A link site request requesting full LSA information from the neighbouring router.
The neighbouring router will send what's called a link state update, which is a packet that contains
links, state advertisements and as mentioned is typically sent in response to a link state request.
This contains detailed information about the link state database rather than just an overview of it
which was contained in the database description.
Links that acknowledgments acknowledge or confirm receipt of a link state update message.
Now an autonomous system is a grouping of routers under a common administrative domain.
So let's assume that the rat is contained within the blue portion of running OSPF within a single autonomous
system.
They may be connecting to other routers under another administrative domain or another company's control
that is running, for example.
OSPF is an IGP or Interior Gateway protocol.
In other words, it's used within an autonomous system.
So within your organization you may have multiple routers running OSPF within the same autonomous system.
Full scalability.
OSPF Networks are broken up into areas.
Now there's debate about this, but Cisco would recommend never more than 50 routers within a single
OSPF area.
In your studies in the future you may come across different figures, but that's a good rule of thumb
to use.
OSPF uses a hierarchical model in that you always have OSPF area zero.
When you have more than one area, it is possible to run OSPF in another area, let's say area one.
But that's only true if you have a single area.
If you've got multiple areas, you have to have area zero, which is known as the backbone area, all
traffic from one area, let's say area one to another area, let's say area two will traverse the backbone.
So what you do is you break up your network into multiple areas following the physical topology with
the view to optimize summarization and reduce routine table updates and link state advertisements.
Certain types of losses can be contained within an area, so the flooding of Lisas throughout the network
is stopped by breaking up the network into multiple areas.
Routers that border the backbone area and another area are known as area border routers or ABS.
The three routers highlighted here are ABS because they have one interface in Area Zero and another
interface in another area.
One of the advantages of ABS is that they allow for summarization of routes.
If Area one contains routes ten 110 up to ten one 100 zero and in Area two, we have networks 10 to
1 zero, up to 10 to 100 zero.
An Area 310 310 up to ten three 100 zero.
Those roots can be summarized on Abydos, so you could say these 100 roots will be summarized as a single
root on this ABR.
So a slash 16 most is used to summarize these 100 roots.
The same could be done here.
10 to 0 zero slash 16 is a summary of these roots.
And lastly, ten 300 slash 16 is a summary of these roots.
This does require careful planning and good IP design.
This Rada is known as an autonomous system border Rada or SVR.
It borders two autonomous systems.
In this case, we've got RIP on the left hand side and OSPF on the right hand side.
Even if all of these routers, including the router, were within your organization, this router would
still be known as an ACR because it's connecting one routing process rip to another routing process
in this case OSPF.
So from an OSPF point of view, this is an autonomous system border router.
These six routers all have interfaces within Area zero and therefore are known as backbone routers.
When traffic is sent from one area to another area, it has to traverse the backbone to reach the destination.
So these three routers in the backbone are used to allow the traffic to flow from area one to area two.
OSPF once again requires a good design with all areas connecting to the backbone area as traffic flows
from one area to another area through the backbone area.
These highlighted routers are known as internal routers.
They are internal to their specific areas.
OSPF routers use halos to form neighbor relationships or adjacencies.
The Hello protocol once again establishes and maintains neighbor relationships by ensuring bi directional.
In other words, two way communication between neighbors.
Bidirectional communication occurs when a router recognizes itself listed in the packet received from
a neighbor.
How those are sent using multicast address 224005 and contain the following information for neighbor
relationships to be formed.
It's important to remember that certain parameters have to match on both routers.
Now the first field containing a hello is the router ID.
A router ID identifies this specific router and is used in various scenarios in OSPF, including the
election of a designated router or backup designated router.
A router ID is chosen per router based on the highest IP address of any configured interface.
When OSPF is enabled on the router or on the highest loopback interface active on the router when OSPF
is enabled or it can be manually specified using the router ID command.
So to demonstrate that you have a router, right.
One.
Show off the interface brief will show me the relevant interfaces on this router.
And what I'm going to do is I'm going to shut down all the interfaces and show you that OSPF is not
able to select a router ID if the interfaces are all shut down.
So show IP interface brief again.
Shows me that all of my interfaces are administratively shut now in global config mode popping the command
router.
In this case, OSPF.
And a process ID will allow me to enable OSPF on this router.
