Cisco CCNA ICND2 640-816

Routing Protocols: OSPF Concepts

by Jeremy Cioara

Start your 7-day free trial today.

This video is only available to subscribers.

A free trial includes:

  • Unlimited 24/7 access to our entire IT training video library.
  • Ability to train on the go with our mobile website and iOS/Android apps.
  • Note-taking, bookmarking, speed control, and closed captioning features.
Video Title Duration

Review: Rebuilding the Small Office Network, Part 1

Review: Rebuilding the Small Office Network, Part 2

Review: Rebuilding the Small Office Network, Part 3

Switch VLANs: Understanding VLANs

Switch VLANs: Understanding Trunks and VTP

Switch VLANs: Configuring VLANs and VTP, Part 1

Switch VLANs: Configuring VLANs and VTP, Part 2

Switch STP: Understanding the Spanning-Tree Protocol

Switch STP: Configuring Basic STP

Switch STP: Enhancements to STP

General Switching: Troubleshooting and Security Best Practices

Subnetting: Understanding VLSM

Routing Protocols: Distance Vector vs. Link State

Routing Protocols: OSPF Concepts

00:00:00 - What better protocol to introduce first in this ICD 2 series
00:00:04 - than OSPF. And I say that only because OSPF is the
00:00:09 - most popular routing protocol in the world.
00:00:13 - OSPF is awesome in the sense that it is one of those protocols that
00:00:16 - just has so much complexity that as you move on into the CCNP
00:00:21 - track, dozens of videos will be dedicated just to this
00:00:25 - protocol and all of its concepts. But, it has been moved and introduced
00:00:30 - into the CCNA criteria because even in all its complexity
00:00:34 - it is the most popular routing protocol in the world. And by
00:00:38 - the time you're done here you will have a very good understanding
00:00:41 - of what it is and how to work with it. The CCNP will just build
00:00:44 - on that. It is kind of like you're.. You're walking up the mountain of
00:00:47 - OSPF and you'll get about halfway up in the CCNA level
00:00:51 - and CCNP just takes you to the peak of the mountain to where you
00:00:54 - know everything. So, in this video we're going to talk about,
00:00:58 - first and foremost, route summarization. The rest of OSPF
00:01:02 - won't make much sense without understanding route summarization.
00:01:06 - We'll then move into the OSPF terms and network design.
00:01:11 - Half of the battle in OSPF is understanding what these different
00:01:15 - terms mean, and why we design our network certain ways. We'll talk
00:01:19 - about that. Finally, we'll look at the OSPF 'hello packet,'
00:01:23 - which is the foundation piece of OSPF that allows routers to
00:01:27 - form neighbor relationships with each other and exchange routes.
00:01:31 - Many of the concepts that we talked about in OSPF will
00:01:34 - not make too much sense without understanding the idea of route
00:01:38 - summarization. Route summarization is all about making routing
00:01:43 - tables smaller. Here's the fact - the larger your routing table
00:01:47 - the more inefficient your router becomes. Your router is slower.
00:01:51 - And the reason why is because the router has more information
00:01:54 - to weed through for every single packet. It receives a packet and it's like ok, you're
00:01:57 - going to here, let me look at this routing table that's just massive
00:02:01 - now this, now this, it looks down through that whole routing table to
00:02:04 - try and find the best route. So what we can do to make our routers
00:02:08 - more efficient is to shrink the routing table. Summarization
00:02:12 - is how that's possible. Here is the idea. Let's say we have
00:02:15 - two routers in our organization. Well, let's say many routers
00:02:18 - but two in this picture. Router 1 over on the left and router
00:02:21 - 2 over here on the right. Now router 1 has a connection
00:02:24 - somehow to all these 192.168.x networks.
00:02:29 - We got 001020, all the way dah dah dah dah
00:02:32 - all the way down to 15.0. Now with every routing
00:02:37 - protocol router 1 is gonna send those routes over to router 2.
00:02:41 - That's what routing protocols do. So router 2 will now have
00:02:44 - all these routes in its routing table and it will say I can reach
00:02:47 - them, and that's how we know routing. That's how routing protocols
00:02:50 - educate each other. But, here's the problem with that. Router
00:02:55 - 2 now has 16 routes, 0 through 15, sitting in its
00:02:59 - routing table. That it.. Number 1 goes all the same direction
00:03:03 - to reach. There's only one way to get to router 1 and all of
00:03:07 - those separate routes, so, you know that's..that's the first..first issue.
00:03:11 - issue. The second issue is, if one of these networks go down,
00:03:15 - then that needs to be updated and replicated to router 2 and router 2 is
