WEBVTT

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 Welcome to this video, which for the
 CCNA Bootcamp is a review of the

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 OSPF Designated Router and Backup Designated
 Router election process.

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 Now before we talk about what DRs and
 BDRs are and how they're elected,

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 we first have to do a review of another
 concept that's actually very important

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 and related to this, which
 is loopback interfaces.

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 So, as a review, interfaces on Cisco devices
 come in two different flavors,

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 physical interfaces that you can actually
 plug a cable into, stick your

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 finger in the hole right there, and
 logical interfaces, which have no

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 cable, it's just a placeholder in the
 mind of the CPU for adding an IP

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 address or doing something.

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 And some logical interfaces come as
 part of the router, for example, a

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 VTY line, a VTY line, which is used to
 terminate telnet and SSH sessions.

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 You don't have to create that.

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 The moment you boot up the router, it
 automatically creates these virtual

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 things called VTY lines.

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 Other virtual interfaces, you do have
 to create, like on your switch,

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 your interface VLAN 2 or interface VLAN
 3 or sub interfaces, or what we're

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 going to talk about, loopback interfaces,
 you have to manually create

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 them. So most logical interfaces have
 to be created via configuration

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 commands. Now, the good thing about a logical
 interface is it's not dependent

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 upon any keep-alives or any electrical
 heartbeats or any kind of line

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 protocol in order to
 maintain its status.

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 So, as long as the device is up, the
 CPU is up in the router or the switch

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 that the logical interface
 will also be up.

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 Now, when switches switched virtual
 interfaces, your interface VLANs,

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 those are a little bit in on your physical
 interfaces, but a loopback

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 interface that we're going
 to talk about here is not.

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 You could have a router or a switch where
 every single interface is disabled

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 and down. Nothing's physically plugged
 into that switch, but your loopback

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 interface could still
 be up and operational.

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 Now, why do we use a
 loopback interface?

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 Well, there's a lot of reasons.

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 So like it says here, we typically used
 to host the management IP address

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 for a device. So a lot of times people
 will say, oh, okay, I want this

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 router to be 111.

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 I want this router to be 112.

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 This one, 1113. So I want to apply an IP
 addressing scheme that's independent

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 of my actual networks are carrying
 data, and I want that to be used to

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 manage to like telnet
 or SSH to the device.

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 So a place in that IP address on a
 loopback interface is a great idea

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 for that. It's also great for lab environments
 to create extra networks

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 for learning routing protocols.

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 Let's say that we have a lab environment
 where I'm a router, you're a

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 router, and that's it.

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 It's just the two of
 us with a cable.

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 And I want to go on my router and I want
 my RIP or OSPF or BGP to advertise

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 like 15 different networks to you.


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 Well, how are we going to do that if
 there's just one cable between us?

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 We can use loopback interfaces because
 when you create a loopback interface

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 and you put an IP address on a physical
 interface, they had a cable.

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 So I can just create 15 or 20 loopbacks
 with different networks on them.

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 And now I've got some stuff I can advertise
 to you to play around with.

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 And some protocols such as OSPF use the
 IP address on your loopback interface

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 as part of their processing rules.


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 So how do we create a
 loopback interface?

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 Real simple. So here it is before.


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 All right. All we do is a global configuration
 level, just type interface,

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 loopback, and then a number.

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 Most people typically start at loopback
 zero, but you don't have to.

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 You could do loopback 200, but the
 number is completely irrelevant.

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 The line, notice the line protocol,
 even before I put an IP address on

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 there, instantly goes up because that
 loopback interface is not dependent

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 on anything to bring up
 as line protocol status.

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 Then I add an IP address of 1111 1111
 and now under show IP interface

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 brief at the bottom, we actually see
 that loopback interface showing up

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 there. And if I looked at the routing
 table, if I did a show IP route,

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 we would now see a connected route
 for the 1111 1111.0 network.

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 All right. So what does this
 have to do with OSPF?

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 Well, OSPF, when you first started
 up, one of the very first things it

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 does, it elects a router
 ID for itself.

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 Think of this as being like
 a descriptive name.

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 So whenever this router creates an LSA
 describing itself, it's going to

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 plug that router ID in there
 as the advertising router.

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 So all the routers out there will
 know about you as your router ID.

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 So the router ID is also
 important in another way.

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 Not only does it describe you to the
 rest of the area so they know who

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 you are, but there's some election processes
 that take place between routers.

