The next topic of discussion
is open shortest path first, which is a routing protocol.
Now, U. S P F is not a distance vector routing protocol. Oh, SPF is our first and only link state routing protocol covered in the C c a.
you don't send routing updates every set, interval or when the topology changes. Routing updates are not sense, like their scent in the i g. R p and rip.
So what does it SPF do? Well,
each router in new SPF upon startup or upon initialization when you start the SP of process describes itself
called Link State advertisements.
Now an L s a r a link state advertisement within it contains the routers links, which means the routers, interfaces and the i p networks attached on those links.
So a routers link, for example, serial 000 on router to Owen and the I P network attached to it, which is 202 102 100 zeros. Last 30.
The neighboring routers, so router to one would also describe router one inside off its L S A.
And the metric to get to those neighbors. So the metric to get from Router two A 12 router one.
Then each router floods these l essays to outlet apology. So l s a flooding done by every router. So each router takes this l s a bills analysis, puts this information in it
and then floods it to everybody else
At the end of the flooding process.
Each router has an L s A from every other router,
and it builds a database. Of all these Ellis is called the Link State did of it so identical links their database built on every router.
The link state database on every router looks exactly like the link state database on every other router.
After the link State database has been built, the shortest path first algorithm is run over
the links, their database and each route. Each router independently calculates routes to remote destinations
and puts them in their routing table or puts them in the rib
or out information base. Now you see the difference here between E. I. G. R p and rip
which route by rumor which basically believe information that the next upstream router is sending them instead of that happening. Oh, SPF independently calculates its own route to every destination with that
doing the routing by rumor approach without getting its routes or its information from somebody else. It automatically independently calculates routes to remote destinations and puts them in the rib. When tourists, we have routers first come up when you first type in the commands and
you bring up the SPF on
one rather and then on another router. The first thing they do is send each other all the l essays, and they synchronize their databases with each other.
Once this database synchronization has happened, these routers are set to be adjacent with each other or an adjacency has formed or their neighbors with each other.
These neighbors are then stored in the neighbor table.
So, as we will see, we will execute a command show I p o S p f neighbor, and we will see all our neighbors in the neighbor table just like it happened with E i g r p.
Now these neighbors are kept alive through the hello message.
Okay, just like with the I g r p, the link is kept alive through the sending off fellows.
The hollow message inside off It contains the dead timer.
So the dead timer is amount of time. I'm going to wait for a neighboring router to send me a hello before considering it down.
The debt timer is usually set to four times or the dead timer by default is set to four times the hello timer.
So what is the low timer? I lose a cent every 10 seconds over monthly access networks like Ethernet
or or broadcast multi access networks like Ethernet and Hollows ascent every 30 seconds over non broadcast multi access networks like train relit,
friend, relatable cover towards the end of the class.
hellos are sent true. The multi cast address to 24 005 Certain fields in the hello packet must match for two routers to become neighbors. The first field that must match is the I. P. Sub net and the sub net mask off that interface that is talking to the other side
or talking to the neighboring router.
Authentication information also must match for the adjacency to form
to form between two SPF neighbors. So if you're using some kind of authentication both authentication,
both sides must have the same type of what indication on both sides must have the same with indicating passwords.
The hollow and dead timers must also match in the low packets for the adjacency to form.
Also, something called the Area I D. And I will get into that in a little bit. Must match for two routers to become Oh, SPF neighbors
Each router picks for itself. Out of all the available I P addresses on the router. Something called a router i d. The router idea is nothing more than an identify her for the router itself.
So in descending order, the router will prefer the router i d. If it is configured with the router I d command
next, the router I d will pick the highest. If the router I d command is not done, then the router will pick the highest configure new back I p.
And if no Lou backs of president than the highest physical interface address.
let's say you have new back.
with an I P address of 1.1, not 1.1 slash 32 you will see later on that Lou back into faces can actually have a slash 30 to mask, which is all to five fives
and then I have new back. I'll just a loop
Between the two of these,
let's say this is Lou back 50
between the two of these, which one will become the routers? Router I d. If the router I d command has not been executed
automatically 1 51 51 51 50 will become the router I d. So it doesn't matter what the loop back interface ideas. It matters with the I P addresses.
Okay, so this type of question might be on the exam. Don't get it wrong.
So your monthly access networks
If every router became neighbors with every other router,
you would end up with a lot of magician sees how many specifically to
into n minus one neighbor relationships.
How do I get that? Well, let's see if I have full routers and these dots represent routers
and every router formed a neighbor relationship or in adjacency with every other router,
123456 adjacent seas or true into N, is the number of routers, which is four minus one
to into three, equals six adjacent seas.
So as the number off routers go up, Jason sees go up, and this is just way too much overhead.
So what happens in a multi access network is multi Axis networks. Elect a router, call the D R or a designated router
and a backup designated router, which, as the name says, is simply a backup for the D. R. If the D R fails and every router becomes adjacent with Onley, the D r and B D r.
Okay, and every router becomes adjacent with adjacent with only the D. R and the BDR. Now, as you will see in most networks, we don't even allow v d our election or backup designated router election. We just elect one designated router and everybody becomes adjacent with that one router. So this picture would look
with this guy being the D R.
Only three adjacent seas instead off six.
Now, how is a designated router elected? Well, a designated router is elected according to something called the SPF router priority.
By default, all routers have a priority off. One
priority values can range between zero and 255
So the higher the priority, the better. So the highest priority router becomes the D R.
Priority zero routers do not participate in designated router and backup designated router election.
So now if everybody's priority is won by default, what's the tiebreaker? Well, the router with the highest router I d becomes the d. R.
However, having said that, it doesn't really work out that way in a real network because you're not going to configure your routers all at the same time. You're one person.
So the router you configure first Alexe itself as the d R. And then you have to go change it around.
D are other routers,
so D are other routers are the routers, as the name implies are not the designated router or the backup designated router.
So D are other outer sender l essays in a big packet
through the multi cast address. True, 24 006 actually
and the only one listening in on this address. The only routers listening in on this address are the designated router and the backup designated router,
and then the designated router and the back of designated router are initially just a designated router reflects these essays back to everybody else. Over to 24 005