave mechanism.

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We'll talk more about IGMP
in some more detail later.

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But version 1 is pretty
much not used anymore.

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Our routers these days
will default to version 2.

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Version 2 is the most
common version that we use.

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However, just to
throw this out there,

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we have mentioned SSM or
Source-Specific Multicast

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a few times so far in our
multicast discussions.

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That requires IGMP version 3.

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This adds SSM into the equation.

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So generally speaking, we're
going to run IGMP version

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2 unless you're using SSM.

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And then you're going to
want to use version 3.

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Now just as a note
here, I have been

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in companies that are running
IGMP version 3 on everything.

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And I'm like, well,
are you guys using SSM?

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They're like, well, no.

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I'm like, well, why are you
running IGMP version 3 then?

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And their answer was, well,
that's the newest version.

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I mean, isn't that what
we should be running?

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I'm like, well, I mean,
it is the newest version.

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Sure.

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You aren't really using
a feature from it.

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But there's no harm
in running it either.

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So IGMP version 3 will do
everything version 2 will do.

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It just adds that
feature of SSM.

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So do we really need it
if we're not doing SSM?

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Not really but again,
the flip side of that

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is it's not going to
hurt anything either.

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So again, in our
environments, we

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should be running either
IGMP version 2 or version 3.

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But version 3 is going to be
required if we need that SSM.

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So now we have
all of our clients

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indicate that they
want this group.

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So now the last hop routers have
these interfaces on their oil.

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It's now up to the last
hop routers to figure out

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how to get this traffic.

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And again, this is where
sparse mode and dense mode

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are going to differ greatly,
how they learn this.

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But again, let's
just go generic here.

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Router 2 would send
a PIM join message

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to get this traffic
flowing to router 2.

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Router 1 will respond
by adding gigabit

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2 to its outbound interface
list and then forwarding

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this multicast traffic
up to router 2.

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Router 2 of course, knows
the client wants it.

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It has that
interface on its oil.

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So it goes ahead and forwards
the traffic to the client.

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And so far, we have one client
receiving our multicast.

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And since router 2 has now
seen the traffic coming

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from our green line here
indicating our traffic flow,

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router 2 now knows
the S comma G entry

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for this particular traffic.

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Remember, the S comma G entry
is this entry over here.

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We're just not going to
write it out every time.

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It'll get rather large.

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But that's the entry
we're talking about,

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the 10.1.1.1 having
the group 224.1.1.1.

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Router 5 is also going to
send an IGMP join message

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towards router 4.

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Router 4 would send
a join to router 3.

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Router 3 send a
join up to router 1.

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And once all these
joins are done,

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well, you can see what's
going to happen now.

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We now add gigabit 3 to our oil.

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Router 1 sends the
traffic to router 3.

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Router 3 now knows
the S comma G entry.

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Router 3 forwards
it to router 4.

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Router 4 now knows
the S comma G entry.

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Router 4 forwards
this to router 5.

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And, of course, router 5
forwards it to the client while

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at the same time, of course,
learning the S comma G entry.

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When router 7 sends its
join message to router 4,

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router 4 is simply
going to graph the tree,

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and add this
interface to its oil,

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and start sending
this to router 7.

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We know, of course, that router
7 forwards this to the client

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as well as learning
the S comma G entry.

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So overall, this is
the process that we

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go through for
all of the routers

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to learn the S comma G entry.

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Now, again, how
this actually occurs

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does vary between sparse
mode and dense mode.

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So again, this is a
generic conversation.

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I just want to make that clear.

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This was a dense mode
conversation here.

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But sparse mode works
in a similar fashion.

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It just involves a
rendezvous point.

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What we drew here is
more of a push model.

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But we'll get into more detail
on those two differences

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in a little bit.

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Our main focus here
is the creation

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of the S comma G entry,
how the oil gets populated.

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And one more thing to discuss,
what if one of these clients--

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we'll take the client up here--

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decides they don't want to watch
this multicast stream anymore,

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whatever it may be?

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So they are going
to leave the group.

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That means that router
5 is going to remove

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this interface from its oil.

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It will no longer forward
out that interface.

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Since router 5 has
no other clients,

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it would then send what's
called a PIM prune message back

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to router 4 saying, hey, I don't
need this multicast anymore.

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I don't have any clients.

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Stop sending it.

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Router 4 would then
remove that interface

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from its oil and our environment
would end up looking like this.

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Now keep in mind router 5
will still have this S comma G

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entry until it times out.

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But again, we'll get into those
details in a future video.

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But this is the idea behind
basic multicast operation,

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so we understand
the moving parts.

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So in this video, we jumped in.

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And we took a look at the
basics of multicast routing.

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We took a look at
how the routers need

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to form an S comma G or a
source comma group entry

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in their multicast routing
table so that they know

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what sources have which
groups and what interfaces

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are on the oil.

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So it knows which interfaces to
forward this multicast traffic

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out.

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This is, of course, all
maintained through prunes

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and joins.

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However, an interesting
question that came up here

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is how did these routers know
which way to send the join?

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How did everybody to send
a join towards router 1

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in our example?

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Now, we didn't make
that real clear.

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Part of that is knowing
the S comma G entry.

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It gets a little complicated.

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But part of the answer
is the same thing

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we use a for loop
prevention here, which

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is reverse path forwarding.

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And that's something we have
to look at in more detail.

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That's exactly what
we're going to do next.

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Until then, I hope this has
been informative for you.

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And I'd like to thank
you for viewing.

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