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So it's next. Let's talk about those OSPF neighborhoodships.

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Certain things have to be correct in order for OSPF neighborhoodships to form.

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First, you have to have reachability between the devices that are going to

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become neighbors.

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Now, most of our technologies today with OSPF network types that we just

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discussed are going to use multicast.

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Specifically, 224.0.0.5 and 224.0.0.6.
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Specifically, 224.0.0.5 and 224.0.0.6.

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However, the .5 is used for our neighborships.

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So we can actually do a ping to that address to see if the other routers are

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responding.

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We'll do this as we go through our troubleshooting.

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Now, something that has to match is the area ID.

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Both sides of a link have to be in the same area in OSPF.

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Now, different interfaces on a router can, of course, be in different areas,
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Now, different interfaces on a router can, of course, be in different areas,

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but between you and your neighbor, you have to be in the same area.

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You also have to be on the same network or subnet, which means your IP address

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and your subnet mask have to match.

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One side can't think it's a slash 30 and the other side thinks it's a slash 28.

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Even if the network numbers are compatible, it exchanges the subnet mask and

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that has to match as well.

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Something that actually has to be different are your router IDs.
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You cannot have duplicate router IDs within the same area.

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And since your neighbors have to be in the same area, you cannot have a

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duplicate router ID with your neighbor.

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The neighborship will fail to come up.

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Also, to prevent neighbors from flapping, hello and dead timers have to match.

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This is simply for practical reasons, and OSPF has a built-in check to make

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sure that these match.

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The MTU or the maximum transmission unit on the segment has to be the same as
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The MTU or the maximum transmission unit on the segment has to be the same as

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well.

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If you're doing authentication, which is, of course, optional, but if you're

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doing authentication, the authentication on both sides, of course, has to match

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.

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Let's be honest, authentication wouldn't mean too much if it didn't have to

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match.

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Now, there are different types of authentication.

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We'll talk about that in a moment as well.

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Now, one thing that we've already discussed, but just to keep it on the list

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here, that stub area flag has to match.

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You cannot have one router in the area think it's a stub area and another
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You cannot have one router in the area think it's a stub area and another

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router think it's not a stub area.

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And since, again, the neighbors have to be in the same area, that means the

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stub flag for that area also has to match.

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And finally, the compatible network types.

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Again, this is for database consistency.

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This, of course, is referring to the fact if there is a DR or no DR.

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So if there's a DR, you can mix and match.

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If there's no DR, you can mix and match, but between a DR and not having a DR,
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If there's no DR, you can mix and match, but between a DR and not having a DR,

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you cannot.

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And again, I'll be honest with you here.

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This can be a very tricky problem to troubleshoot.

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We're going to see this in one of our examples.

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This is tricky because the neighborship itself, and I know you might be

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thinking, well, why did you put it under OSPF neighborships with what you're

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about to say?

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Well, because it doesn't really necessarily fit anywhere else, but the

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neighborships will come up.
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neighborships will come up.

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But the database will be invalid.

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Therefore, OSPF will not use the database.

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Well, therefore, you might as well not have neighborships.

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But again, this one can be a little bit tricky. Maybe it should be in a

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category of its own because, again, the neighborship will establish.

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But either way, be aware that this must match.

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Next up, we have OSPF authentication types.

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For the authentication types, the default is actually type zero, which means we
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For the authentication types, the default is actually type zero, which means we

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're actually not doing any authentication.

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So that's effectively off.

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Then there's type one, which I really wouldn't recommend.

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This is, of course, just plain text authentication, which means your password

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is going to be sent over your network in plain text.

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Anybody doing packet capturing or anything like that could easily catch this

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password.

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You could make the argument.

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It's better than nothing.

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But, you know, it's really about the same to set up plain text authentication
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But, you know, it's really about the same to set up plain text authentication

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versus type two.

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So in most cases, I would, of course, recommend going to type two.

