Multiarea OSPF and LSA Types

0:00[AUDIO LOGO]

0:07Welcome to multi-area OSPF and LSA types.

0:11In this skill, we're going to jump in and look

0:15at some of the advantages and, well,

0:17a couple of the disadvantages to running

0:19OSPF using multiple areas rather than just a single area.

0:25We're also going to take a look at the LSA types that

0:29operate within the area, between the areas, some of our extras.

0:33So we're going to talk about all the different LSAT types

0:37and their role in our OSPF environment.

0:40So we have a lot of stuff here we need to cover.

0:42Let's go ahead and jump in and get started.

0:44I'll see you in the next video.

0:00[AUDIO LOGO]

0:07In this video, we're going to start off

0:09by looking at single area OSPF.

0:13We'll take a look at what the database looks like,

0:15as well as what happens if there's

0:17a failure in single area OSPF.

0:21We'll also, of course, talk about some

0:23of the advantages and disadvantages of running

0:26OSPF in a single area.

0:28So let's go ahead and jump in and get

0:30started by talking about some of the attributes of single area

0:34OSPF.

0:36So the first thing we want to consider

0:38when talking about using, particularly single area

0:42OSPF is it's main issues come into play with scalability.

0:49The real issue here is when our network

0:52starts getting very large, single area just

0:56really doesn't cover it.

0:57There are, of course, a few reasons for this.

1:00The first is that the Dijkstra algorithm really

1:04can cause high CPU utilization.

1:07Remember, it has to run against the entire database

1:12for the area.

1:14And because of that, it actually makes the running

1:17of OSPF proportional to the square

1:21of the number of routers.

1:24So the more routers there are in an area,

1:26that means the database is going to be larger.

1:29And overall, it means that Dijkstra

1:33is going to take a lot more horsepower

1:35to run on our devices.

1:37And going along with that, since each router must

1:42hold the complete link state database for that area,

1:47it means that it can also cause high memory use.

1:52Now, just as a note on this topic, when we say high memory

1:55use on today's devices, honestly this is probably not

1:59going to be an issue.

2:01But it is something to keep in mind, particularly if you have

2:04a very large environment or if maybe you

2:07have some dated routers that are still in your environment.

2:11But overall, really neither one of these two issues

2:15is really going to be a huge concern on today's routers

2:19until our network starts getting very large.

2:22But these are some things we need

2:24to keep an eye on as our OSPF environment gets larger.

2:28So we're going to jump over to the command line

2:31and take a look at what the database looks like

2:34and what failures look like in an OSPF single area.

2:38To do that, we're going to be using this topology.

2:42As we can see, this is all of our routers,

2:45router 1 through router 10, all being in a single area zero.

2:50So this is not an exceedingly large environment.

2:53Again, it's just 10 routers.

2:55However, we'll still see how every single one

2:59of these devices is going to be receiving all of the LSAs

3:03from all of the other routers.

3:06So every single router is going to have every other routers LSA

3:09information in its database.

3:12So we'll see this as we look at some of our different routers.

3:16Let's jump over to the command line and take a look.

3:19So let's start off here on router 10,

3:22and let's just start with a show IP route OSPF.

3:26And what we see here is that we have all OSPF intra area

3:31routes.

3:32We can tell because our key here is just

3:36an O where it would be followed by something else like an IA

3:41or an E1 or any E2, if this was external and so on.

3:46So we can see by the fact that there's nothing after the O,

3:50that these are all routes from within the same area.

3:53And this applies to all the routes.

3:55If we do a show IP OSPF database,

3:58we can see that all we have are type 1 and type 2 LSAs.

4:05These are known as router links states and network link states.

4:10We don't have any LSAs that would

4:12be coming from other areas.

4:14This is, of course, because everything's in this area.

4:18We will talk more about these link

4:20state types in an upcoming video.

4:24So now let's take a look at what happens if we

4:27trigger SPF to actually run.

4:30Let's start with a show ip ospf.

4:33And what we want to note here is that for area zero,

4:36currently SPF has executed 17 times.

4:41And this is simply how many times

4:43it executed while this particular lab environment was

4:47being built. So let's take a look

4:49at what happens when a change occurs in OSPF.

4:53Let's turn on a debug ip ospf spf.

4:58So we have this on.

4:59Let's now switch over to router 5.

5:02Here we'll go into configuration mode.

5:04And we'll simply go in to say it's loopback

5:07and shut that interface down.

5:09We'll switch back over to router 10,

5:11we'll scroll up a little bit here.

5:13And what we see as we start going down

5:16through the database, obviously a lot is going on.

5:20There are several pages here.

5:21We're not going to look at every detail of this.

5:23But what we want to see is that SPF is definitely executing,

5:28and it's checking every single route in the database.

5:32So we can see here, as we scroll down through this,

5:35this is covering the entire OSPF domain

5:40because it's all in one area.

5:42So we're forcing SPF to run against the entire database

5:47here.

5:48And as you can see, this is quite a bit of work.

5:51There's quite a bit going on here in the debug.

5:54And if we look at the very end, we

5:56can see that this actually took on this equipment

5:5917 milliseconds.

6:01Now, that's of course not a whole lot.

6:03But let's see how that stands up when we get to multi-area OSPF

6:09next, and we'll see if it's any faster when it's converging

6:13on a much smaller area.

6:15Now, it might not be much faster, again,

6:17this is a relatively small lab environment.

6:20But let's see if we get any performance increase

6:22when we go multi-area.

6:24The main thing, of course, however, regardless

6:27of how long it actually took, is if we do our show ip

6:31ospf again, notice that now, SPF algorithm has executed

6:3618 times, which means it did in fact run again

6:40in response to this change.

6:42And of course, if we bring that interface back up, so

6:45back over here on router 5, we'll simply do a no shut.

6:49Back over on router 10, we can see that it once again

6:52executed SPF.

6:54We don't have to scroll through the whole thing again.

6:57I think you get the idea.

6:59This time it only took 14 milliseconds,

7:01so it was a little bit faster.

7:03And again, if we look at our show ip ospf,

7:06we see that it's now executed 19 times.

7:09So this is definitely executing SPF any time anything changes

7:15in the area.

7:16And this is part of what leads to that scalability problem.

7:21In this video, we talked about, well, mostly

7:25the disadvantages of running a single area in OSPF, mostly

7:31focused around that scalability issue.