00:03:19 - gonna have to replicate that to other routers in the corporate
00:03:21 - network because they all have the same routes, and they all need to have
00:03:24 - a synchronized routing table. Now, honestly, when you will when
00:03:28 - we look at this picture does router 2 really need to know
00:03:33 - that the 1.0 network went down?
00:03:36 - Now, initially my thoughts go to well, yes it needs to, because
00:03:39 - maybe it has traffic that's going to that. Well, if router 1 knows that it's
00:03:43 - down router 2 will send a packet, the very first packet to router 1,
00:03:47 - and router 1 will then just reply and say ICNP unreachable.
00:03:51 - You can't get there from here. So, router 1 will take care
00:03:55 - of any packets that are going to that network, because it knows
00:03:58 - that it's down, so in the big picture, it really doesn't make
00:04:02 - too much sense for router 2 to know the specifics about those
00:04:06 - networks. So here's what we can do.
00:04:09 - Route summarization is the process of summing up all these routes
00:04:13 - into fewer advertisements. I'll give you what I will call
00:04:17 - cheap summarization. Here's an example. I can go on router 1, and I
00:04:22 - could say, I am going to advertise
00:04:33 - Meaning, I've pulled this subnet mask back to be a class B subnet mask.
00:04:37 - So, here's what in essence router 1 is saying, it's saying router 2
00:04:41 - I know about every single network that starts with
00:04:45 - 192.168. Router 2 puts that in it's routing tables and says,
00:04:49 - wow, that's a huge router over there. It's got all
00:04:52 - of my 192.168 networks. Now it only really has these
00:04:56 - 16 of them, but it's advertising the whole scope. Now when you
00:04:59 - do that, the routing protocol will automatically suppress all the more
00:05:04 - specific routes. So there's no need to go and send
00:05:08 - or 1.0 or 2.0 or 3
00:05:11 - because all of those are encompassed in
00:05:15 - It's the one big advertisement.
00:05:19 - Now that's.. that's like I said what I call
00:05:23 - cheap summarization, because
00:05:26 - it's not very efficient. Meaning, if I send that route advertisement,
00:05:32 - that means that this router, router 1, claims to have all 192.168 networks
00:05:37 - and I can no longer use 192.168
00:05:40 - networks anywhere else my network because router
00:05:43 - 1 has laid claim to them all. says that I have them
00:05:46 - you cannot use them. So, it only has 16 networks but it
00:05:50 - claims to have them all, and we've now wasted a whole chunk of IP
00:05:53 - addresses that could be of use to us. So here's how you can do
00:05:57 - router summarization more efficiently. We need to get back to working
00:06:01 - in binary, and working in our subnetting mindset. Here's the idea.
00:06:05 - I've got all of these works. They all start with 192.
00:06:08 - They all start with 168. Now, we see the difference
00:06:13 - in the third octet. Let's break that into binary. 192.168.0
00:06:16 - is one, two, five, six, seven eight zeros,
00:06:20 - then all zeros over here, right? 192.168.1
00:06:23 - is 00000001 dot, and all zeros over here.
00:06:28 - Two, I'll just kinda, parenthesis here, is all zeros 1 zero. Three,
00:06:34 - is all zeros 1-1. Four is all zeros 1-0-0. Now we're all
00:06:40 - thinking in binary. And the, and the same trend goes down you know
00:06:43 - da..da..da..da..da. Let me get to, ah, I'm running out of space here.
00:06:46 - So, ah, let me, let me go to we'll say 13. Thirteen in binary
00:06:51 - is 128-64-32-16-8-
00:06:56 - 4-2-1, that's thirteen. Fourteen is 1110; Fifteen 1111.
00:07:03 - So, looking at all this, I know it's a little
00:07:07 - difficult to see because my binary's not perfect. I should actually,
00:07:10 - you know what, I should have typed these out. Hold on...
00:07:16 - Shazam. Take a look at that. Through the magic of video, I have already typed
00:07:21 - all of these up into binary. Now, let me go back to what I was talking about.
00:07:25 - We've got 192.168, and then here's our binary bits
00:07:28 - broken up, and I put the decimal number next to it. You see
00:07:31 - 0,1,2,3,4,5, and then down here I've got
00:07:36 - 13, 14, 15.
00:07:39 - Alright, the concept of route summarization is to take the bits that
00:07:44 - you have found that are similar, that are the same between
00:07:48 - all of these routes and group them together. So we look
00:07:53 - at our..our routes right here. We've got 192. That would
00:07:57 - be in binary, 8 bits that are all the same, right? 168,
00:08:00 - eight bits that are all the same, because every single one of
00:08:04 - these networks start with 192, and every single one starts
00:08:06 - with 168. Now we come to the third octet, and we look