The process ID allows for the differentiation of multiple OSPF instances running on the same router
that can become important in multi protocol label switching environments or MPLS environments.
For this course you'll only run a single OSPF process on a router, but note multiple processes can
be enabled on errata.
Now notice what the router says.
No router id ospf process one failed to allocate a unique router id and therefore cannot start.
Topping the come on do show IP protocol shows that the writing protocol in use is OSPF one, but the
router ID is 0000.
OSPF is not running correctly on this router.
Show IP interface brief.
Let's enable this interface.
So interface.
If zero zero.
No.
Shut.
So what I'll do is I'll remove OSPF.
And then I'll re-enable, OSPF.
Notice there is no complaint.
Do show IP protocols.
Shows me that the router ID is ten 111.
Do show up to interface brief.
Shows me that this IP address has become the router ID.
The reason why is the router ID is chosen based on the highest IP address of any active interface when
the writing process is enabled.
So if I enabled, for instance, these two interfaces.
As follows.
And then top the command.
Do show IP interface brief.
You'll see that these interfaces, the fast Ethernet interface and the two serial interfaces are up,
up.
Do show IP protocols still shows that the router ID is ten 111.
I've just changed the encapsulation back to its default so hdl-c and the interfaces come up again.
But what I'd like you to see is that the Ryder ID still remains as ten 111.
And I can do that command again.
Do show IP protocols.
Notice the Rada IDs ten 111.
I can do this.
Come on.
Clear.
IP ospf process to clear the OSPF process and let's see if that makes any difference.
So show ip protocol.
And as you can see, the root of ID remains the same.
But if I removed OSPF.
And then re enabled ospf.
Notice the router ID has changed to ten 151.
So what the Rada has done is it looked for the highest IP address on any active interface when OSPF
was enabled.
This time, when OSPF was enabled, ten 151 was the highest IP address on any active physical interface.
So it was chosen as the router ID.
Now what happens if we enable the loopback?
You'll notice the loopback has the lowest IP address.
One is far lower than ten.
So no.
Shut that you show IP protocol.
You can see the Rada ID is stole.
Still ten 151.
I'll do it again.
Rada ID is still ten 151.
But if I remove OSPF.
And then re-enable it.
Notice the radar ID is now quadruple one.
So the rule again.
OSPF router ID is chosen based on the highest IP address of any physical interface that is active when
the OSPF process is enabled.
But if there is a loopback that is active, the loopback overrides the physical interfaces and the loopback
is used as the router.
ID loop backs have the advantage that they never go down unless manually shut down.
This has multiple advantages, including stability.
If my router ID was originally ten 111 and the router reloaded, assuming that there were no loop backs
enabled the router, id would have changed to ten 151.
So there are two ID number could have changed, but with a loopback the router ID would remain as quadruple
one as long as there were no other loopback configured on this router.
So it doesn't matter what the physical interfaces are set to.
It's recommended to set the router ID manually using this command and set it to a relevant loopback.
In this case, I'm just going to set it to some arbitrary value to show you that you can.
So I'm going to set it to 1921681.1.
Which is not an IP address on my local router do show.
IP protocol allows me to see that the router ID is 1921681.1.
Now it's recommended practice to set the router ID to one of the loop backs on your router.
So I'm going to set the router ID to quadruple one.
As you can see here, the Rada ID is changed to quadruple one.
The Hello packet then contains the hello and dead intervals, which must be the same on both routers.
Otherwise, an adjacency or neighbor relationship will not be formed.
It then contains the list of neighbors that thereafter knows about.
That's how a router knows if there's two way communication, because it recognizes itself in the list
of neighbors that it receives in the hello packet.
It then contains the area ID, which must also match on both routers.
It then contains a right of priority which can be used in designated router and backup designated router
elections.
It then contains the designated router or D or IP address backup designated router or BTR IP address.
We're going to talk more about designated routers and backup designated routers in a moment.
It then contains the authentication password.
Now, there are various ways to set up authentication, including clear text and MD5 hashing.
We'll talk more about those later.
The authentication password needs to be the same.
Otherwise, the relationship will not be formed.
And then lastly, the stub area flag needs to be the same.
The stub area flag denotes whether this is a stub area or a normal area.
We'll talk more about stub areas once again in later slides.