WEBVTT

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 Hello and welcome this video on a review
 of how to implement OSPF in Cisco

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 IOS. I'm going to show you the basic
 commands to turn it on and verify

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 that it's working.

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 All right, so step number one, there's
 actually a couple of ways you can

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 go about this for a couple of decades
 after OSPF first came out.

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 The way to do it which we now consider
 as a legacy method, was you first

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 went to your global configuration level
 and you started your OSPF process.

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 So you would say router OSPF and
 then type in a process ID.

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 Now this process ID, people typically
 start with number one, like router

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 OSPF one. It can be
 anything you want.

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 I mean within reason if you do router
 OSPF question mark, it'll show you

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 the range of numbers
 you can select from.

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 But the point is, whatever process ID
 you select, this is not something

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 you exchange with other routers.

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 This number is only something that
 your own local router knows about.

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 We call that a locally
 significant value.

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 So one router could be router OSPF
 one, another router could be router

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 OSPF seven. It doesn't matter, it's
 not going to influence how they do

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 OSPF. But from a practical standpoint,
 if you start doing that having

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 router OSPF one, router OSPF seven, router
 OSPF 32, if anybody else jumps

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 on your routers and they do a show run
 and they look at the show run output,

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 it might confuse them.

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 They might think, is there some reason
 why this one is one, 17, 32, 220?

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 So you don't want to confuse them.


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 You don't want to make them think that
 there's something more to that

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 number than there actually is.

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 So most people just usually pick a
 number, like I said, usually number

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 one and you just have all the routers
 configured as router OSPF one.

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 Once you do that, then it drops you
 down into config-router mode.

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 And now here is where you activate
 OSPF on your various interfaces.

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 So for example, if this was my router
 right here and you had two interfaces,

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 let's say this was the 2.2.2.2.2
.24 and this was 70.70.1.1.1.16.

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 Well, if I want to activate OSPF on
 the 2.2 interface right there, then

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 I would say network.

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 Now you have a couple
 of options here.

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 You could actually type in the network
 itself, like we know that with

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 this subnet mask, the
 network is 2.2.2.0.

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 You could do that.

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 And then if you want to do that, you
 would add a wildcard mask after that,

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 which would be 0.0.0.255.

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 Remember that a wildcard mask is basically
 the inverse of a subnet mask.

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 Slash 24 means 255, 255, 255, 0.

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 Well, a wildcard mask is just
 the opposite of that.

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 You can absolutely do that.

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 Or another viable alternative that
 a lot of people do is they actually

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 put in the actual host IP address on
 their router and then they put in

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 a wildcard mask of all zeros.

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 That will work as well.

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 So you've got to put in something in
 a wildcard mask, but how specific

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 or how less specific that wildcard
 mask is is completely up to you.

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 The main point is the network statement
 is what OSPF uses to look around

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 and say, OK, do I have any interfaces
 that have that network?

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 If I do, I can activate OSPF on those
 interfaces, and I can include that

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 network in my LSA.

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 Now, like I said, that's more
 of the legacy method.

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 Typically, the newer and more current
 method is that you don't even have

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 to do this stuff at all.

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 You just go right to the interface,
 like interface gigabit 0 slash 1,

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 and you activate OSPF right there.


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 You say IP OSPF 1 area 0
 or something like that.

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 And if you don't already have a router
 OSPF process, this will dynamically

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 create one for you.

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 And this way, you don't have to worry
 about wildcard masks either, which

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 is kind of nice.

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 OK, now, like I mentioned, when you
 when you enable OSPF directly on an

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 interface or use the network command
 to reference an interface, that means

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 that OSPF is going to start sending
 out hello packets on that interface.

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 Now, sometimes you don't
 want OSPF to do that.

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 For example, let's say that you had
 two routers right here and they were

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 connected via a trunk.

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 So these routers are using multiple sub
 interfaces to talk to each other.

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 Maybe this line right here represents
 fast ethernet 0 slash 0 dot 1.

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 This line right here represents fast
 ethernet 0 slash 0 dot 2 and so on

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 and so forth. Fast ethernet
 0 slash 0 dot 3.

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 OK, now, let's just put a couple
 of networks on here.

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 Let's say that the the pink line is
 the 1 dot 1 dot 1 dot 0 network.