7:34But keep in mind, like most things,

7:36there are advantages to running single area OSPF as well.

7:41First, it's definitely easier to configure.

7:44And it's certainly easier to design,

7:47since it's literally just a single area.

7:50And it also has some advantages because certain technologies

7:55such as MPLS traffic engineering actually function better

7:59in a single area environment.

8:02And when you go multi-area, it gets a bit more complicated.

8:06So like most things, it certainly

8:09has advantages and disadvantages.

8:12And we'll talk about this a little bit more

8:14in our next video as we jump into multi-area OSPF.

8:20I hope this has been informative for you.

8:22And I'd like to thank you for viewing.

0:00[AUDIO LOGO]

0:07So now, let's jump in and talk about multi-area OSPF.

0:12This is going to be used two address our scalability issues

0:16with OSPF.

0:18Now understand, this still won't scale as large as something

0:21like BGP.

0:23It still can't carry hundreds of thousands or millions

0:26of routes.

0:27However, it will let us scale it up to a much larger environment

0:32than we could with just a single area.

0:35It also brings to the table the ability

0:38to do route summarization.

0:40Keep in mind as we talked about in the previous video,

0:44all of the routers in an area have

0:47to have the same exact link-state database.

0:50And, of course, a side effect of that is I can't have one router

0:55have a /16 for a network while another router has a /24.

1:00So since they have to have identical databases,

1:04we cannot summarize within an area.

1:07So we have to have multi-area OSPF in order

1:11to do proper summarization.

1:13That also implies that our summarization

1:16points in our network also have to fall on area boundaries.

1:21So let's go ahead and take a look at some of these things.

1:24We'll start by taking a look at some

1:26of the attributes of multi-area OSPF.

1:28And then, of course, we'll jump in

1:30and take a look at what our databases look

1:32like with multiple areas.

1:34Let's get started.

1:36So as we said during the introduction,

1:39the biggest thing that multi-area OSPF brings to OSPF

1:44is really that scalability.

1:46And again, the reason this helps is because remember,

1:50that all of the routers in a given area

1:52have to have identical link-state databases.

1:56And by having multiple areas, that

1:59means that SPF is only run for database changes

2:05within that area.

2:07So the way that it works in between areas

2:10is we only carry prefix information

2:14between those areas.

2:16We do not carry the full database information.

2:22Now, we'll talk a lot more about these LSAs, the different types

2:26of LSAs within the area and between the areas

2:29in an upcoming video.

2:31But for now, just realize that the LSAs sent between areas

2:37carry less information.

2:39And we don't actually run Dijkstra against those LSAs.

2:44And we'll see this in action in just a few moments.

2:49Now, one of our requirements when using multiple areas is we

2:53do have to follow a very strict hierarchy.

2:58The reason for this is as we mentioned

3:01during the introduction, the only place

3:04we can summarize in OSPF is at these area borders.

3:09And because of that, this has to follow our IP structure

3:14as well.

3:15So that where our areas cross, this

3:18is also the place that we can actually

3:20do our network summarization.

3:23So this does require a very specific design.

3:27Now, not only does this have to follow

3:29our IP addressing scheme, but this hierarchy

3:32does have some rules of its own.

3:34The first, of course, is that there must be a backbone area.

3:39This is an area that all of the other areas have to connect to.

3:43This has a few guidelines.

3:45First, it must be area zero.

3:48This is not an option in OSPF.

3:51As soon as you have more than one area, one of them

3:55has to be area zero.

3:57It also must be contiguous.

4:00In other words, you cannot have two separate area zeros.

4:04So you cannot have, for example, an area zero here,

4:09have a net router between them, have this connected to, say,

4:12area 21 with another router.

4:15And then over here, do another area zero.

4:19This would be a split area zero.

4:22This is not allowed.

4:24Now, there are ways we can make this work, which we will

4:26talk about in future skills.

4:29But in making it work, what we're really doing

4:32is fixing the fact that it's split

4:35and making it so that it's not split.

4:37That's the actual fix for it.

4:39And the last point here is not so much a rule as a guideline.

4:43We don't generally put end user devices or resource networks,

4:48such as servers, or printers, or things

4:51like that in the backbone area.

4:55It certainly can be done.

4:57Area zero is a full blown OSPF area.

5:00So you can do this.

5:02But area zero is very often reserved to just be a backbone

5:06and, therefore, not actually have any resources in it.

5:10All of the other areas are then non-backbone areas.

5:14Now this, of course, can use any 32-bit number.

5:17They all, as we said earlier, must be connected to area zero.

5:21And this is usually where we provide those connections

5:26for the end users.

5:28And those resource networks that we spoke of up above.

5:32So quite commonly, what you would have,

5:34you would have an area zero, connect it out

5:37to some other area, say, 21 again.

5:40And this is where we would actually

5:42connect in our resources.

5:44Let's take a look at this in a little bit more

5:46of a structure diagram.

5:48So here, we have a typical multi-area OSPF design

5:52where, of course, we have our backbone area, area zero,

5:55as well as two other areas, area 100 and area 200.

6:00So the main thing we want to look at here

6:02are the different roles of the routers when we start getting

6:06into a multi-area topology.

6:08So the first thing we have is something

6:11called a backbone router.

6:14That would be this router here, which

6:16is a router that has all of its interfaces in the backbone

6:20area.

6:21Other routers would be things like internal routers, which

6:25are routers such as these that have all of their interfaces

6:30in an area other than area zero.

6:33And in case you're wondering here, yes, a backbone router

6:37is also an internal router.

6:40We just give it a more specific name

6:42because all of its interfaces happen

6:44to be in the backbone area, area zero.

6:46But yes, it is still an internal router.

6:49Now, the other routers here are a little more specialized.

6:53The first thing we have is an ABR or an Area Border Router.

6:58And that would be this router and this router.

7:02These are routers that have one interface at least in area zero

7:06and one interface in some other area, possibly more than one.

7:12But nonetheless, these are then performing

7:14the job of an ABR or an area border router.

7:18This, by the way, is where we can actually

7:21do our network summarization.

7:23So the ABR is where we can summarize internal routes.

7:28Now, we have another type of router here bringing

7:31in routes from the outside.

7:33So maybe, for example, this would be something like BGP.

7:38Maybe this would be something like EIGRP

7:41or something like IS-IS.