00:08:10 - and we see, ok looks like, all those are the same, all these
00:08:15 - are the same, all these you can see my little lines going down; let me,
00:08:18 - let me, actually draw a perfect line right here, right...kachunk...kachunk.
00:08:24 - There. You see my magic dividing line? That is where the bits
00:08:30 - start to go different. So, what I could say is these have
00:08:35 - 8, 16, 17, 18, 19, 20. So, four extra bits here. Twenty
00:08:41 - bits that are all the same. So the perfect summary route that
00:08:45 - router 1 can advertise to router 2, is 192.168.
00:08:50 - .0,.0, slash 20.
00:08:55 - Notice I started with the very first
00:08:59 - network in my list. so that that
00:09:04 - is going to be what I start with and 20. Now if you're thinking
00:09:07 - about this in terms of subnetting, if I had a slash 20, that
00:09:12 - would be,
00:09:16 - if I were to break this into binary my subnet mask would be
00:09:20 - 11110000. My increment, thinking
00:09:24 - back to subnetting here, would be 16.
00:09:27 - So, if I were to take and I were reverse engineering this
00:09:31 - if you will to find out what my network ranges
00:09:34 - are, the first one would be Second one
00:09:37 - would be 16.0, 32.0, and so on. So
00:09:42 - a /20 represents 0.0 through
00:09:46 -
00:09:52 - Perfectly encompassing all of these routes that I have behind router 1.
00:09:55 - So router 2 now has the perfect route on its routing table.
00:09:59 - All of these can be suppressed. Now before I expand more on...on
00:10:03 - the specifics of the summarization, and kind of
00:10:06 - growing it a little bit;
00:10:07 - I want to mention what that does. Number one, it accomplishes
00:10:10 - our goal - larger routing tables equal slower routers, router 2
00:10:14 - now has a smaller routing table. Second, is it suppresses
00:10:18 - updates. If one of these networks go down, as I was mentioning
00:10:22 - before, router 1 no longer sends an update to router 2, and router
00:10:26 - 2, flooding the rest of the corporate network with that update,
00:10:28 - because router 2 doesn't care. It doesn't know
00:10:33 - It just knows the big summary, so there's
00:10:36 - no purpose in sending the notification.1 is down,
00:10:39 - because router 2 doesn't really even know about
00:10:43 - the.1 network. It's all hidden. So we accomplish our two major objectives
00:10:48 - by putting that perfect summary route in there. Now, this
00:10:54 - summarization example
00:10:56 - is perfect. Meaning in the real world, I mean it would be awesome if
00:11:01 - router 1 had those networks behind it, but unfortunately, router 1
00:11:05 - or our organization, just went through a growth spurt, and they
00:11:08 - just added
00:11:11 - behind router 1. D'oh, totally goofs things up. Because 16
00:11:16 - in binary, if I were to put it in here, would be
00:11:20 - 00010000. Totally goofs up my summarization route.
00:11:26 - Now, some of you might be thinking, well, we can fix that. Right?
00:11:29 - We can, we can take that line of yours right there... Let's see if this
00:11:33 - works. Grab my line, and we could move it over. Look at that.
00:11:39 - The power of animation. We can move that line over and
00:11:43 - now you've got, that would be the first three bits..chum chum... are the same.
00:11:48 - So my new subnet mask would be
00:11:53 - because we've moved our line back a
00:11:58 - bit to catch our extra 16 network that we added down
00:12:02 - here. That would be a solution and you're absolutely right
00:12:06 - in thinking that way that router 2 would now still only have
00:12:10 - one route on its table. But when you move that line back you
00:12:14 - didn't just catch the 16 route,
00:12:17 - you caught 17 (0001). You caught 18 (10010).
00:12:22 - You caught 19 (10011). Because
00:12:26 - all of those have the same three first digits, or three
00:12:30 - binary bits and the last five are different. So you've encompassed
00:12:35 - a lot more networks than just sixteen by doing that. Now that
00:12:38 - may be okay, but that's also I mean this this by the way
00:12:41 - will go down,, all the way to 32 because
00:12:45 - by moving our line back one, our new increment has become 32.
00:12:48 - So, we've encompassed networks all the way up to, I guess you could
00:12:52 - say, 31.255 would be the last network
00:12:56 - encompassed in this. 32 would start the next range.
00:13:00 - Now that's a lot of networks to do. So let me show you what
00:13:02 - most people will do in the real world. If you have growth, router 1
00:13:07 - will say, ok, I've got 16. I'm going to keep advertising
00:13:11 - that summary route this /20... sorry it's getting a little scribbly here...
00:13:14 - this /20 will be advertised to router 2, so it encompasses