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 And let's say that the blue line was
 the 2 dot 2 dot 2 dot 0 network.

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 OK, if I went into each of these sub
 interfaces and I did IP OSPF 1 area

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 0, the next sub interface, IP OSPF 1
 area 0, or I used a couple of network

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 statements. Network 1 dot 1 dot 1 dot
 0 and then my wild card network

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 2, 2, 2, 2, then that means that OSPF
 Hello's in this case, because I

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 have three sub interfaces.

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 If I did that on all three of them, that
 means OSPF Hello's would go across

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 that one physical
 link three times.

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 We'd have an OSPF adjacency formed across
 the 1, 1, 1 network across the

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 2, 2, 2 network and across
 the 3, 3, 3 network.

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 On the bottom. Well, certainly we want
 those routers to, you know, they're

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 directly connected, right?

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 They don't need to learn that, hey, I'm
 direct, that my neighbors connected

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 to this. We really just need one OSPF
 neighbor relationship between these

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 routers, because there's
 just one link here.

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 OK, as long as one set of Hello's is
 going back and forth, that's good

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 enough. If they form, for example, if they
 form just one neighbor relationship

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 across the pink sub interface, the 1,
 1, 1, 0 interface, once that neighbor

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 relationship is formed, and let's
 just say router 1 sends an LSA.

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 So let's just say right here, going
 across the pink sub interface, this

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 is a link state update.

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 And inside there, he's got his
 router LSA describing himself.

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 Well, if we assume that all these sub
 interfaces are in the exact same

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 area, which they probably are, that
 one router LSA will say, hey, router

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 2, let me tell you about myself.

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 My name is router 1, and I have these
 links I'm connected to, the 1, 1,

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 1, the 2, 2, 2, the 3, 3, 3.

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 Then router 2 will say, oh,
 hey, how funny is that?

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 I've got the same links.

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 So they will learn about each other,
 they will learn about what links

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 they have, and they did not need three
 simultaneous neighbor relationships

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 across one physical cable
 in order to do that.

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 So how do we suppress those
 neighbor relationships?

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 How do we tell OSPF, for example,
 on router 2 over here?

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 How do we tell him, hey, I want you
 to form a neighbor relationship on

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 fastEthernet 0 slash 0 dot 1, but don't
 send out hellos on 0 slash 0 dot

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 2 We use a feature that's common to all
 routing protocols, which is called

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 passive interface.

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 When you make an interface passive, it
 suppresses hellos from going across

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 that interface. Like in this example right
 here, on the router X is fastEthernet

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 0, 0, we do want that network advertised
 up to the cloud, the cloud needs

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 to know about that within an LSA, but
 router X doesn't really have any

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 need to send an OSPF
 hello down this way.

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 There's just PCs and
 laptops down there.

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 So when we make passive interface fastEthernet
 0, 0, now that says, okay,

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 you can still advertise that.

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 Right now we're assuming we had
 a network statement, right?

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 If this was the 3, 3, 3, 0 network,
 we're assuming that we did one of

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 two things. We either went interface
 fastEthernet 0 slash 0, and then

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 we enabled OSPF.

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 Area 0. Okay, we did that way, or we
 did router OSPF 1, and then we used

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 a network statement network
 3, 3, 3, 0, 0, 0, 0, 2, 55.

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 Area 0. We have to do one of those two
 things to tell OSPF, hey, include

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 this network in your LSA,
 in your link state update.

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 But by making it passive, we say, hey,
 just don't send hellos out there.

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 There's nobody to talk to, there's nobody
 to form a neighbor relationship

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 without there, just don't do it.

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 So just like in my previous whiteboarding
 example, we could do the same

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 thing on our subinterfaces.

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 I could say passive dash interface fastEthernet
 0 slash 0, 0 dot 2, passive

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 dash interface fastEthernet
 0 slash 0 dot 3.

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 And now we would not form multiple simultaneous
 OSPF adjacencies between

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 the same two routers across
 the same cable.

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 All right, we talked
 about the router ID.

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 So within your router OSPF process,
 here's how you can deterministically

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 set the router ID
 with the command.

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 You can also do the show IP OSPF command
 to see what your router ID is.

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 And that last bullet point
 is also important.