7:45The router performing these functions

7:47is called an ASBR or an Autonomous System Boundary

7:51Router.

7:53This is a router that's doing some form of redistribution

7:57from these external protocols into OSPF

8:02bringing external routes into our network.

8:05This is where we can actually summarize external routes.

8:10So technically, we can summarize at the ABR or at the ASBR,

8:15depending on which type of route we're talking about.

8:18The ABR, again, is for internal routes going between areas.

8:24The ASBR can only summarize the external routes

8:28as they're being brought into OSPF during the redistribution.

8:33So now, let's wrap up with a look at some advantages

8:37and disadvantages of multi-area versus single-area OSPF.

8:42So let's begin with the advantages.

8:45First, we have that it allows for filtering and summarization

8:51between the areas.

8:53Now, we've been talking a bit about the summarization

8:55as we go here.

8:57We haven't mentioned a whole lot of filtering.

9:00But for the exact same reason that we can only

9:03summarize between areas, the ABR is also

9:07the only place we can filter.

9:09So again, with multi-area we can do

9:11route filtering between the areas as well as summarization.

9:15It also localizes the impact of any topology changes.

9:21So if an interface goes up, an interface goes down,

9:23or more importantly, if an interface is flapping,

9:26it won't affect the entire OSPF domain.

9:30It would be localized just to the single area where

9:34the problem is occurring.

9:36Part of that also relates to the containing

9:39of the flooded detailed LSAs.

9:41But we're really talking about containing

9:43the flooding of those detailed LSAs regardless

9:47of whether it's because of a topology change,

9:49but just in general.

9:51This is what causes that larger database

9:53and for the longer execution of Dijkstra's algorithm.

9:57Now, these are really good advantages.

10:00Very, very important that we keep these in mind.

10:02And quite frankly, these are most of the reasons

10:05that we're going to go with multi-area OSPF.

10:08Now, of course, scalability is also a huge advantage.

10:11But I was trying to go for some more specifics here.

10:14Now, there are some concerns, however,

10:16with running multi-area OSPF as well.

10:19You could also refer to these as advantages

10:21to running single area, however you want to look at this.

10:24But the first one is that the routers no longer have

10:30full visibility of the network.

10:32Now, we'll get into this more as we go through future skills.

10:36But since the routers no longer have full visibility,

10:40this can actually lead to routing loops.

10:43Link-state routing protocols are immune to routing loops.

10:49Because they have that full visibility.

10:52But without that full visibility,

10:54you lose that immunity.

10:56So as we start digging into more detail with multi-area

11:00and how our ABRs work and such, we'll

11:02get more into how this actually has to do loop prevention

11:06and implement some rules on our ABRs.

11:10And further talking about those ABRs,

11:12we need to make sure that they don't become

11:14a single point of failure, which means

11:17we have to worry about redundancy with those ABRs.

11:20Meaning, we should have at least two ABRs between our areas

11:24in most cases.

11:26And finally, there are some technologies

11:29that just don't work as well without having a single area.

11:34One of those as an example would be MPLS traffic engineering.

11:39Can we get it to work between areas?

11:43Yes.

11:44Is it much more difficult to configure and set up?

11:47A bit.

11:48So it's certainly more preferred to have

11:51a single area for things like traffic engineering,

11:55if it can be accomplished.

11:57In this video, we jumped in.

11:59And we took a look at some of the advantages

12:02and a few of the disadvantages of running multi-area OSPF.

12:07Keep in mind its main advantage is scalability.

12:11Because at a certain point, we simply

12:13can't run Dijkstra against a really large database

12:17and still have an efficient network.

12:21I hope this has been informative for you.

12:23And I'd like to thank you for viewing.

0:00[AUDIO LOGO]

0:07So now that we've taken a closer look at Multiarea OSPF,

0:11let's jump into the command line and take a look

0:14at some of our database commands to see what this actually looks

0:18like in the OSPF database.

0:20So we'll start off by looking at our topology, how

0:24our areas are currently set up.

0:25Then we'll jump in, we'll take a look at the databases,

0:28as well as watching how Dijkstra's algorithm actually

0:32runs now that we have different areas in our OSPF environment.

0:38So let's start off by looking at that topology.

0:41So for this demonstration, we will be using OSPF topology 2.

0:45And what we see here is now do we not only have area zero,

0:51but we also have area 100 and we have area 200.

0:55The way things are configured we have

0:57router 6 is acting as an ABR.

1:00So is router 1 and router 3.

1:03We also have router 2 over here, which is an ASBR bringing

1:08in external routes from EIGRP.

1:12Now in this demonstration, we're simply

1:14going to be focusing on what's happening inside of OSPF

1:18with our different areas.

1:20So we won't really be looking at the external routes coming

1:23from router 9 at this point, we'll be using that later.

1:28But for now we're going to focus on what happens when failures

1:32occur in these different areas now that we're not

1:36in one big area.

1:38So let's jump into the command line,

1:41and we're going to start by looking at area 200

1:44since this is a very small area and let's see what happens

1:48when a failure occurs there.

1:52So let's start here on router 6.

1:54And let's turn on a debug of IP OSPF SPF

2:00and we'll go over to router 10, and let's shut down

2:03one of its interfaces.

2:05Specifically, we'll just shut down its loopback 0 interface.

2:10Now let's go back over to router 6,

2:12we'll turn off our debugging.

2:13And what we want to notice here that SPF ran and notice that it

2:18only took 8 milliseconds for this to occur,

2:23which is significantly faster than what

2:25happened when we ran this earlier when everything

2:29was one big area.

2:30So even in an environment as small as this,

2:34we're definitely getting a performance increase

2:36by having multiple areas.

2:39So let's bring that interface back up on router 10.

2:48So now let's take a look at what happens when

2:51a failure occurs between areas.

2:54For this demonstration, let's keep an eye

2:57on what happens on router 7 when we

3:01create a failure on router 5.

3:04So we'll start by doing our observations on router 7.

3:10Let's start with the show IP route OSPF.

3:13Notice that we do have an entire area route for 10.5.5.5

3:19which is router 5's loopback.

3:22If we do a show IP OSPF database,

3:25notice that we also have numerous type III LAs here,

3:30which are the summary net link states.

3:32And notice that we in fact, have one for 10.5.5.5.

3:37In fact, we have two, one coming from each of our ABRs.

3:42And finally, let's do a show IP OSPF.

