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(lightning crackling jingle)

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- In this video, I'm going to go into

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the OSPF designated route
and backup designated router

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in theory and in practice

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and talk about why these
things were developed for OSPF.

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So, to begin with, let's go ahead

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and go to the whiteboard first,

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before we go into our slides,
to talk about this concept.

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Like I frequently like
to do, I like to contrast

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OSPF and EIGRP so you can see
some of their similarities

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and some of their differences.

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Here, we have four routers,

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and all four routers are
connected to the same switch,

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and they're all in the
same broadcast domain,

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as you can see from their IP address.

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If we were talking about
EIGRP, as an example,

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router one would list R2, R3,

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and R4 as neighbors.

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Now, let's say that R1 had an EIGRP update

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that it needed to send.

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We know that EIGRP updates
go out as multicast packets.

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What do switches do with multicasts?

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They flood them, just like
they flood broadcasts,

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so that multicast packet would
be flooded to R2, R3, and R4.

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Now, because R1 is neighbors

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with all three of those routers,

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he would wait and he'd say,

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"Well, I need to get an
EIGRP acknowledgement

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"from R2, R3, and R4."

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All of a sudden, here's R2,
sends in EIGRP hello packet

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with the sequence number
saying, "I acknowledge."

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He says, "Okay, R2, you're good to go.

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"Thank you for acknowledging."

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Here comes R3 sending a hello packet,

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and we check that off.

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Hopefully, eventually, R4
would send his own hello packet

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with that same sequence number in it.

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R1 would say, "Okay, all
three of my neighbors

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"have acknowledged this update."

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In the event that something went wrong

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and one of these routers did
not send an acknowledgment,

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then R1 would resend his EIGRP update

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as a unicast packet
directly to that neighbor.

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What does that have to do with OSPF?

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Now, remember that with OSPF,

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we saw that when we were
talking about the process

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of building a neighbor
relationship that it was important

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that all the OSPF routers
who were neighbors

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had a common database, that
their LSAs were the same.

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So, there were about six different steps

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we had to go through to get to that point.

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We had to go to init,
2-way, exstart, exchange,

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loading, and then finally
full to do all that.

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In this type of an environment,

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in this type of a network
where a router says,

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"Okay, there could be more than
one router on this network."

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For example, think about Ethernet.

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The nature of Ethernet is that

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you can put one frame on the wire,

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and you don't know how many
people are going to see that.

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You don't know how many people are on

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that same Ethernet cable as you.

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The nature of Ethernet
is that it could be two,

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three, four, 44, you don't know.

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Ethernet is what we call
a multi-access medium.

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The desires of OSPF, they said,

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"Well, we could, on a
multi-access medium like Ethernet,

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"where there's a potential
of lots of different routers

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"out there on this wire,

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"we could do like EIGRP does.

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"We could have all the routers go through

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"all five or six steps with each other.

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"All the routers become fully adjacent."

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They said, "Then, when an OSPF router

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"sends out a link-state update,

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"we could do like EIGRP does

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"and all the other routers
have to acknowledge it back."

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But the designers of OSPF
took a different approach.

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They said, "Here's what we're gonna do.

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"On a multi-access network
segment like Ethernet,

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"we're gonna have one router elected

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"as a very special position,
which is gonna be called

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"the designated router."

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Let's just say it's R4, as an example.

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During the exchange of hello packets

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is when this election takes process.

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As these four routers
are exchanging hellos,

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remember, their hellos are
going out as multicasts.

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What's the multicast address
that OSPF hellos go out to?

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224.0.0.5.

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Hellos always go to that address.

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Inside that multicast
hello, there are some fields

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that help these routers to figure out

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who this designated router is going to be.

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What's the significance
of the designated router?

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Well, once we know who
the designated router is,

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router one will go through all
six steps with router four.

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Init, 2-way, exstart,
exchange, loading, full.

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Router two will become fully
adjacent with router four,

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and router three will
become fully adjacent

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with router four.

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Notice, R1,

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R2, and R3,

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they're not becoming fully
adjacent with each other.

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They see each other.

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They're still seeing each other's hellos,

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because those hello packets are multicast,

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so they're seeing that.

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But when R1 looks at R2 and says,

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"Oh, neither one of us
is a designated router,

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"we don't need to go
beyond the 2-way state.

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"I see you and you see me.

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"Good, we're in the 2-way state,

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"but that's as far as we need to go.

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"We can stop there."

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With the DR, everybody
becomes fully adjacent.

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What's the purpose of that?

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Well, now if R1, let's
just take R1 as an example.

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Let's say that R1 has a link-state update

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that it needs to send

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containing one or more
new LSAs inside of it.