00:13:19 - the first 15, and then I will advertise
00:13:22 -,
00:13:28 - as the separate route. So, in that logic, it's a lot better to
00:13:32 - have two routes in router 2's routing table than it would
00:13:35 - be have seventeen, 0 through through 16, so we're still
00:13:38 - accomplishing running efficiency, but we're not grouping a bunch
00:13:42 - of networks that router 1 does not have. So as router 1
00:13:46 - keeps growing, you know later on they add.17 and
00:13:49 - .18, and.19, those routes will be advertised individually
00:13:54 - until the organization reaches the point that they say,
00:13:57 - okay, we've got enough networks behind router 1 now. We can
00:14:02 - safely move that binary line back in our summarization to a
00:14:06 - /19, and encompass 0 through 31 networks,
00:14:11 - in that one summary route. So that is the idea of summarization and
00:14:15 - that key concept is what lights the way in OSPF, and why OSPF
00:14:21 - is such a powerful routing protocol.
00:14:24 - Now that you have the concept of summarization under your belt,
00:14:27 - we can move into the ideas and terminology behind OSPF.
00:14:33 - Half the battle in learning this protocol and this is a complex
00:14:36 - protocol, is understanding the terminology and the whats and whys
00:14:40 - that are used in OSPF, and the most foundation of all the terms
00:14:44 - is the concept of area. An area in OSPF is a group of
00:14:51 - routers that all have the same routing information. See here's the idea.
00:14:55 - When you have a network that continues to grow bigger
00:14:58 - and bigger and bigger, the routing tables on all the routers
00:15:01 - begin to grow bigger and bigger and bigger as well. So what
00:15:04 - we can do is split our network into groups of routers, like I can have
00:15:08 - you many routers within an area here.
00:15:12 - And all of those routers would have the exact same routing
00:15:16 - information. Here's a good analogy to describe it. In the trunk
00:15:20 - of my car I have a Rand McNally's road map of Arizona.
00:15:27 - Now also on the wall in my office right here, I have a world
00:15:32 - map. It''s actually an area code tracker that shows me area codes
00:15:36 - all around the world. And it''s this big world map that I can
00:15:39 - just look at and see area codes. Now if I were trying to figure
00:15:43 - out how to get to
00:15:46 - the mall, which is about 15 miles from my house. Would it
00:15:50 - be easier to use the Rand McNally road map in my trunk or
00:15:54 - would it be easier to use the world map sitting right here on
00:15:57 - the wall?
00:15:58 - Well, it would be easier to use the one in my trunk because it's focused
00:16:02 - on specific areas of Arizona. And instead of looking at the
00:16:06 - world map and going, man, all these roads are so small. I can't...I can't see
00:16:10 - you know, could I use the world map? Yeah, maybe, you know if
00:16:13 - I can look close enough, and they actually put the road small enough
00:16:15 - on there. I might be able to do it but it would be a lot harder because
00:16:18 - there's so much more information I have to weed through to get there.
00:16:21 - But the one in my trunk is just more focused.
00:16:25 - That's the idea of areas. Once your organization grows too big
00:16:30 - all of your routers will have to process all that information.
00:16:33 - And every packet that they get, it's like they're looking at a world
00:16:36 - map of your organization. And it's going to slow them down. So, by breaking
00:16:39 - it into areas, you could say, okay well area 0 represents
00:16:43 - we'll say Arizona. Area 2 represents Florida. Area 1 over here
00:16:48 - represents California and the United States. And we have these
00:16:51 - specific areas that group together similar routers.
00:16:55 - Now, just to give you a random guideline. CISCO recommends that
00:16:59 - an area never be more than 50 routers.
00:17:03 - So, as your network grows, you can begin dividing into areas.
00:17:07 - Now, that is...that is a guideline. That is not a hard and fast
00:17:10 - rule. So now that we see what areas are, let's talk about the
00:17:15 - routers that make-em up. Inside of the areas, and I'm gonna stray
00:17:19 - straight from talking specifically about the backbone and types
00:17:22 - of areas and stuff like. I'm gonna first talk about the routers.
00:17:26 - Inside of an area, you'll have internal routers. And these are routers
00:17:30 - that belong to an area. That internal router connects to area
00:17:33 - 0 and knows nothing but area 0. In area 2, I have
00:17:37 - another router. That router is an internal router. In area 2, and
00:17:41 - it' knows nothing but area 2. These routers that sit
00:17:45 - between areas are known as ABRs. Area Border Routers.