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This is either going to be MD five authentication or HMAC Shaw authentication,

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depending on the version of code you're using.

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Most of the newer code, of course, is using HMAC Shaw.

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Next, let's discuss the OSPF route types.

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Here, we're talking about the different OSPF entries that we would see when we
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Here, we're talking about the different OSPF entries that we would see when we

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do a show IP route.

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The first one is just a standard O by itself.

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This indicates an intra area route, meaning within this area.

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And that also indicates that it came from either a type one or a type two LSA

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because those LSA types stay within the area.

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Next, we have the OIA, which is the inter area route.

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This would be routes to networks in a different area.
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This would be routes to networks in a different area.

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These are, of course, coming from an LSA type three.

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Next, we have the O E one or E two.

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These are external routes.

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And remember that the externals can be either a type one or a type two external

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, but they're carried in a type five LSA.

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So don't confuse the external type with the LSA type that carries it.

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Next, we have the counterpart to these, the O with an N one or an N two.
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Next, we have the counterpart to these, the O with an N one or an N two.

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These are NSSA external.

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So the equivalent of the type five, but inside of the NSSA itself.

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So therefore, we carry it in a type seven.

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And to wrap up, let's talk about some good tools that we have for troubles

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hooting in OSPF.

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This is just for the purpose of having a good list of commands that we can use.
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We'll be seeing all of these on the command line as we actually do troubles

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hooting in the next video.

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So the first one is show IP protocols.

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Great command that shows us information about all of the IP protocols that we

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're running OSPF, of course, being one of them.

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You can also get more specific information about OSPF itself with a show IP OSP

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F.

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We can verify our neighbors with a show IP OSPF neighbor.
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Also, we can verify our interfaces.

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We do this with a show IP OSPF interface.

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If you're looking for detail, you just hit enter. If you just want a really

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good summary, you can add brief to this just to make sure all your neighbors

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are up.

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It's a really good short summary.

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We mentioned this one before, but you can ping the multicast address for OSPF

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neighborships.

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And this will tell us if our neighboring routers are actually listening for OSP
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And this will tell us if our neighboring routers are actually listening for OSP

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F properly.

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Maybe the neighboring routers not configured correctly.

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The show IP OSPF database will let us look at, of course, the database itself.

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This command by itself will give you a nice overview.

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Then you can drill down by doing a show IP OSPF database and then specifying an

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LSA type.

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So you specify it by the names I gave us earlier.

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So we could say summary network router and so on.
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We also get some more detailed information by adding the keyword internal.

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We'll be taking a look at this as well.

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Now, of course, the database is used to build a topology and we can look at the

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topology directly that's been built off the database.

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Once again, we can get some more information by using internal.

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Then we have some really good debuggs.

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First, we have debug IP OSPF packet.

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Very useful command. However, be aware that on a production network with a lot
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Very useful command. However, be aware that on a production network with a lot

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of OSPF neighbors and fast OSPF timers.

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This could actually create a decent amount of debugging information.

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So this one you want to use with a little bit of caution in a lab.

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It's going to be fine in production.

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Be careful.

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Debug IP OSPF adjacency is supposed to show us things that prevent OSPF adjac

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encies from occurring.

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However, if I'm being perfectly honest, there's a lot of things about adjac

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encies that this debug actually misses.
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encies that this debug actually misses.

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So this debug by itself is often not enough.

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I find that anything that adjacencies doesn't show you is usually going to be

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shown by the debug IP OSPF hello.

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This one is useful in conjunction with adjacency. So between packet adjacency

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and hello, you should be able to find out what is preventing your OSPF

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neighborships from coming online.
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So in this video, we just jumped in and got what was hopefully a reminder about

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all the different OSPF attributes, such as what needs to match or not match for

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OSPF neighborships, the different LSA types, the different area types,

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different network types and so on.

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And then of course we wrapped up here with some good tools to keep handy as we

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move into our next video, where we're going to actually be doing some troubles

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hooting with OSPF.
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