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R1 is gonna say, "I can only
send my link-state updates

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"to OSPF neighbors that
I'm fully adjacent with,

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"that I'm in the full state,

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"I went through those
five or six steps with."

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Which, in this case, is
the designated router.

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He says, "Hm, I know there's other routers

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"here on this segment besides R4, the DR.

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"How do I get this link-state
update just to him?

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"Ah, here's what I know I'll do.

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"I'll send my link-state update

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"to a special, reserved
OSPF address of 224.0.0.6."

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Only the designated router
is listening to that address.

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So now, when this link-state
update packet goes in,

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it is a multicast, so it will be flooded.

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R2, R3, and R4 will see it,

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but R2 and R3 will discard it.

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They'll say, "Oh, I'm
not a designated router.

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"This is not for me."

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Now, that link-state update will only be

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seen and processed by R4.

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Now, R4 gets it.

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Here it comes in.

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Now, R4 says, "Great.

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"My link-state database
is synchronized with R1.

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"R1 and I are in sync.

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"I got his link state.

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"Oh, by the way, let me
acknowledge that back."

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So R4 will do that.

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He'll send a link-state
acknowledgement packet back.

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Now R4 will say, "But I
have two other neighbors

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"on this wire, R2 and R3."

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Now what R4 will do is he'll turn around

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and he'll send a link-state update

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to R2 and R3, link-state update.

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He'll reflect that
link-state update right back,

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going to 224.0.0.5.

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Now, you might be thinking wait a second,

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if R4 does that, isn't R1

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gonna see his very own

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link-state update packet
coming back to him?

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As a matter of fact, yes he will.

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Remember, that link-state
update has an LSA inside of it,

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maybe more than one LSA.

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Remember, every LSA has a field that says

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who does this LSA belong to?

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Who created it?

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Who's the originator of the LSA?

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In this particular case,

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when R1 gets that link-state update

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and he looks inside of it, he'll say,

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"Oh, here's an LSA that
was advertised by R1.

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"Oh, that's me.

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"Okay, I can ignore that.

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"Here's another one that
was advertised by R1.

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"Oh, okay, that's me.

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"I can ignore that."

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In this regard, all these routers

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will become fully synchronized.

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Once again, just to repeat the process,

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the DR is elected.

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All these routers exchange hello packets,

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and within the hello
packets are some fields,

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which I haven't talked about yet,

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which will help them to figure out

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who the designated router should be.

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Once that takes place, all of the routers

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become fully adjacent with
the designated router.

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They go through all five or
six of those steps with him.

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After that point,

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if anybody needs to send
a link-state update,

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they will send it to the DR,

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and they'll send it to 224.0.0.6.

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That is an address that
only the DR is listening to.

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The DR, in turn, will reflect
that link-state update

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right back again

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so that all the other routers can get it,

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including the originator,
who will say, "No."

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That will go to 224.0.0.5.

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The idea being here is that
if I have an Ethernet switch

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that, let's say, has 20 routers on it,

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they don't all have to
become fully adjacent

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with each other.

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They only have to become fully adjacent

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with the designated router.

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They send their updates to him,

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and then they don't
have to worry about it.

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He's the one that's going to turn around

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and reflect it to everybody else.

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The designated router's
job is to make sure

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that everybody is synchronized
with everybody else.

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That is his job.

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By the way, there's one other router here

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who is also elected as

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a backup designated router, or a BDR.

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Just like the DR is
listening to 224.0.0.6,

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the BDR is also listening to 224.0.0.6.

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When R1 sent his initial link-state update

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to 224.0.0.6,

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both R3 and R4 got it.

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But it was only R4's job,
as the designated router,

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to turn around and
reflect that back so that,

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in this case, R2 could get it.

224
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R3 is sort of just sitting there silently.

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He is taking stuff in,
because he's listening to .6,

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but it's not his job to turn
around and reflect stuff back.

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Only if the designated router
fails will R3 take up the job

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as the new designated router.

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Now that we know that,

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let's go into these slides.

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A DR and a BDR are elected in broadcast

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and non-broadcast multi-access networks.

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Now, we haven't really talked about

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those two terms yet.

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That's coming up in a little
bit in a different video.

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Just think about it this way.

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We know that at layer 2,

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at the data link layer of the OSI model,

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there are several different
encapsulation types:

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Ethernet, PPP,

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HDLC, frame relay.

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You could probably think
of one or two or three more

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in addition to that.

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What's one thing that makes
them different from each other?

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Some of those encapsulation types

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were built around the assumption

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that I could put one frame on the wire

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and multiple devices
could see that one frame,

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like Ethernet.