3:45And when we want to look at specifically

3:48is for area zero how many times SPF has actually executed.

3:54At the moment it's 129 times.

3:58So let's take a look at what happens now if we shut down

4:02that interface on router 5.

4:04Let's turn on our debug.

4:06So we'll debug SPF here on router 7

4:09then we'll go over to router, 5 and we'll

4:11shut down that interface.

4:13Let's go back over to router 7.

4:15We notice here that it definitely did calculations.

4:20We can see where the network is being removed,

4:23and we see that it processes things.

4:25But what's not here is any calculation for how long it

4:30took to run SPF because it didn't.

4:34If we say show IP OSPF again, notice

4:38that the execution of the SPF algorithm is still at 129 times

4:44even though we recognize the failure.

4:46And if we do a show IP route OSPF,

4:49we'll see that the 10.5.5.5 is now in fact missing,

4:53and we don't have it in our database either.

4:56So we've removed it from our database,

4:58we've removed it from our routing table

5:01but we did not execute SPF.

5:05This is the big advantage of having multiple areas in OSPF.

5:10When a change or a failure or anything

5:13happens in another area, yes we learn about it.

5:17Of course we do, this wouldn't be much of a routing protocol

5:19if we didn't, but we're not running SPF.

5:24We still remove it from the database,

5:26we still remove it from the routing table,

5:28but we don't have to run SPF every time something changes

5:32in another area, that would even include the route coming

5:35back up.

5:36Let's go back over to router 5 and bring that interface back

5:39online.

5:40So we'll pop over here, we'll do a no shut

5:43to bring the interface back up, back over to router 7.

5:46Notice once again, we're adding the routes back

5:50into the routing table.

5:52We do a show IP route OSPF, there's

5:55the 10.5.5.5 back in our table again.

5:59We do a show IP OSPF database, the 10.5.5.5

6:02is again here in the database.

6:06However, if we look at our show IP OSPF, notice

6:10still the SPF algorithm has only executed 129 times.

6:17We're not running SPF because the changes are not

6:21occurring in this area.

6:24In this video, we took a look at how the database reacts

6:29and how the execution of SPF reacts

6:33when changes occur both within our smaller areas,

6:37as well as what happens between areas

6:40when we have a failure that's not

6:42in our directly connected area.

6:44So we saw that within the area things

6:47happen faster because it's a smaller area.

6:50And we saw that between areas although we of course

6:54lose the database entry and therefore the route,

6:57we don't run SPF when something changes in a different area

7:02when we're doing Multiarea OSPF.

7:05And this is what adds to the scalability

7:08of doing Multiarea OSPF.

7:11I hope this has been informative for you,

7:13and I'd like to thank you for viewing.

0:00[AUDIO LOGO]

0:07So now that we've gotten some more detail on multiarea OSPF,

0:11now let's jump in and look at our different LSA

0:15types and the role that they play in our multiarea SPF

0:19topology.

0:21So we're going to start off, of course,

0:22by looking at type 1 and type 2 LSAs that work within the area.

0:27Then we'll look at our type 3 LSAs between the areas.

0:31We'll look at our external LSAs, our LSAs for our ASBR--

0:36so our summary ASBR entries.

0:38And then we'll finally wrap up by taking a look at LSAs

0:43that we use in a not so stubby area or an NSSA.

0:47So we'll take a look at each of these.

0:50And then in our next video, we'll

0:52actually jump in and see these in action on the routers.

0:56So for now, let's jump in and take a look

0:58at the role of each LSA in the multiarea OSPF topology.

1:05So our first LSA type is of course

1:08going to be LSA type 1, otherwise known as the router

1:12LSA.

1:14So the type 1 LSA is generated by all routers in OSPF.

1:19And as such, this serves as the fundamental building

1:23block for our OSPF database.

1:26This is what contains all the information

1:29about the router and its links.

1:32So this is going to include things like the router ID

1:35to identify the router, as well as the current status of all

1:39of its interfaces.

1:41Now, since this is the detailed information for the database,

1:46this is going to be contained within an area.

1:50So if we were to, for example, pick on router 7 here,

1:54it's type 1 LSA will be sent over here to router 8.

1:58It would be sent up to router 6, to router 1,

2:02and over to router 3.

2:04However, it won't go any further than that.

2:07Router 6 would not flood it into area 200.

2:11Router 1 and router 3 would not flood it into area 100.

2:17Type 1 LSAs are completely confined within the area.

2:22And again, they're used to define the routers themselves.

2:27Next is the type 2, or the network LSA.

2:32Now, this is generated by the DR, or the Designated Router,

2:38on a broadcast network type.

2:41In our example, we actually have two broadcast network types.

2:45So let's take a look at the one in area 0.

2:48We have three routers connected to this--

2:51router 6, router 7, and router 1.

2:56These are all connected to this common subnet 50.0.167.0/24.

3:04One of these routers will be elected as the DR.

3:08Keep in mind that the default for this election

3:12is to go with the router with the highest priority.

3:16All routers, by default, have a priority of one.

3:20So if we as administrators don't take control over this,

3:24it will then go to the router with the highest router ID.

3:27The router ID is generally based on its loopback interface.

3:31So therefore, it would probably go

3:33to the router with the highest loopback address.

3:36In our example, that would, in fact, be router 7 here.

3:39So let's go ahead and label that as our DR,

3:42assuming that that's the router that would have won.

3:45So if router 7 becomes the DR, that

3:48means that router 7 is going to be the one

3:50to generate this LSA.

3:53This would get sent to router 3, to router 6, to router 1,

3:58and of course, over here to router 8.

4:01Once again, it gets flooded through the whole area.

4:05This contains information about that multiaccess network

4:10segment.

4:11So this contains information about this 50.0.167.0/24

4:17network.

4:18Now, just like the type 1 LSAs, these

4:21are also going to be contained within the area, which means,

4:25once again, router 6 will not send this into area 200.

4:30And router 1 and router 3 will not send this into area 100.

4:37The main purpose of doing this is

4:39so that all of the routers connected

4:41to this multiaccess segment don't

4:44have to generate the type 1 LSAs for that link individually.

4:49So the idea is we send this information out.

4:53The DR tells everybody else which routers are connected

4:58to this multiaccess segment.

5:00We'll see this in the next video when

5:02we look at these in a little more detail.