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 So when I'm forming a neighbor relationship
 with somebody, that router

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 ID is critical into determining who's
 going to play certain types of OSPF

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 roles. So how does OSPF come
 up with a router ID?

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 Well, this is the process
 that it goes through.

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 Number one, you can configure it.

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 Under the OSPF process, there's a router
 ID command where you can type

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 in the router ID.

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 So that's the first preference.

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 OSPF will look to see if you've
 manually added a router ID.

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 If you have not, then OSPF will look to
 see if you have any loopback interfaces

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 like we were just talking about.

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 And it'll pick the highest, the numerically
 highest IP address from among

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 your loopbacks. Now, let's say you
 don't have any loopback interfaces,

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 then OSPF will look at your
 physical interfaces.

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 Any physical interface that is not administratively
 down will be a candidate.

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 And so if you have no loopbacks, you'll
 pick the highest IP address from

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 any of your physical interfaces or your
 sub interfaces as long as they're

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 not shut down. As I mentioned, you can
 configure the router ID using the

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 router-id command under
 the OSPF instance.

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 Okay, so in broadcast and non-broadcast
 multi-access networks.

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 Now, you're probably not going to see
 non-broadcast multi-access networks

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 very much. If you're still dealing with
 frame relay networks, which are

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 a type of a WAN network, those are
 considered non-broadcast by OSPF.

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 But more and more if time goes on, Ethernet
 and all the flavors of Ethernet

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 are becoming more and more
 ubiquitous among networks.

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 And that's becoming like the only network
 type that people use these days.

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 And Ethernet is considered
 a broadcast network type.

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 But either way, OSPF elects
 a DR and a BDR.

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 So what is the purpose of this?

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 How does this work?

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 Well, here's how it works.

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 So imagine your typical environment
 where you have an Ethernet switch

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 and you have several routers
 connected to Ethernet switch.

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 Let's say router one, router two,
 router three, and router four.

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 Now, let's assume that the links to
 those routers are all in the same

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 VLAN. So we'll say this guy is 1.1
.1.1, 1.1.1.2, 1.1.1.3, and 1.1.1.4.

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 That's the actual IP address on their
 actual interfaces themselves.

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 OK, now when you enable OSPF on an
 interface, one thing the OSPF does

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 is OSPF has a priority,
 an interface priority.

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 Now, by default, the
 priority is one.

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 So I'll just put that right here.

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 OSPF interface priority is
 the same on all these guys.

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 OK, so what's going to happen here is
 that as these routers boot up, one

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 of them is going to be elected
 as the designated router.

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 Now, why do we need
 a designated router?

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 Well, let's contrast this with
 EIGRP as a routing protocol.

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 Now, in the world of routing with EIGRP,
 if all these routers here were

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 running EIGRP, they would all form
 neighbors with each other.

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 So if I was on router one, for example,
 I would see that I had three neighbors,

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 router two, three, and four.

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 And in the world of EIGRP, if router
 one learned of some new network via

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 that way, he would send that update
 into the switch, which would get to

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 router two, router three,
 and router four.

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 And then each one of those routers
 would be responsible for sending an

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 acknowledgement back to router
 one, saying, yep, I got it.

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 If one of the routers did not send an
 acknowledgement, router one would

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 resend that update specifically to that
 EIGRP neighbor, saying, hey, never

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 got an acknowledgement from you.

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 Here's some new information.

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 The desires of OSPF took
 a different approach.

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 So the desires of OSPF said, hey,
 here's what we're going to do.

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 One router is going to be elected
 as the designated router.

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 Another router is going to be elected
 as the backup of the designated

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 router. Then all the routers in this
 environment are going to form a fully

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 synchronized neighbor relationship
 with the DR and the BDR.

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 Not amongst each other.

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 They'll see each other's hello packets,
 but they're not going to exchange

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 database descriptors.

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 They're not going to change link state
 requests and link state updates

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 with each other.

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 They're only going to do that
 with the DR and the BDR.

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 Now, in this environment, how do we
 determine who that's going to be?

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 Well, the default is whoever's got the
 highest OSPF priority, that's number

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 one. So if one of these guys had a priority
 other than one, if somebody

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 was higher than that,
 they would win.

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 They would take on the role
 of the designated router.

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 If their priorities are the same, then
 it falls back to that router ID

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 we were just talking about.

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 So it's not the IP address.