00:17:52 - Now these are usually the beefier routers in your network. The ones
00:17:55 - that have a little more processing power a little more memory
00:17:57 - than the rest, because these routers have the road maps
00:18:02 - for two or more areas in the routing table, so they have to be
00:18:05 - able to process and look through, it's almost like two page
00:18:08 - of pages of the map, rather than one page. The big point about
00:18:13 - an ABR that
00:18:15 - you'll want to know, is that an ABR is the one that is able
00:18:20 - to summarize. Summarization. Now you know why I covered that
00:18:27 - key concept on on, a page ago because if you didn't know
00:18:30 - what summarization was all about, the whole design of OSPF wouldn't
00:18:33 - make any sense. And when you're designing these areas, it has to be
00:18:38 - a hierarchical design. And what that means is that you group similar
00:18:43 - subnets in similar areas. So, for example, in area 1 you
00:18:47 - know I've got my 50 routers, and maybe over here I do
00:18:50 - 172.16.1,.2,.3, all of these
00:18:53 - different subnets, all /24s we'll say for ease.
00:18:57 - And I've got all these different subnets. And the ABR
00:19:00 - as it advertises area 1 to area 2, can sum that up and say, oh
00:19:05 - area 1 is all about
00:19:09 - Yes I'm using some cheap summarization,
00:19:13 - just for easing here, some easy stuff. But at the same time,
00:19:16 - you get the concept. The area 0 backbone routers, they don't
00:19:21 - need to know anything more about area 1 then that one route.
00:19:25 - So 50 plus routes summed up in one. Same thing with area
00:19:28 - 2. Maybe this is 172.17.1,.2,.3, and so on...
00:19:33 - And we sum that up as it comes in the backbone. And the backbone
00:19:36 - has a hierarchical network of its own. If you don't design your
00:19:40 - network right with OSPF, it will tear you apart, because the ABRs
00:19:45 - will not be able to summarize, and there's no point in
00:19:48 - dividing into multiple areas. And let me emphasize that. The
00:19:51 - whole reason that we even use multiple areas is to summarize.
00:19:57 - If you don't summarize when you break into multiple areas,
00:20:00 - you're defeating the whole purpose and you're just causing more
00:20:02 - processor cycles on all of the routers. So, when you're setting
00:20:06 - up an OSPF network, you have to be very careful where to place things.
00:20:10 - If I were to go to area 1, and just say, ah, ah, let's you know let's 192.168.1 over here.
00:20:14 - Ah, let's throw that.10 network...
00:20:17 - over there. You''re shooting yourself in the foot, because you
00:20:20 - can't summarize that in a hierarchical format. That's known
00:20:24 - as an IP address hierarchy. So, let me hit the specifics that I have
00:20:29 - here. All areas, the rule of OSPF, all areas must connect
00:20:33 - to area 0. That's what's so special about area 0. It is considered
00:20:38 - the backbone of your network and all other areas must connect
00:20:42 - over here as your network grows larger and larger, things must
00:20:46 - tie into that area. All routers within an area have the same
00:20:50 - topology table. And if you want to emphasize that, that means
00:20:54 - road map. All of the routers in area 0 know everything
00:20:59 - about area 0. Even the routes that they're not using. The backup
00:21:02 - routes, you know, and they know everything. And that's fantastic
00:21:05 - because if a route goes down in that area, the routers, wham
00:21:09 - converge in a snap. They're able to find those backup routes
00:21:12 - that they have in their road map. They just pull the road map back out and
00:21:15 - regenerate the routing table. Now let me emphasize the
00:21:18 - difference here. All routers in the same area
00:21:22 - have the same topology table. Or they all have the same roadmap.
00:21:25 - But, every router within the area will have a different
00:21:32 - routing table.
00:21:34 - Hmm, let's talk about that. We've got, we'll say, this...this router
00:21:38 - up here. This area 0 is represented as Arizona. Maybe this router
00:21:42 - is what connects to Phoenix. This router over here connects to
00:21:45 - Tempe, that's another city here. And this router over here connects to Tucson.
00:21:51 - So we've got, you know, these...these three different routers. Now, in
00:21:54 - Tempe it'll have the road map of the entire area 0, the entire
00:21:58 - backbone. Tempe knows how to get to Tucson. It knows that it
00:22:02 - has a backup route to go to Phoenix to go to Tucson, but it can
00:22:05 - also go to whatever the city is. Scottsdale we'll say. And get to Tucson. It's got
00:22:09 - all this information so if its primary route through Scottsdale