5:05Keep in mind, as well, in our example,

5:07this same thing would happen for this segment over here

5:11in area 100, which is also a multiaccess segment.

5:16In other words, it's network type broadcast.

5:20Next up is the type 3 LSA, or the summary LSA.

5:26This is the LSA that's actually generated by ABRs to represent

5:32the prefixes in this area.

5:35So say, for example, router 7 has a loopback interface.

5:40That interface will get sent with a type 1 LSA to router 6.

5:47Since router 6 is in fact an ABR,

5:50it will then generate a type 3 LSA into area 200,

5:55representing router 7's loopback.

5:58At the same time, router 7 is also

6:01going to send this to router 1 and router 3 with type 1 LSAs.

6:06And both router 1 and router 3 will generate a type 3 LSA

6:12into area 100, again, representing

6:16router 7's loopback.

6:19This happens, of course, for every prefix in every area.

6:23We're just picking on router 7's loopback.

6:27But we would have the exact same thing

6:29going the other direction.

6:31So if we were talking about router 4's loopback,

6:33for example, once again, it would go as a type 1

6:37over here to router 3.

6:39And router 3, of course, would generate that type 3

6:42into area 0.

6:44So again, this is going to be for all prefixes

6:47in all directions between all the areas.

6:50And that's what this point is that it's

6:53used to advertise the prefixes from one area

6:58into another area.

7:00As we just demonstrated, this is going

7:02to be flooded both into the backbone and into other areas.

7:08Now, remember what we learned in our previous video

7:12that when we get these type 3 LSAs in another area,

7:17yes, we have to update our database,

7:19either adding or removing the prefix.

7:22And of course, that causes us to update our routing table,

7:25either adding or removing the route.

7:27But we do not run SPF, Shortest Path First,

7:31or Dijkstra's algorithm in response

7:34to receiving these type 3 LSAs.

7:37Also keep in mind that although these are called summary LSAs,

7:42it's referring to the fact that it's a database summary.

7:45Don't look at this as a network summarization.

7:49This is where we can create a network summarization,

7:53but we have to do that manually as administrators.

7:56Now, we can do that to help reduce or control

8:00the flooding of these type 3 LSAs into other areas.

8:05And we'll look at that in a future skill

8:07when we get to summarization and filtering between areas.

8:11But keep in mind that that's going

8:13to be talking about controlling these type 3 LSAs.

8:17By default, all prefixes are sent into all other areas

8:21with a type 3 LSA.

8:25Next is the type 5 LSA, or the external.

8:29Maybe you're thinking to yourself, wait a minute,

8:31what happened to the type 4?

8:33We're going to get to the type 4.

8:35But since the type 4 is to support the type 5,

8:39it actually sort of makes sense to talk about the type 5 first,

8:43even though, numerically, that's not exactly in order.

8:46So let's talk about the type 5 first.

8:49This is going to be generated by the ASBR That's

8:53doing redistribution into OSPF.

8:57In our example, that would be R2.

9:00This is our ASBR.

9:03And what it's doing is taking the EIGRP prefixes

9:06and redistributing them into OSPF.

9:10This is going to come in as a type 5.

9:13So again, this is going to be advertising those prefixes

9:17from an external protocol in this example, such as EIGRP,

9:22into the OSPF domain.

9:25This is then going to be flooded through the OSPF domain.

9:29Well, what that means is when R2 sends this in,

9:33it's going to go to R4.

9:35It's going to go to R5.

9:38It's going to go to R1.

9:414 is going to send it over here to 3.

9:43Router 1 is going to send it into 6.

9:466 is going to send it to 10.

9:481's going to send it to 7, and so on.

9:50Of course, 7 sends it to 8.

9:523 sends it to 7.

9:53You get the idea.

9:54It's going to be flooded everywhere.

9:58Now, the reason this is important to understand

10:00that we're flooding the type 5 LSA

10:03unaltered through our network is this

10:07helps us understand why we need the type 4.

10:10For example, in this case, this being flooded from router 2,

10:15it's going to have an advertising router

10:17ID of router 2's loopback, which, in our example,

10:21would be 10.2.2.2.

10:24That's all fine when it gets to router 1 and router

10:273 and router 4, router 5 even, because they're

10:31in the same area as router 2.

10:34And they would have a type 1 LSA for router 2.

10:37But what happens when that information

10:40goes into another area, such as, in our case, area 0?

10:44It gets to say router 7.

10:46Router 7 has no idea where router 2 is.

10:51And that is what we need the type 4 LSA for.

10:54Let's take a look.

10:56So again, our next type is the LSA type 4.

11:00This is called an ASBR summary.

11:04Now, this is going to be generated

11:06by the ABRs for the area, much like a type 3, actually.

11:11However, instead of advertising prefixes

11:15from one area to another, it's instead

11:18advertising the router ID of the ASBR.

11:23Remember what we said when we were talking about the type

11:265's.

11:27When this type 5, again, reaches, say, router 7,

11:31it's going to have on it the router ID of 10.2.2.2.

11:37The problem is router 7 literally has

11:40no idea where 10.2.2.2 is.

11:44So how could I possibly use this type 5 LSA

11:49if I don't know where the advertising router is?

11:52Because it's in a different area.

11:54And this is where the type 4 comes in.

11:57The type 4's are going to be generated by the ABRs

12:01and sent out into the area, advertising the router ID

12:06of the ASBR.

12:08This will get forwarded through the whole network.

12:10Once I receive this type 4 LSA--

12:13and again, everybody will get it because it is flooded,

12:16just like the type 3's into the backbone and the other areas.

12:22Now, all of a sudden, router 7 knows exactly where

12:2710.2.2.2 is.

12:28It's no longer a question.

12:30And now router 7 can use the type 5 LSA.

12:35Our next LSA type is the LSA type 7.

12:40This is NSSA external.

12:43Now, this one is going to be generated by an ASBR

12:47but specifically in a not so stubby area.

12:51So for example, let's say that area 200 up here is an NSSA.

12:56Router 10 has a loopback interface on it

12:59that it is redistributing into OSPF.

13:03This will be sent as a type 7 to get it through the NSSA.

13:10Now, once it gets to router 6, which is acting as the ABR--

13:16remember, router 10 is acting as the ASBR in this case--

13:19router 6, being the ABR, will then

13:23translate this and send it into the rest of the network

13:26as a typical type 5 LSA.