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 So if I put in some different numbers
 in here for router IDs, let's say

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 we've got seven, seven, three, three,
 let's say we've got one dot two

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 dot one dot seventy seven.

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 Let's say we have nine dot nine dot
 one dot one and two dot two dot two

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 dot two. So in this environment, because
 they all have the same interface

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 priority, our designated router
 would end up being router one.

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 He's got the highest router ID.

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 So he would be my designated router
 and router three would end up being

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 my backup designated router because
 he has the second highest router ID.

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 Now that we know that, what we're going
 to see is, let's start with router

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 two. Router two will form a fully synchronized
 neighbor relationship with

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 router one and with router three.

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 Same thing same thing is going
 to happen with router four.

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 He's going to form a fully synchronized
 neighbor relationship with router

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 one and router three.

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 Matter of fact, router one and router
 three will form fully synchronized

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 neighbor relationships
 with each other.

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 Now, as far as what's going on between
 router two and router four, they

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 will see each other's hellos, but that's
 as far as they're going to go.

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 They will not exchange data based
 descriptors with each other.

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 They will not exchange
 link state requests.

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 So, if I was in, for example, router
 four, and in router four, I did the

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 command show IP OSPF neighbor.

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 Let's just get rid of
 some of this here.

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 So, I've got a little
 bit of space.

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 So, router four, I would see this.


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 I would see router one which would display
 as the designated router and

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 I am in the full state fully
 synchronized with him.

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 All of his LSAs match
 all of my LSAs.

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 Router three would display as
 the backup designated router.

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 Also in the full state, router two
 would display as a DR other, which

0:12:38.660000 --> 0:12:42.840000
 means he is neither the BDR.

0:12:42.840000 --> 0:12:47.060000
 He could become that in the future
 if the DR, the BDR drops out of the

0:12:47.060000 --> 0:12:51.420000
 picture, router two could take their
 spot, but he is right now neither.

0:12:51.420000 --> 0:12:55.220000
 And I am in the two-way state with him,
 which means I'm just exchanging

0:12:55.220000 --> 0:12:55.980000
 hellos with him.

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 I have not become fully synchronized
 with that guy.

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 And so now if, let's just take
 router two here for a second.

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 Let's say that router two is on the
 back end a neighbor with router X.

0:13:13.160000 --> 0:13:16.740000
 Okay? And router X sends him some sort
 of a link state update about some

0:13:16.740000 --> 0:13:18.120000
 new information.

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 Router two is going to forward that
 link state update to the DR and the

0:13:24.520000 --> 0:13:29.660000
 BDR. And to make sure that only they
 get it, he's going to send that to

0:13:29.660000 --> 0:13:37.120000
 224.006. That's a special multicast
 address that only the DR and the BDR

0:13:37.120000 --> 0:13:39.360000
 are actually listening to.

0:13:39.360000 --> 0:13:43.540000
 So router four is not going to pay
 attention to that because he's not

0:13:43.540000 --> 0:13:44.660000
 one of those roles.

0:13:44.660000 --> 0:13:48.480000
 So router one and router three will
 get that link state update to 224

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.006. Now that they've gotten it as
 it is the DR's responsibility, you

0:13:55.700000 --> 0:13:58.320000
 know, the router three will
 basically say, thank you.

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 And then just go silent.

0:13:59.680000 --> 0:14:03.480000
 Router one who's the DR will say, okay,
 I have to make sure that everybody

0:14:03.480000 --> 0:14:08.240000
 else in this particular broadcast domain
 in this VLAN also knows about

0:14:08.240000 --> 0:14:13.960000
 this. So he'll turn around and reflect
 that update right back out to 224

0:14:13.960000 --> 0:14:18.820000
.005. Well, it'll get
 to router two.

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 Router two will say, I've
 already got this.

0:14:20.740000 --> 0:14:23.260000
 Ignore it. It'll get
 to router three.

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 Route three will say,
 I've already got this.

0:14:24.960000 --> 0:14:30.000000
 Ignore it. But now router four
 can hear it for the first time.

0:14:30.000000 --> 0:14:36.480000
 So that's the idea behind a DR and
 a BDR in a broadcast environment, i

0:14:36.480000 --> 0:14:42.280000
.e. Everybody synchronizes
 with the DR and the BDR.

0:14:42.280000 --> 0:14:45.260000
 And when they learn of new information,
 they send it to the DR and the

0:14:45.260000 --> 0:14:49.360000
 BDR. And that's the responsibility
 of the designated router to reflect

0:14:49.360000 --> 0:14:58.480000
 that back and make sure that everybody
 is synchronized with that information.