250
00:11:40,079 --> 00:11:41,639
That's the nature of Ethernet.

251
00:11:41,639 --> 00:11:43,793
That's the nature of,

252
00:11:43,793 --> 00:11:45,202
well, let's just stick with Ethernet.

253
00:11:45,202 --> 00:11:46,685
That's Ethernet, you can
put one frame on there,

254
00:11:46,685 --> 00:11:48,671
and everybody will see it.

255
00:11:48,671 --> 00:11:51,019
Other layer 2 encapsulation types say,

256
00:11:51,019 --> 00:11:52,785
"Well, if I put one frame on the wire,

257
00:11:52,785 --> 00:11:55,137
"it's only going to go one place,"

258
00:11:55,137 --> 00:11:59,017
like the point-to-point protocol or HDLC.

259
00:11:59,017 --> 00:12:00,559
Those are layer 2 protocols that say,

260
00:12:00,559 --> 00:12:03,409
"This was designed for a cable
that's just got two devices.

261
00:12:03,409 --> 00:12:06,013
"Put something on the cable,
it only goes over here.

262
00:12:06,013 --> 00:12:07,533
"This guy puts something on the cable,

263
00:12:07,533 --> 00:12:08,967
"it only goes over here."

264
00:12:08,967 --> 00:12:11,278
So, there's really no need for an address,

265
00:12:11,278 --> 00:12:13,075
because there's just the two of us.

266
00:12:13,075 --> 00:12:15,325
PPP and HDLC are like that.

267
00:12:16,159 --> 00:12:18,481
Then, there's other types
of encapsulation types,

268
00:12:18,481 --> 00:12:19,946
like frame relay, that say,

269
00:12:19,946 --> 00:12:23,729
"Well, this particular cable
could go multiple places."

270
00:12:23,729 --> 00:12:26,239
For example, think about your
connection to the Internet.

271
00:12:26,239 --> 00:12:27,409
You're sitting at home.

272
00:12:27,409 --> 00:12:28,837
You have a connection to the Internet.

273
00:12:28,837 --> 00:12:31,828
That one cable you have
leading out of your house

274
00:12:31,828 --> 00:12:35,716
can reach billions of remote destinations.

275
00:12:35,716 --> 00:12:37,673
It's sort of like Ethernet in that way.

276
00:12:37,673 --> 00:12:39,307
You could put one thing on there,

277
00:12:39,307 --> 00:12:41,881
one cable that gets you multiple places.

278
00:12:41,881 --> 00:12:45,722
But is it possible for you to
put something on that cable

279
00:12:45,722 --> 00:12:49,282
that all those remote
destinations are going to see?

280
00:12:49,282 --> 00:12:51,041
No, it's not.

281
00:12:51,041 --> 00:12:53,593
In the case of your Internet connection,

282
00:12:53,593 --> 00:12:56,038
you have to address everything
you put on that cable

283
00:12:56,038 --> 00:12:57,844
to who you want to send it to.

284
00:12:57,844 --> 00:12:59,605
It doesn't support broadcast.

285
00:12:59,605 --> 00:13:02,201
You can't put a broadcast
on your cable modem

286
00:13:02,201 --> 00:13:05,830
or your DSL connection to
Time Warner Cable or Comcast

287
00:13:05,830 --> 00:13:08,728
and expect that all the billions
of devices in the Internet

288
00:13:08,728 --> 00:13:10,205
are gonna see it.

289
00:13:10,205 --> 00:13:12,560
It doesn't work that way.

290
00:13:12,560 --> 00:13:13,664
That's what we consider

291
00:13:13,664 --> 00:13:16,783
a non-broadcast multi-access network.

292
00:13:16,783 --> 00:13:19,219
In all these networks where I
can out something on a cable

293
00:13:19,219 --> 00:13:20,386
and I can say to myself,

294
00:13:20,386 --> 00:13:23,564
"Well, there could be more than one device

295
00:13:23,564 --> 00:13:25,316
"reachable on this cable."

296
00:13:25,316 --> 00:13:27,263
In that particular case,
a designated router

297
00:13:27,263 --> 00:13:31,180
and a backup designated
router will be elected.

298
00:13:32,257 --> 00:13:35,753
We've talked about these last
two bullet points right here.

299
00:13:35,753 --> 00:13:40,072
How do we determine which
router that will be?

300
00:13:40,072 --> 00:13:43,201
Well, by default, the very first router

301
00:13:43,201 --> 00:13:46,907
that comes online on a cable
will be the designated router.

302
00:13:46,907 --> 00:13:48,454
That's pretty much it.