13:30So again, this is to advertise the prefixes

13:34from that external protocol into the NSSA itself.

13:38And this one is contained within the NSSA

13:43because it's translated to a type 5 at the first ABR.

13:48Now, I realize we haven't really gotten into OSPF area types

13:52yet.

13:53And you may not fully understand what an NSSA is at this point.

13:58And that's OK.

14:00I only bring it up here so that we've

14:02at least seen that there's something called a type 7 LSA

14:05and that we use it in an NSSA.

14:08NSSA will be covered in a future skill.

14:12And we will be jumping into it in much more detail.

14:16Now, you might have noticed as we

14:18were going through our discussion

14:19there that we actually skipped quite a few LSAs.

14:23For example, we skipped right over type 6.

14:27And that's because type 6 is used for multicast.

14:31Quite frankly, it's not a very popular implementation.

14:35And Cisco has actually never supported multicast with OSPF.

14:41We're not talking about OSPF using multicast.

14:45We're talking about multicast using OSPF.

14:48Type 8 is used for BGP interaction

14:52in the case of OSPF version 2.

14:55And in the case of OSPF version 3,

14:58these are used for link LSAs.

15:01Remember that OSPF version 3 is specifically for IPv6.

15:07As such, it has to deal with link-local addresses.

15:12Type 8's are used to carry link-local information just

15:17between directly connected neighbors.

15:20Type 9's, in the case of OSPF version 2,

15:24these are just referred to as opaque LSAs.

15:28Opaque LSA is really just a way of saying

15:31that it has no specific or specified function in the RFCs.

15:36Now, it can be used for other things.

15:39And we'll see some uses for these opaque LSAs

15:42as we move through this entire course.

15:45Now, the type 9's in the case of OSPF version 3,

15:49these are your interarea prefixes.

15:53The type 9 LSA in IPv6 actually carries

15:57information that's usually carried in a type 1

16:01in OSPF version 2.

16:04One of the big changes that they made in OSPF between version

16:082 and version 3 is they actually separated

16:12the advertising of the topology from the advertising

16:16of the prefixes.

16:18So in OSPF version 2, the prefixes are advertised,

16:23of course, in a type 1, where in OSPF version 3 for IPv6,

16:30we actually separate that out into these type 9's.

16:34So the type 1's advertise the topology.

16:37The type 9's advertise the prefixes for that topology.

16:42Now, you might notice that type 10 and type 11 for both OSPF

16:47version 2 and OSPF version 3, they're

16:51all listed as opaque just because, again, they

16:54don't have a specific use according to the OSPF RFCs.

17:01In this video, we went through and we

17:03talked about all of the different OSPF LSA types,

17:07their general function on our network,

17:10and where they get flooded as far as whether they

17:13cross area boundaries, what routers generate them,

17:16and so on.

17:17So hopefully, we now have a clearer understanding

17:20of how these LSAs and the different types

17:23operate in our OSPF network.

17:26I hope this has been informative for you,

17:28and I'd like to thank you for viewing.

0:00[AUDIO LOGO]

0:07So now that we've looked at the different type of LSAs

0:10in more detail, now it's time to jump onto the command line,

0:13take a look at these LSAs in the database

0:17so we can see what they actually contain,

0:19and take a look at the different fields,

0:21and what they represent in our OSPF environment.

0:24So let's start by looking at our diagram.

0:27So remember what our topology looks like

0:29and what our different area types are.

0:31And then we'll jump in to the command line

0:33and take a look at that database.

0:36So for this demonstration, we'll again,

0:38be using OSPF topology 2.

0:41And just some things to keep in mind

0:43as we're going through this, area 200

0:47is configured as an NSSA.

0:49We have router 6 acting as the ABR

0:53between area 0 and area 200.

0:56We have router 1 acting as an ABR,

0:59as well as router 3 for area 100 going into area 0.

1:03And we have router 2 who's an ASBR, redistributing the EIGRP

1:09routes into area 100.

1:12Also as a note, router 10 has a loopback interface

1:17that is also being redistributed into OSPF in the NSSA.

1:23This topology is going to allow us to look

1:25at all of our LSA types.

1:29We'll see 1 and 2 looking in area 0.

1:32We'll see type 3 LSAs coming from areas 200 and 100.

1:37We'll also see some type 5 LSAs, along with type 4 LSAs

1:43coming for the routes from router 9 and from Router 10.

1:47Then we'll take a look at our type 7

1:49LSA in our NSSA area, area 200.

1:54So let's jump in and start looking at the different LSAs.

1:57And we'll be looking primarily at router 7.

2:00But, of course, we'll have to jump up to router 6

2:03to look at our type 7.

2:05But let's begin on router 7.

2:08So let's start by just looking at the routing table.

2:10Just do a show ip route ospf.

2:13And what we see here is we do, in fact,

2:15have a combination of inter-area routes, intra-area routes,

2:21and external routes.

2:23Another good command to keep an eye on is show ip ospf route.

2:29This actually shows the OSPF routing table.

2:33These are the routes that OSPF has chosen as best,

2:37and is trying to put into the routing table.

2:40We can actually see here the routes that

2:42have been chosen as best, and it's trying

2:44to put into the routing table.

2:47Let's talk just for a moment about where

2:49these different components fall into the OSPF decision process.

2:53The first thing is the link state database

2:56with its entries.

2:58This, of course, is what we're going

2:59to be looking at mostly in this demonstration.

3:03The best routes from the link state database

3:06are then put into the OSPF RIB.

3:11This is the O-S-P-F RIB.

3:14And that's what this command we just looked at showed us.

3:18This is what we see with the show ip ospf route.

3:22The routes that are actually marked for entry

3:25into the routing table then get put into the global RIB.

3:30This, of course, is what we see with the show ip route.

3:33And not that it's incredibly important to this discussion.

3:36But just to wrap this up, of course,

3:39all of the entries in the global RIB

3:41then get installed into the FIB, or the Forwarding Information

3:47Base for CEF or Cisco Express Forwarding.

3:51So ultimately, this is the table used for the actual packet

3:54forwarding.

3:55So again, just as a review.

3:57If we do a show ip route, we see this table.

4:00If we do a show ip ospf route, that's when we see this table.

4:04That's the last command we just did.

4:06And, of course, for the rest of this discussion,

4:09we're going to be looking at the database.