00:22:13 - fails, it just looks back at the road map and says oh, I've got another
00:22:16 - route through Phoenix. That's meaning the same topology table. All
00:22:20 - routers area 0 will have a different routing table
00:22:23 - because they all start from different points.
00:22:26 - So, Tempe will say, my best route to get to Tucson might be
00:22:31 - through the Scottsdale router. Whereas Phoenix will say well I've got a
00:22:34 - direct connection to Tucson down here. I'm going to use that as my best
00:22:37 - route to get to Tucson. So even though they have the same road
00:22:40 - map, they all generate different routing tables. It's just like,
00:22:44 - I mean think about it logically. If you have the same road map
00:22:46 - in your trunk that somebody else has and they live some completely
00:22:50 - different place than you, their best routes around the state
00:22:53 - are going to be different because they're starting point is
00:22:55 - different. They...they are in a different place in the
00:22:58 - state, so they will generate a different routing table.
00:23:02 - The goal of OSPF is to localize updates within an area.
00:23:06 - Whenever something happens in area 0 everybody knows about it.
00:23:09 - But,
00:23:11 - area 1 should not. Because we should be summarizing to area 1.
00:23:16 - And area 2 should not. So only things that happen in area 0
00:23:20 - will stay in area 0. It's like our Las Vegas mantra.
00:23:23 - What happens in Las Vegas, stays in Las Vegas. Same thing here.
00:23:26 - What happens in an area stays in an area. Finally, and I talked about this;
00:23:31 - requires a hierarchical design. You must design your network right.
00:23:36 - Oh, there is one more thing. Let me erase all this chicken scratch. The last
00:23:40 - term I want to throw out at you
00:23:42 - is this one over here tucked in the corner. The
00:23:45 - Autonomous System Boundary Router or ASBR. The Autonomous System Boundary
00:23:50 - Router is the router in OSPF that connects to networks outside
00:23:55 - of its own. This is not another area. This is a completely different
00:23:59 - network. So it could be a network that is running Rip over here.
00:24:03 - It could be the internet and I would say that's the most common
00:24:06 - network that the ASBR connects to. Others...there's many different
00:24:09 - things that...that this could be. But the ABR and the ASBR are
00:24:14 - the only two routers in OSPF that can do summarization.
00:24:18 - Between areas and between completely different routing systems.
00:24:23 - The last OSPF concept I want to talk about is how OSPF
00:24:27 - forms neighbors. Unlike the Rip protocol, OSPF will form
00:24:32 - direct neighbor relationships with the routers it wants to
00:24:35 - speak with. Rip just walks up to the ethernet line and says, hello
00:24:39 - everybody, sends out a broadcast to everybody. These are the routes
00:24:42 - I know about. It doesn't actually form neighbor relationships, it's just
00:24:45 - the other routers happen to hear it saying hello everybody,
00:24:48 - and adds those routes to its routing table. They don't know about
00:24:51 - each other directly. So, in OSPF, routers come up to each
00:24:56 - other and say, hello router, hello, and they start exchanging
00:25:00 - routes between each other and then they maintain that...that
00:25:02 - neighbor relationship using something known as the 'hello protocol.'
00:25:07 - It's not just what I'm saying. That's the technical name of the protocol. Now
00:25:10 - hello messages are sent, when you configure OSPF on whatever
00:25:14 - ...whatever interfaces you designate. So, if I say head out serial
00:25:18 - zero zero then it will start saying hello and trying to form
00:25:21 - neighbors on that interface. If it does, these neighbors will
00:25:25 - meet and they will exchange routes and now we have a synchronized
00:25:28 - routing table. In OSPF, these hello messages are sent
00:25:32 - once every ten seconds on broadcast or point to point networks
00:25:36 - And once every thirty seconds on non-broadcast multi-access
00:25:41 - networks. Those are things like frame relay which we'll talk
00:25:43 - about later. The idea is that the more often you send hello
00:25:48 - messages, the sooner you will know if a neighbor is down, because
00:25:52 - they'll stop responding to your hellos, and the faster you
00:25:54 - can change over to a backup route.
00:25:57 - Now, a lot of people in the OSPF world will tune this
00:26:01 - hello timer down, to a second or maybe two seconds. So you're just
00:26:05 - sitting there going, hello hello hello, making sure that they're
00:26:09 - online because you want to be able to detect that failure extremely
00:26:12 - fast. Now, when you and I say hello we think of a greeting.
