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So, now let's jump in and take a look at some characteristics and attributes of

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OSPF

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that are going to aid us in troubleshooting OSPF.

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Now, keep in mind, first, this is not a full coverage of OSPF here.

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This is meant to serve as a reminder to knowledge which hopefully you already

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have.

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If you don't, I would encourage you to go back and review a different course

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that would
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that would

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offer more coverage of OSPF such as Cisco's Enterprise Core, which of course is

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available

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here on CBT Nuggets.

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The second thing to keep in mind is that the best way to troubleshoot something

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is to know

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it very well.

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The better you know something, the easier it's going to be to troubleshoot it

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because

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you'll know how it's supposed to act versus how it's currently acting and given

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that difference,

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it'll help you come to the conclusion of what might be wrong.
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it'll help you come to the conclusion of what might be wrong.

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So, all of that said, let's jump in here, review some of the critical things

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about OSPF

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that are going to help us with our troubleshooting and we'll take a look at

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some very useful both

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show and debug commands that we can use with OSPF.

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So as we're jumping into OSPF, let's start by talking about the different LSA

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types.

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Remember, LSA's are the packets that are used to exchange the information
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Remember, LSA's are the packets that are used to exchange the information

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between the

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routers to build the database.

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There are six major LSA types that we want to be aware of.

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The first one of course is the LSA Type 1.

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This is referred to as a router LSA and this represents all of the interfaces

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on a single

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router and this type of LSA is generated by all routers.

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The LSA Type 2, which is referred to as a network LSA, this represents a shared
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The LSA Type 2, which is referred to as a network LSA, this represents a shared

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network

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segment.

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So for example, an ethernet segment.

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This will be referring to any network that has three or more OSPF routers

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connected to

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it.

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These are the same segments that are going to elect a designated router, a

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backup designated

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router and then as we say here, it's actually the designated router that

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generates this type

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2 LSA on behalf of that network segment.
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2 LSA on behalf of that network segment.

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The Type 3 LSA or a summary LSA represents a network that's in a different area

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.

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So this is a network outside of your current area and this is generated by the

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ABR or the

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area border router that's sitting between those two areas.

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Very similar to the Type 3 is the Type 4 except that this one is an ASBR

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summary and instead

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of representing a network, this actually tells us the ASBR router ID.
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of representing a network, this actually tells us the ASBR router ID.

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Again, it's for a different area than the area that we're currently in.

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Since it's information for a different area, this is also going to be generated

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by the

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ABR and this is how the ABR lets us know how to get to the ASBR in another area

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.

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And of course we have Type 5 LSA's, these are for our external routes.
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And of course we have Type 5 LSA's, these are for our external routes.

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This can either be a Type 1 or a Type 2 external which is a little bit beyond

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our current scope.

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But nonetheless, it's going to be created by the ASBR or the autonomous system

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boundary

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router.

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This would be any router that's doing redistribution into OSPF.

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Very similar to the Type 5 is the Type 7, this is specifically for NSSA extern

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als.

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So this would be external routes as they're traversing an NSSA or a not so stub
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So this would be external routes as they're traversing an NSSA or a not so stub

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by area.

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The reason for this is that Type 5s are not allowed in an NSSA.

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We'll be covering this in just a moment in more detail.

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Once again these are going to be external networks, once again they can be Type

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1 or Type 2,

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but this is while the networks are traversing the NSSA since their external

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still generated

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by the ASBR.
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by the ASBR.

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Up next let's talk about those OSPF area types.

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So our first area type is of course the backbone area.

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This is always going to be Area 0 and the LSA types that are allowed in this

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backbone

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area are 1, 2, 3, 4 and 5.

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It also does not automatically generate a default route.

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Let's be very clear when we say default route here, we mean automatically

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generated.
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generated.

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You can tell any of these areas or routers specifically to generate a default

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route.

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So when we say default route yes or no here, we simply mean is the default

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generated by

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default.

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Next is will this area type set the stub flag and in the case of the backbone

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area no the

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stub flag is not going to be set.

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A normal area is just like the backbone area except that it's not Area 0.
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A normal area is just like the backbone area except that it's not Area 0.

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So any other area that's not Area 0 you can see it's almost the same here in

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that well

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it allows LSA types 1, 2, 3, 4, 5.

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It does not automatically generate a default and it does not generate the stub

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flag.

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This again is a little bit beyond our scope but just to give a quick definition

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the difference

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between normal and backbone the backbone area can be used to connect other
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between normal and backbone the backbone area can be used to connect other

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areas and a normal

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area cannot.

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All areas must connect to the backbone area.

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Next up we have a stub.

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The purpose of a stub network is to not allow external routes.

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So since it's not allowing external there's no need for the type 4 or the type

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5.

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The type 5 is the external and the type 4 tells us how to get to the ASBR

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neither of

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which would be necessary if we don't have externals.
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which would be necessary if we don't have externals.

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So this is only going to allow types 1, 2 and 3.

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Now since we are now filtering routes specifically external routes from that

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area it will automatically

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generate a default because it knows that the other routers in that area would

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be lacking

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information.

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Now of course since our database has to be the same on all routers and we are

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now limiting

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which LSA's are in the area we set this stub flag to make sure we do not become
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which LSA's are in the area we set this stub flag to make sure we do not become

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neighbors

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with a router that does not have the stub flag set in a stub area.

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Next we have a total stub.

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The primary difference here is we don't send all the routes from the other

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areas.

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Notice the stub allowed type 3's.