4:11So when we do a show ip ospf database,

4:14we'll be looking at the link state database.

4:16And just for completeness, if you want to look at the FIB,

4:20on most platforms, it would just be show ip cef.

4:24But let's go ahead and take a look now

4:26in more detail at the actual database.

4:30So let's just start with the show ip ospf database.

4:34This just gives us a summary of all

4:36of the different LSA types that are currently in our database.

4:40We'll be looking at these in more detail.

4:42But this is just like a summarization.

4:44And if you actually just want a count,

4:47there's also other options here for filtering this.

4:51One of them is database summary.

4:54So it's sort of funny because the command can actually be

4:57show ip ospf database database.

5:01And what this does is just show us

5:04a counter of literally how many of each

5:07type are in the OSPF database.

5:10What we're going to do, though, is focus more

5:12on the individual types.

5:14So let's start by taking a look at the type LSA or the router

5:19LSAs.

5:20So we'll take database off of here, and we will say router.

5:23Now this first entry happens to be

5:25the entry coming from router 1.

5:27The first thing we see is the LS age.

5:30The LS age here is how long this has been in the database.

5:35We also have some options.

5:36We have no TOS capability.

5:39TOS is type of service.

5:41This is simply not supported on Cisco routers.

5:44So this will pretty much always just say no TOS capability.

5:48And then we also see DC here, which is demand circuit.

5:53It just means that this router is

5:54capable of doing demand circuits.

5:57We see the LS type is router links.

6:00That make sense, because that's what we asked to look at.

6:03We see the link state ID, which is the router ID.

6:06And the advertising router is also the router ID.

6:10These are going to be the same on type 1 LSAs.

6:13They'll be different on other types of LSAs.

6:17We also see the sequence number, which

6:18we use to ensure that the database is up to date.

6:21The checksum, of course, for error correction,

6:24the length in bytes.

6:26And we also see that router 1 is announcing itself as an ABR

6:30into the environment.

6:32Then we see that it has four links.

6:35And then it starts breaking them down for us.

6:37So we have one link that's connected to a stub network.

6:40That stub network is 10.1.1.1 with a mask of a /32.

6:46Now just based on our topology and our knowledge,

6:49we happen to know that that's its loopback interface.

6:53Our second link here is connected to another router.

6:56It's a point-to-point link.

6:58And that's connected to router 3.

7:00We also see its IP address on that interface.

7:04If we scroll down, we see that it has another connection

7:07to a stub network.

7:08Notice that it's also listing that as the link 2 router 3.

7:13These point-to-point links are listed

7:16both with a topology entry, as well as a network advertisement

7:20being a stub network.

7:22Then we have the transit network,

7:24which is the multi-access network between routers 1, 6,

7:29and 7.

7:30This is listed as the transit network.

7:32We see who the DR is.

7:33In this case, the DR is actually .7,

7:36which makes sense because it's the highest IP address.

7:39And our interface on that is .1.

7:43Now as we can see, we have other entries

7:45here from other routers, here starting with router 3.

7:48But we're not here to look at the whole database.

7:51We just want to look at the different LSAs.

7:54So next, since we know we have this transit network,

7:57let's take a look at our type 2's.

7:59So we'll take off router, and we'll put in network.

8:02So here's that type 2 LSA.

8:05Notice that it's coming from the DR. Notice

8:09the link state ID is the address of the DR,

8:12and it even tells us that.

8:13Also we see that the advertising router

8:16is in fact router 7's router ID, which make sense since he's

8:21the DR.

8:22And then notice down here it lists the subnet mask in use

8:27on that network.

8:28So the network mask, along with the link state ID would

8:33actually tell us what the network is, 50.0.167.0/24.

8:40Those two pieces of information together define the network.

8:44And then, of course, here at the very bottom,

8:46it lists all of the router IDs attached to that segment.

8:52Notice that the other fields that we didn't mention

8:55are just the same as they are in a type 1.

8:59Next up would be the type 3's.

9:01These are called summary LSAs, of course.

9:03The first thing to notice here is

9:05that the type 3 LSA has this new option on it that says upward.

9:11This is referred to as the OSPF downward bit.

9:14And I know that seems odd since it says upward.

9:18But the thing is the option being

9:20set to upward simply as a way of saying that the downward bit is

9:25not set.

9:26So we generally call this the downward bit.

9:29And we'll talk about this more when we get into MPLS.

9:33This is for MPLS loop prevention when

9:35we're dealing with duplicate PES and things like that.

9:39So we'll get more into this later.

9:41But just for now, be aware that this is where

9:43it's carried in the type 3 LSA.

9:46And if we're not dealing with MPLS and VRFs,

9:49then this is going to be set to upward,

9:52meaning again the downward bit flag is not set.

9:57So next, we see the link state ID,

10:00which in these first two examples here,

10:02notice that they're both 10.2.2.2.

10:06That's actually router 2's loopback interface,

10:09which is over in area 100.

10:12Notice that we're learning this from two different advertising

10:16routers because we have two different ABRs--

10:1910.1.1.1 and 10.3.3.3.

10:24The only other thing to note here

10:25is the network mask is carried down here at the bottom.

10:29So again, if we apply the network mask

10:31with the link state ID, which is the actual network being

10:34advertised, this tells us the actual network.

10:38If we scroll down to something that's a network rather than

10:41a loopback address.

10:42So for example, these entries.

10:45We have an entry here for the network 50.2.61.0.

10:50Now we can see that that's a /24.

10:53So this is telling us that, that network

10:55is available through router 6, which of course, is also

10:59an ABR.

11:01Next up, let's take a look at our type 5's.

11:04So these are going to be external.

11:06And for now, we're just going to focus here

11:08on router 9's loopback.

11:10It happens to be our first entry.

11:12So first thing to note is it again,

11:14has that upward bit or the downward bit that's not set.

11:17So listing it as upward.

11:20And then notice that once again, just like a type 3,

11:23the link state ID is the actual network being advertised.

11:27So it's 10.9.9.9.

11:30But here's the important part.

11:32The advertising router is 10.2.2.2.

11:37That is a router not in our area.

11:41We would have no idea how to get to 10.2.2.2, because we do not

11:46have a type 1 LSA for it.

11:48We'll demonstrate that in just a moment.

11:50But notice we also have the network mask here.

11:53And external type 5 LSAs do carry some extra information

11:57here.