00:26:18 - Like, hello how are you doing, or...or something to that effect. But
00:26:21 - when OSPF sends a hello, I want you to think about it like
00:26:24 - an envelope with hello written on it. And when that hello message comes
00:26:29 - across, the router opens that hello envelope and sees all of these specifics
00:26:34 - inside of it. It will see things like the router ID, which
00:26:38 - is the name of the OSPF router over here. It says hello, my name
00:26:42 - is, and the router IDs is an IP address. It might say
00:26:44 - and the router will say, well hello, I'm
00:26:48 - thats...thats the router ID. In that hello envelope
00:26:51 - will be that hello and dead timers, meaning how often they're
00:26:54 - saying hello, and it's kinda rude, but that's alright. It's soon
00:26:58 - until they believe you are going to be dead. You know how many
00:27:01 - hellos they can miss before they say that person must be down.
00:27:05 - They will advertise their subnet mask in that hello packet.
00:27:08 - They will advertise what area they are in in that hello packet.
00:27:11 - Now you notice some of these
00:27:15 - messages or I should say pieces of that hello envelope have
00:27:20 - little stars by them.
00:27:22 - The stars, I'll put a little key, star equals must match.
00:27:31 - Right? They have to match in the hello packets between the neighbors,
00:27:36 - or else these guys will not end up forming neighbor relationships.
00:27:41 - I think about it this way. My old roommate that that I used
00:27:44 - to live with. Ah, actually met his wife online. He...he had one
00:27:50 - of those
00:27:51 - dating, um, dating sites. I don't know what they're called,
00:27:55 - where match up. Right? And when you go to this dating site...
00:27:59 - I actually was working with him because I'm the computer
00:28:02 - guy of the house. And I was showing him how to get on there, and you
00:28:05 - know, log in and all that. And, ah, on this dating site you have
00:28:08 - criteria. And what you do in your criteria, you say,
00:28:11 - okay, this must match. For instance, you know this...this...this person
00:28:16 - or this mate that I'm looking for must have blue eyes.
00:28:20 - They...they must have an adventurous spirit. Meaning they like
00:28:24 - to do adventurous things. They must have, you know, and you list
00:28:27 - your must-haves. And then you list your, well you know, it would be nice if
00:28:31 - they had, um,
00:28:34 - pink painted toenails. I don't know. I'm just throwing stuff out there.
00:28:37 - You know. But you know if those don't match it's okay. In the
00:28:39 - same way, our routers are online daters. They are sending
00:28:43 - their little hello messages with each other and inside
00:28:46 - of there are criteria that must match. For instance, if this
00:28:50 - router 2 over on the righthand side says hello once a second and
00:28:54 - the one over on the left says hello once every ten seconds then
00:28:58 - it's gonna say, I'm...I'm sorry. We don't match. We're not compatible
00:29:03 - with each other. And at that point they will choose not to form
00:29:07 - a neighbor relationship. Now, a lot of these things that you
00:29:10 - see in this hello packet we haven't talked about yet. And we will talk
00:29:13 - about but the ones with stars, if they're not matching, the
00:29:16 - neighbor relationship will not work. And that's the number
00:29:19 - one trouble shooting criteria we have with OSPF, is
00:29:23 - making sure that all these things match. Otherwise routes won't
00:29:27 - be exchanged.
00:29:29 - That should be enough of the OSPF concepts to get us going.
00:29:33 - As we continue this in the next video we'll be able to see
00:29:37 - how these concepts apply in our configuration. But before we
00:29:41 - do let's wrap things up here. We first off looked optimization
00:29:45 - at its best, or the best way to optimize a router is through
00:29:48 - route summarization. We then moved into the OSPF terms and
00:29:52 - network design. And to hit the high ones, area is the most critical. Area
00:29:57 - defines routers that have the same topology database or road
00:30:01 - maps. ABRs, Area Border Routers, is what moves you between areas. And
00:30:06 - ASBRs, Autonomous System Boundary Routers, are the routers
00:30:10 - that move you outside of your own OSPF network, maybe to access
00:30:14 - the internet. We've then finally analyzed the OSPF hello packet.
00:30:20 - The hello packet is the foundation language that these
00:30:23 - these routers will use to communicate with each other
00:30:26 - and form neighbor relationships. If the relationships don't form
00:30:30 - routes won't be exchanged. I hope this has been informative for you and
00:30:34 - I'd like to thank you for viewing.