0:14:58.480000 --> 0:15:01.960000
 So that's what we discussed
 right there.

0:15:01.960000 --> 0:15:04.580000
 And I've already mentioned this.

0:15:04.580000 --> 0:15:10.620000
 That the router with the highest priority
 becomes the designated router.

0:15:10.620000 --> 0:15:15.740000
 And the way you can influence that is
 if I was under router, I said, hey,

0:15:15.740000 --> 0:15:18.740000
 I really do want this router to
 become the designated router.

0:15:18.740000 --> 0:15:22.880000
 Then you would go on to an interface
 like interface, get the data, and

0:15:22.880000 --> 0:15:24.220000
 then you would go on
 to an interface.

0:15:24.220000 --> 0:15:30.800000
 And the command would
 be ipospf priority.

0:15:30.800000 --> 0:15:35.460000
 As long as he gave it something higher
 than one, he'll beat all the other

0:15:35.460000 --> 0:15:37.500000
 routers that have the default.

0:15:37.500000 --> 0:15:43.020000
 Now one thing to be aware of.

0:15:43.020000 --> 0:15:45.720000
 Now let me just see if this is coming
 up in the next slide before I talk

0:15:45.720000 --> 0:15:54.900000
 about it. Okay, so we're not it's not
 in the slides there so one thing

0:15:54.900000 --> 0:15:57.060000
 to be aware of is that.

0:15:57.060000 --> 0:16:01.960000
 Once the designated router and the backup
 doesn't eat router is elected

0:16:01.960000 --> 0:16:04.340000
 but this is focused
 on the DR right now.

0:16:04.340000 --> 0:16:09.600000
 It's kind of sticky in other words what
 I mean by that is, let's say that.

0:16:09.600000 --> 0:16:13.320000
 Let's say that you and I were connected
 to a switch and it's just you

0:16:13.320000 --> 0:16:17.180000
 and I and so we decide that you'll be
 the designated router I'll be the

0:16:17.180000 --> 0:16:19.540000
 backup because you have a
 higher router ID than me.

0:16:19.540000 --> 0:16:23.740000
 Great now let's say that your router
 ID is one dot one dot one dot four

0:16:23.740000 --> 0:16:27.860000
 and I'm one one one one one that's
 why you won because you're one one

0:16:27.860000 --> 0:16:29.600000
 one four okay great.

0:16:29.600000 --> 0:16:32.360000
 Let's say that in a few minutes or
 a few hours all of a sudden another

0:16:32.360000 --> 0:16:37.760000
 router joins the same switch and his
 router ID is higher than yours.

0:16:37.760000 --> 0:16:39.880000
 He's like one one one five.

0:16:39.880000 --> 0:16:44.580000
 It's too late. Once you're elected as
 the DR anybody else that comes in

0:16:44.580000 --> 0:16:48.060000
 after that point in time whether they've
 got a higher priority or they

0:16:48.060000 --> 0:16:52.540000
 have a higher router ID they cannot
 take over that role from you.

0:16:52.540000 --> 0:16:55.760000
 That's why I said it's sticky that
 roles are like sticks to you.

0:16:55.760000 --> 0:17:00.240000
 Now clearly if your interface goes down
 or if you crash and die well then

0:17:00.240000 --> 0:17:05.060000
 that guy now has the opportunity to
 take over the role but the DR sticks

0:17:05.060000 --> 0:17:11.640000
 to whoever gets elected
 to that position.

0:17:11.640000 --> 0:17:16.240000
 All right so here are some show commands
 you can use to view who your

0:17:16.240000 --> 0:17:22.180000
 various neighbors are and what their
 what their state is show IP OSPF

0:17:22.180000 --> 0:17:26.660000
 database will show you your LSA's show
 IP route OSPF will display the

0:17:26.660000 --> 0:17:29.780000
 routing table but only the
 OSPF learned routes.

0:17:29.780000 --> 0:17:36.920000
 And these are a couple of useful debug
 you can use to troubleshoot OSPF.

0:17:36.920000 --> 0:17:41.720000
 So that concludes this particular video
 on OSPF designated routers backup

0:17:41.720000 --> 0:17:44.920000
 designated routers and
 how they are elected.

0:17:44.920000 --> 0:17:45.780000
 I hope you found it useful.