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 Once a router figures out his router
 ID, even if you try to reconfigure

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 it, he's not going to
 change it on the fly.

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 For example, let's just
 take a look right here.

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 Show IP OSPF. All right, so we can
 see this router has formed a router

0:10:43.800000 --> 0:10:47.740000
 ID of 122.122.2.

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 I say, oh, no, I don't want that.

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 I want you to be a 777.

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 So I say router OSPF1
 router dash ID 777.

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 Now, this is kind of interesting because
 usually what I see is the moment

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 I give it a router ID command, it'll
 give me a warning and say, you know,

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 you must clear the OSPF process or
 restart in order for new router ID

0:11:20.800000 --> 0:11:23.900000
 to take effect. Here's what
 I think is happening.

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 This particular router doesn't
 have any neighbors.

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 See, no neighbors.

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 So as long as he doesn't have neighbors,
 I think I can change the router

0:11:35.720000 --> 0:11:38.120000
 ID. Yep, see right here?

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 I can change on the fly
 without a problem.

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 But the moment he forms a neighbor relationship
 with anybody else, then

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 he will not allow me to do that.

0:11:48.580000 --> 0:12:08.340000
 So let's just quickly form a neighbor
 relationship so we can do this here.

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 All right. Show IP
 interface brief.

0:12:18.020000 --> 0:12:44.480000
 Okay, I think I've got the wrong
 IP address on somebody.

0:12:44.480000 --> 0:12:47.480000
 So just to show you what
 I'm doing at this point.

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 Okay, neighbor relationship
 is built.

0:12:50.000000 --> 0:12:53.020000
 So we had router two.

0:12:53.020000 --> 0:12:59.980000
 And I had enabled OSPF on his
 gig zero slash one interface.

0:12:59.980000 --> 0:13:04.860000
 Which was configured for something
 like 122 dot something.

0:13:04.860000 --> 0:13:08.240000
 Well, he hadn't formed any
 neighbor relationship yet.

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 That's why he was allowing me to change
 the router ID without any warnings

0:13:11.560000 --> 0:13:15.620000
 or complaints. So I decided, oh, okay,
 well, I know on the other side

0:13:15.620000 --> 0:13:18.180000
 of this link is another router.

0:13:18.180000 --> 0:13:24.000000
 CSR one. Let's just form a neighbor
 relationship with him.

0:13:24.000000 --> 0:13:27.580000
 So I activated OSPF on
 this interface as well.

0:13:27.580000 --> 0:13:29.660000
 Neighbor relationship
 was not forming.

0:13:29.660000 --> 0:13:32.640000
 And the reason for that was their
 IP addresses were mismatched.

0:13:32.640000 --> 0:13:37.020000
 This guy was configured
 as 21 21 21 dot one.

0:13:37.020000 --> 0:13:39.080000
 Which was completely wrong
 with the other side.

0:13:39.080000 --> 0:13:43.060000
 So I had to go to this side
 and make him match.

0:13:43.060000 --> 0:13:48.020000
 And then the OSPF neighbor
 relationship came up.

0:13:48.020000 --> 0:13:53.180000
 Okay, right now we're having a network
 type mismatch because in a previous

0:13:53.180000 --> 0:13:58.600000
 video. I actually changed the
 network type on his interface.

0:13:58.600000 --> 0:14:03.640000
 The default is broadcast and I told
 him it should be point to point.

0:14:03.640000 --> 0:14:06.140000
 So they do need to match.

0:14:06.140000 --> 0:14:24.340000
 Okay. So now if you do
 show IP OSPF neighbor.

0:14:24.340000 --> 0:14:27.460000
 We see we do have a neighbor as it
 so happens the neighbor on the left

0:14:27.460000 --> 0:14:30.640000
 CSR one is our designated router.

0:14:30.640000 --> 0:14:37.400000
 And so now if I try to change the router
 ID router OSPF one router dash

0:14:37.400000 --> 0:14:40.240000
 ID, let's just make
 it two two two two.

0:14:40.240000 --> 0:14:42.440000
 Now I get that warning.

0:14:42.440000 --> 0:14:46.800000
 It says, Hey, everybody already knows
 me as seven seven seven seven.

0:14:46.800000 --> 0:14:49.000000
 Everybody's already got
 my LSA out there.