303
00:13:48,454 --> 00:13:50,870
Now, if two routers come
online at the same time,

304
00:13:50,870 --> 00:13:53,729
if they start exchanging
hellos at the same time,

305
00:13:53,729 --> 00:13:56,730
then the router with the
highest interface priority

306
00:13:56,730 --> 00:13:58,580
will become the designated router.

307
00:13:58,580 --> 00:14:00,989
This is the first time
I've talked about this.

308
00:14:00,989 --> 00:14:04,821
In OSPF, when you use the
network command under OSPF,

309
00:14:04,821 --> 00:14:07,560
and OSPF says, "Oh, here's
an interface right here

310
00:14:07,560 --> 00:14:09,582
"that falls in line with that network.

311
00:14:09,582 --> 00:14:12,605
"I'll start speaking, I'll start
sending out hello packets."

312
00:14:12,605 --> 00:14:16,232
On this interface, he gives
it an OSPF priority value.

313
00:14:16,232 --> 00:14:18,570
It's actually the value of one by default.

314
00:14:18,570 --> 00:14:20,903
In my hello packets I send out,

315
00:14:20,903 --> 00:14:22,871
that's one of the things I put in there is

316
00:14:22,871 --> 00:14:24,795
what my interface priority is.

317
00:14:24,795 --> 00:14:26,381
If I'm sending hello packets to you

318
00:14:26,381 --> 00:14:28,271
and you're sending hello packets to me,

319
00:14:28,271 --> 00:14:31,652
if one of us has a higher
interface priority than the other,

320
00:14:31,652 --> 00:14:32,688
we're done.

321
00:14:32,688 --> 00:14:34,571
The highest interface priority will win,

322
00:14:34,571 --> 00:14:38,492
and that will become
the designated router.

323
00:14:38,492 --> 00:14:41,975
When you do the command
show ip ospf neighbor

324
00:14:41,975 --> 00:14:43,559
to view your neighbors,

325
00:14:43,559 --> 00:14:45,620
you will see, on an Ethernet segment,

326
00:14:45,620 --> 00:14:48,308
you'll see at least one
neighbor shown as DR,

327
00:14:48,308 --> 00:14:51,980
for designated router, possibly
another one that says BDR,

328
00:14:51,980 --> 00:14:54,957
and then others that will say DROTHERS.

329
00:14:54,957 --> 00:14:58,436
DROTHERS mean routers that currently

330
00:14:58,436 --> 00:14:59,990
are not the DR,

331
00:14:59,990 --> 00:15:01,811
not the BDR,

332
00:15:01,811 --> 00:15:03,941
but they could be in the future.

333
00:15:03,941 --> 00:15:06,644
If the DR and BDR fail,
these other routers

334
00:15:06,644 --> 00:15:10,311
could potentially become
designated routers.

335
00:15:11,156 --> 00:15:13,453
Now, let's say that you have not

336
00:15:13,453 --> 00:15:16,375
manually configured
the interface priority.

337
00:15:16,375 --> 00:15:17,816
Remember, it's interface priority.

338
00:15:17,816 --> 00:15:18,994
It's not a global priority.

339
00:15:18,994 --> 00:15:21,704
On a router, if he's got five interfaces,

340
00:15:21,704 --> 00:15:23,986
if you wanted to, you
could have each interface

341
00:15:23,986 --> 00:15:26,050
have a different priority value.

342
00:15:26,050 --> 00:15:28,085
By default, they'll all be the same.

343
00:15:28,085 --> 00:15:29,740
They'll be the value of one.

344
00:15:29,740 --> 00:15:31,682
If you and I exchange OSPF hello packets

345
00:15:31,682 --> 00:15:33,001
over an Ethernet segment

346
00:15:33,001 --> 00:15:36,014
and we both have the exact
same interface priority,

347
00:15:36,014 --> 00:15:38,081
then we're going to
compare each other's names,

348
00:15:38,081 --> 00:15:41,268
our router-ids, our OSPF router-ids.

349
00:15:41,268 --> 00:15:45,790
Whoever has the highest
router-id will be the winner.

350
00:15:45,790 --> 00:15:48,262
That will become the designated router,

351
00:15:48,262 --> 00:15:50,044
and the second highest will become

352
00:15:50,044 --> 00:15:52,461
the backup designated router.

353
00:15:54,477 --> 00:15:57,942
A lot of times, people don't
really necessarily care

354
00:15:57,942 --> 00:16:00,363
which router will be the DR or the BDR.

355
00:16:00,363 --> 00:16:03,062
They just let OSPF take
care of that by itself.