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Now the total stub allows a type 3 as well however notice I have an asterisk on
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Now the total stub allows a type 3 as well however notice I have an asterisk on

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that.

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And the reason is yes this does generate a default and yes that default is

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going to

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be a type 3 LSA but that default is going to be the only type 3 LSA allowed

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into that

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area.

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No type 3 LSA's from other areas come in.

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So yes the type 3 is allowed but it's going to be limited to just the default

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route.

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And once again since we are in fact limiting the LSA's in the area we are going
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And once again since we are in fact limiting the LSA's in the area we are going

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to set

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that stub flag to make sure that we don't get inappropriate neighbors.

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Next up is the NSSA or the traditional long name of the not so stubby area.

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This is an area that is a stub so we could jump to the end here and say yes we

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are setting

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the stub flag here however it does allow externals.

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Now this presents a problem because type 4 and 5 LSA's are not allowed in stub
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Now this presents a problem because type 4 and 5 LSA's are not allowed in stub

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areas.

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So instead you could say that we circumvented the rules and instead created the

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type 7.

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So now we allow types 1, 2 and 3 so those are of course the internal LSA's.

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The type 3 being the LSA from other areas and the type 7 being the external

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route being

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transported through the NSSA.

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Now the NSSA does not by default create a default route.
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Now the NSSA does not by default create a default route.

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However if you as the administrator choose to create a default route and we do

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this using

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a command called default information originate if you do that it will be a type

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7.

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So just a piece of information to be aware of.

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Like the stub to the total stub we also have the NSSA to the total NSSA.

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The difference being that again we don't allow type 3's.
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The difference being that again we don't allow type 3's.

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We filter those out coming from other areas but notice on our list we still

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have 1, 2,

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3, 7.

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We probably should have put an asterisk on that 3 like we did for the total

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stub because

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once again the only reason that we're allowing that one is because we are going

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to generate

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a default route automatically and it's now going to be a type 3.

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Now there's a lot of design reasons behind why one of them is a type 3 and one
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Now there's a lot of design reasons behind why one of them is a type 3 and one

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of them

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is a type 7 it has to do with which default route you want to be able to follow

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.

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It's a whole big discussion that's a little bit beyond our scope here.

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Just be aware that they are two different types of defaults and there's a

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really good

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reason behind it.

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Once again this is a stub area so we are going to set the stub flag.

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So these are OSPF area types.

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Next up we have OSPF network types.
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Next up we have OSPF network types.

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The network types determine how OSPF is going to operate on a single network

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connection.

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Now I'm listing three of them here.

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These are the most common network types.

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There are a few more.

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There's a non-broadcast.

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There's also a point to multi-point non-broadcast.

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So there's a few other ones but these are the main ones that you're probably

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going to run

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into in modern networks.

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So the first one here is the broadcast network type.
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So the first one here is the broadcast network type.

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This is going to be the default on segments such as Ethernet, any kind of multi

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-access network.

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And since it is a multi-access network it's going to be permitted to have many

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neighbors

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on that segment.

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I saw a limitation once I believe it's supposed to be 256 routers on a single

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segment.

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I've personally never run into that particular limit so I just got that out of

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documentation.

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I've never actually tested it.

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But unless you're on something like a really large multi-access like DMVPN
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But unless you're on something like a really large multi-access like DMVPN

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network you should

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never get close to that anyway.

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And the solution of course for something like DMVPN is we can just switch over

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to point

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to multi-point but we'll get to that in a moment.

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This network type will elect a DR and a BDR.

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Therefore it will generate type 2 LSAs and the default hello and dead timers

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are 10 and

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40.

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Next is point to point.

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The main difference here of course is you're only allowed to have one neighbor
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The main difference here of course is you're only allowed to have one neighbor

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which means

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there will only ever be two routers on the segment, the router you're looking

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at and

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one neighbor.

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That's it.

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Now since there's only two devices on the segment there is literally no reason

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to have

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a DR or a BDR.

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Remember that the job of the DR is to do efficient packet exchange between well

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many

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devices on a single segment.

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The DR's job is to organize the communication.
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The DR's job is to organize the communication.

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Well if there's only two devices on a segment we don't exactly need to organize

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that communication

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so we don't use one.

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Default timers on this are also 10 and 40.

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Then we have point to multi-point.

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Point to multi-point also allows many neighbors.

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This network type was designed specifically for large hub and spoke

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environments such

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as things like DMVPN, possibly MPLS and other such WAN technologies.
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as things like DMVPN, possibly MPLS and other such WAN technologies.

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This just like point to point does not create or use a DR but since it is meant

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for use on

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the WAN it does default to slower timers of 30 and 120.

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Now something to note here.

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When it comes to OSPF, neighborships, neighborships can actually form between

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any of these if the

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timers match.
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timers match.

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But here's a critical factor.

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For the database to be consistent you cannot have a network type that uses a DR

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mix and

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match with network types that do not.

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Therefore you can have valid neighborships between point to point and point to

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multi-point

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but not between any of the points to the broadcast.

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In other words if it starts with the word point you can use them together.
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In other words if it starts with the word point you can use them together.

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If instead it starts with the word broadcast you cannot.

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So you can only mix and match these within these categories of whether they use

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a DR or

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whether they don't.

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Also keep in mind for any of this to actually come up as neighborships the

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timers are going

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to have to match anyway so these default timers would have to be adjusted.

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But we'll talk about what has to match for neighborships next.
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But we'll talk about what has to match for neighborships next.

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