Routing Protocols: OSPF Configuration and Troubleshooting

Routing Protocols: EIGRP Concepts and Configuration

Access-Lists: The Rules of the ACL

Access-Lists: Configuring ACLs

Access-Lists: Configuring ACLs, Part 2

NAT: Understanding the Three Styles of NAT

NAT: Command-line NAT Configuration

WAN Connections: Concepts of VPN Technology

WAN Connections: Implementing PPP Authentication

WAN Connections: Understanding Frame Relay

WAN Connections: Configuring Frame Relay

IPv6: Understanding Basic Concepts and Addressing

IPv6: Configuring, Routing, and Interoperating

Certification: Some Last Words for Test Takers

Advanced TCP/IP: Working with Binary

Advanced TCP/IP: IP Subnetting, Part 1

Advanced TCP/IP: IP Subnetting, Part 2

Advanced TCP/IP: IP Subnetting, Part 3

Please help us improve by sharing your feedback on training courses and videos. For customer service questions, please contact our support team. The views expressed in comments reflect those of the author and not of CBT Nuggets. We reserve the right to remove comments that do not adhere to our community standards.

comments powered by Disqus

Course Features

Speed Control

Play videos at a faster or slower pace.


Pick up where you left off watching a video.


Jot down information to refer back to at a later time.

Closed Captions

Follow what the trainers are saying with ease.

Offline Training

Our mobile apps offer the ability to download videos and train anytime, anywhere offline.

Accountability Coaching

Develop and maintain a study plan with assistance from coaches.
Jeremy Cioara

Jeremy Cioara

CBT Nuggets Trainer

Cisco CCNA, CCDA, CCNA Security, CCNA Voice, CCNP, CCSP, CCVP, CCDP, CCIE R&S; Amazon Web Services CSA; Microsoft MCP, MCSE, Novell CNA, CNE; CompTIA A+, Network+, iNet+

Area Of Expertise:
Cisco network administration and development. Author or coauthor of numerous books, including: CCNA Voice 640-461 Official Cert Guide; CCNA Voice Official Exam Certification Guide (640-460 IIUC); CCENT Exam Prep (Exam 640-822); CCNA Exam Cram (Exam 640-802) 3rd Edition; and CCNA Voice 640-461 Official Cert Guide.

Stay Connected

Get the latest updates on the subjects you choose.

  © 2015 CBT Nuggets. All rights reserved. Licensing Agreement | Billing Agreement | Privacy Policy | RSS