11:58They have the metric type, which can either

12:00be a type 1 or a type 2.

12:02We'll cover that in more detail in a future skill.

12:05We also have a metric listed here, the forwarding address,

12:09which we will also cover in more detail,

12:11as well as any external route tag

12:13that may be set on this route.

12:16Now again, the problem with getting to 10.2.2.2

12:19is if we do our show ip ospf database again,

12:22and this time we say router 10.2.2.2,

12:25notice that there's no entry.

12:27Again, 10.2.2.2 is in a different area.

12:30It's over in area 100.

12:31And we're currently in area 0.

12:34This router, router 7 would have no idea

12:37how to get to this advertising router.

12:39And this is exactly what the type 4's are going to give us.

12:43Let's take a look.

12:44It's actually ASBR summary.

12:47I usually just type ASBR.

12:48But there's the whole command for you.

12:51And notice exactly what this is telling me.

12:54Notice what the link state ID is.

12:57And we don't have to go into a whole lot of detail on these.

13:00Notice that they're almost identical to type 3's

13:04that we just talked about a moment ago.

13:06The only thing that makes these different from a type 3

13:10is what's being advertised.

13:12The link state ID.

13:14And it even tells you in parentheses

13:16afterwards what it is.

13:17It's the AS boundary router address.

13:20Specifically, it is the router ID of the ASBR.

13:26And notice that just like with the type 3's,

13:28we're getting this information from both of our ABRs--

13:3210.1.1.1 and 10.3.3.3.

13:36So finally, let's look at type 7's.

13:39But as I mentioned before, we can't exactly

13:41look at them here because if we say NSSA here,

13:44we're going to have no entries.

13:47Remember that the type 7 only lives inside of the NSSA.

13:52Router 7 here is not in the NSSA.

13:56So we're going to have to jump over

13:57to a router that is in there.

13:59We'll just use router 6.

14:01So on router 6, we'll do our show ip ospf database.

14:04And we'll do the NSSA here.

14:07And we'll see that now we have a type 7 for that loopback

14:10interface on router 10, which happens to be 100.10.10.0.

14:16And as we can see by the network mask here, it's a /24.

14:21The advertising router, of course,

14:23is router 10's router ID.

14:25Notice that this is almost identical to a type 5 LSA.

14:30And it doesn't really need to be any different,

14:32because we're just using the type

14:347 to get this through the NSSA.

14:37We'll be covering this in more detail in a future skill

14:41when we cover OSPF area types in greater detail.

14:45But all we really need to see for right now

14:47is the fundamental difference on this LSA from a type 5,

14:52is this option here that says type 7/5 translation.

14:57It's basically the originator, in this case router 10,

15:02asking our ABR, in this case, router 6,

15:06to translate this into a type 5 LSA.

15:10And in fact, if we say show ip ospf external

15:15and filter it by self originated,

15:18notice we are, in fact, translating it.

15:21There it is.

15:22This is us generating the type 5 on behalf of the type 7.

15:27We are doing the translation.

15:31If we do a show ip ospf, and if we

15:33look at our information for area 200,

15:36notice it even says perform type 7 to 5 LSA translation.

15:41And we can see that router 6 is.

15:44Again, we'll be covering this in more detail in a future skill.

15:47But I at least wanted you to see the type 7's for now.

15:51So you can get an idea of how these work.

15:55In this video, we jumped into the command line,

15:58and really got a detailed look at each of these LSA types,

16:02what they look like, what information they contain.

16:05We even got to see how our ABR between an NSSA

16:10and a normal area, in our case, area 0,

16:13how that ABR is translating the LSA from a type 7 to a type 5.

16:20I hope this has been informative for you,

16:22and I'd like to thank you for viewing.

0:00[AUDIO LOGO]

0:08Congratulations on completing another skill.

0:11Now it's time for a review in the form of a quiz.

0:15Let's jump in with our first question.

0:19Which OSPF network design works best

0:23with MPLS traffic engineering?

0:32So as we look over our answers, really only two of these

0:36are sort of valid.

0:37Multi-tier sort of isn't even a thing,

0:40and never used OSPF for traffic engineering.

0:44That's certainly not correct.

0:45It's actually one of the few protocols

0:47that actually supports it.

0:49So that certainly wouldn't be correct.

0:51So which of the two multi-area or single area

0:55is going to be better?

0:56And the fact of the matter is when you do multi-area,

1:00you break the traffic engineering path,

1:03and you have to go through other methods to get it to work.

1:06So the best answer here is going to be single area, therefore

1:11making three our correct answer.

1:15Next question.

1:16Which two LSA types carry external routes in OSPF?

1:27So once again, let's go down our list.

1:30Type 1 and type 2 would both be clearly incorrect,

1:35since not only are these not external,

1:37these are LSAs for internal routes within an area.

1:42Type 3 is for internal routes between areas.

1:47So again, none of those would be correct.

1:50That would leave us with type 5's and type 7s.

1:55Remember that type 5s are external for a normal area,

1:59and type 7s are specifically for an NSSA or the not

2:05so stubby area.

2:06But both of them are for external routes,

2:09just in different area types.

2:12Therefore, our answers are number 4

2:15and number 5 type 5 and type 7.

2:19Final question, which LSA type lists all the routers connected

2:25to a multi-access LAN segment?

2:33Now the type 1 LAS, remember, is information

2:37about the router itself, and its directly connected links.

2:41So type 1 is not about a list of routers connected

2:45to a multi access LAN segment.

2:47Type 2 sounds like a good answer,

2:49but let's just make sure we're clear on the other ones first.

2:52Type 3, this, of course, is for prefixes between areas.

2:56So that's not correct.

2:58The type 4, remember, is for ASBRs from other areas.

3:02So that's not right.

3:03And type 5 LSAs are, of course, for external routes,

3:08also not correct.

3:10So that, of course, makes type 2 our correct answer.

3:13Remember specifically that these are created by the DR

3:18on that multiaccess LAN segment.

3:21So this makes answer 2, the type 2 our correct answer.

3:27So that concludes this quiz.

3:29If you find that you were unable to answer

3:31any of these questions and want to review the information,

3:34please see the reference sources at the bottom of each question.

3:38Otherwise, congratulations on completing multi-area OSPF

3:43and LSA types.

3:44I hope this has been informative for you,

3:47and I'd like to thank you for viewing.