0:14:49.000000 --> 0:14:53.300000
 If I change my router ID, that means
 I'm going to have to tell them that

0:14:53.300000 --> 0:14:56.280000
 old LSA is bad in what's
 called poisoning it.

0:14:56.280000 --> 0:14:58.240000
 I'm going to poison the old LSA.

0:14:58.240000 --> 0:15:01.160000
 Now I'm going to have to resend
 a new LSA with my new name.

0:15:01.160000 --> 0:15:04.720000
 That's going to be very intrusive
 for me and everybody else.

0:15:04.720000 --> 0:15:08.640000
 If you want me to do that, you really
 got to be sure and you have to clear

0:15:08.640000 --> 0:15:09.840000
 the OSPF process.

0:15:09.840000 --> 0:15:12.160000
 You have to restart everything
 from scratch.

0:15:12.160000 --> 0:15:16.160000
 I say, Okay, see right
 now do show IP OSPF.

0:15:16.160000 --> 0:15:20.880000
 He's still using seven seven seven seven
 seven even though I configured

0:15:20.880000 --> 0:15:25.300000
 this. Yes. So in order for him to take
 that over, I have to type in clear

0:15:25.300000 --> 0:15:27.460000
 IP OSPF process.

0:15:27.460000 --> 0:15:30.160000
 Well, I got to get out
 of config mode first.

0:15:30.160000 --> 0:15:37.940000
 Yes. So now he's going to tear down
 his relationship with CSR one.

0:15:37.940000 --> 0:15:41.420000
 There it goes. He'll
 rebuild it again.

0:15:41.420000 --> 0:15:45.340000
 And while he's doing that.

0:15:45.340000 --> 0:15:48.960000
 Oh, that's not what I want.

0:15:48.960000 --> 0:15:54.800000
 All right, let's just do it.

0:15:54.800000 --> 0:15:57.440000
 So show IP OSPF.

0:15:57.440000 --> 0:16:02.280000
 Now we can see that because
 I restart the process.

0:16:02.280000 --> 0:16:13.420000
 Now he was able to take on that new router
 ID that we were talking about.

0:16:13.420000 --> 0:16:17.200000
 Who talked about using the
 IP OSPF priority command.

0:16:17.200000 --> 0:16:20.580000
 But like I said, that
 is not preemptive.

0:16:20.580000 --> 0:16:28.060000
 What do I mean by that show
 show IP OSPF neighbor.

0:16:28.060000 --> 0:16:31.440000
 Notice that my neighbor
 on gig zero one.

0:16:31.440000 --> 0:16:33.280000
 He is the designated router.

0:16:33.280000 --> 0:16:37.080000
 I say, well, I'll show you interface
 gigabit zero slash one.

0:16:37.080000 --> 0:16:39.440000
 Okay. So he's got
 a priority of one.

0:16:39.440000 --> 0:16:43.080000
 All right. IP OSPF priority.

0:16:43.080000 --> 0:16:51.940000
 About 200. That should make me
 the designated router, right?

0:16:51.940000 --> 0:16:57.120000
 Nope, it does not.

0:16:57.120000 --> 0:17:00.680000
 And if we go over to the other guy, you
 say, well, hey, Keith, maybe he's

0:17:00.680000 --> 0:17:02.380000
 not seeing you as 200.

0:17:02.380000 --> 0:17:05.140000
 Right? Maybe he maybe he
 didn't take that command.

0:17:05.140000 --> 0:17:08.720000
 Yeah, he took the command.

0:17:08.720000 --> 0:17:13.480000
 Right on this guy says, Hey, all the
 router, you're the backup Disney

0:17:13.480000 --> 0:17:16.960000
 router. Hey, even though you got a higher
 priority than me, I beat you.

0:17:16.960000 --> 0:17:18.920000
 I became the Disney
 router before you.

0:17:18.920000 --> 0:17:23.320000
 So you can't take it over for me.

0:17:23.320000 --> 0:17:28.840000
 And we've just looked
 at those commands.

0:17:28.840000 --> 0:17:33.380000
 So thank you for watching this video
 that concludes this video on the

0:17:33.380000 --> 0:17:37.440000
 initial implementation
 of OSPF in Cisco iOS.