356
00:16:03,062 --> 00:16:05,267
Let's say you wanted to
have some control over that.

357
00:16:05,267 --> 00:16:07,752
As the network administrator,
you wanted to be

358
00:16:07,752 --> 00:16:10,458
in control over which router
was going to become the DR

359
00:16:10,458 --> 00:16:12,084
and the BDR.

360
00:16:12,084 --> 00:16:15,210
The easiest way to do that
would be to go the interface

361
00:16:15,210 --> 00:16:17,892
and change the OSPF interface priority.

362
00:16:17,892 --> 00:16:18,861
In a video coming up

363
00:16:18,861 --> 00:16:21,405
where we actually go over
the configuration commands,

364
00:16:21,405 --> 00:16:23,136
I'll show you how to do that.

365
00:16:23,136 --> 00:16:24,516
The second way you could do it

366
00:16:24,516 --> 00:16:28,266
is by manipulating the
OSPF router-id itself.

367
00:16:30,036 --> 00:16:31,166
As soon as you type

368
00:16:31,166 --> 00:16:35,567
router OSPF and then some
process ID and hit enter,

369
00:16:35,567 --> 00:16:37,494
it elects a router-id for itself.

370
00:16:37,494 --> 00:16:39,532
It looks at all of its
interfaces and says,

371
00:16:39,532 --> 00:16:41,578
"I need to pull an IP address here

372
00:16:41,578 --> 00:16:45,161
"and use that as my
name, as my router-id."

373
00:16:46,024 --> 00:16:47,121
What's the order?

374
00:16:47,121 --> 00:16:48,061
How's it going to do that?

375
00:16:48,061 --> 00:16:50,362
Number one, first thing
OSPF is going to say is,

376
00:16:50,362 --> 00:16:51,901
"All right, under my configuration,

377
00:16:51,901 --> 00:16:54,535
"under my router OSPF configuration,

378
00:16:54,535 --> 00:16:58,702
"did some human being actually
type the router-id command?

379
00:17:00,239 --> 00:17:04,738
"If they did, that's what
I'll use as my router-id."

380
00:17:04,738 --> 00:17:07,927
What if you did not type
the router-id command?

381
00:17:07,927 --> 00:17:09,676
Well, then the router will look to see

382
00:17:09,676 --> 00:17:11,956
if you have any loopback interfaces.

383
00:17:11,956 --> 00:17:14,482
If you do, whatever the
highest IP address is

384
00:17:14,482 --> 00:17:16,864
of all of your loopbacks,

385
00:17:16,864 --> 00:17:18,883
that will be your router-id.

386
00:17:18,883 --> 00:17:21,787
Lastly, if there are no loopbacks

387
00:17:21,787 --> 00:17:25,286
and there's no OSPF router-id
that was manually configured,

388
00:17:25,286 --> 00:17:27,439
it'll just look around at your interfaces

389
00:17:27,439 --> 00:17:29,848
and select an IP address
from your interface,

390
00:17:29,848 --> 00:17:32,987
the highest IP address from
among your physical interfaces,

391
00:17:32,987 --> 00:17:36,140
and use that as its router-id.

392
00:17:36,140 --> 00:17:38,462
Now, interesting little factoid here.

393
00:17:38,462 --> 00:17:41,053
A lot of times, people
think only interfaces

394
00:17:41,053 --> 00:17:45,247
that are actually up/up are candidates

395
00:17:45,247 --> 00:17:46,511
for the router-id.

396
00:17:46,511 --> 00:17:48,287
That's not actually true.

397
00:17:48,287 --> 00:17:51,511
Even an interface that
is up/down or down/down

398
00:17:51,511 --> 00:17:53,383
is a candidate for router-id.

399
00:17:53,383 --> 00:17:56,249
The only time there's an
IP address on an interface

400
00:17:56,249 --> 00:17:57,863
and OSPF says, "I can't use that.

401
00:17:57,863 --> 00:17:59,654
"I can't even consider that,"

402
00:17:59,654 --> 00:18:03,754
is if the interface is
administratively disabled,

403
00:18:03,754 --> 00:18:05,672
if it is shut down.

404
00:18:05,672 --> 00:18:09,485
Other than that, the IP address
could potentially be used

405
00:18:09,485 --> 00:18:13,925
as the router-id depending
on if it's not a loopback

406
00:18:13,925 --> 00:18:17,121
and if it's the highest
IP address on the box.

407
00:18:17,121 --> 00:18:21,501
So, that ends this video on
the OSPF designated router

408
00:18:21,501 --> 00:18:24,584
and backup designated router process.

409
00:18:34,346 --> 00:18:38,096
(lightning crackling jingle)