This is one of multiple videos discussing OSPF, D.R. and PDR or designated router and backup designated
router elections and selection criteria.
On Cisco, we have this document that says that DIERS and BTR elections are done via the hello protocol.
Hello packets are exchanged via multicast packets on each segment.
The router with the highest priority on a segment will become the DAR for that segment.
The process is repeated for the BDR.
In the case of a Thai, the rider with the highest rider ID will win.
The default priority on an interface is one.
If you set the priority to a zero, it means the router will not become a D.R. or a PDR.
Remember that the D.R. and PDR concepts are pur multi access segment.
Setting the OSPF property on an interface can be done using the IP OSPF Property Command.
Now let's look at the segment that has router three, router to router one and router four connected
to it.
On this segment.
Rather one has neighbor relationships.
To write a to write a three and rather four note to the interface gigabit zero.
The stop entry is for the segment on gigabit zero one, but we are concentrating on gigabit zero zero
for the moment so we don't see a door.
Show IP OSPF interface brief.
What we do see is that the local Rada Rada one is the designated Rada.
Now, that goes against what we've just read.
The Rondo idea of the strata is lower than the other riders on the segment, and this is where real
world comes into play versus theory.
The reason why Rider One was elected as the designated rider is that in the previous video I configured
Rider 1/1 before I configured the other riders.
So Rider one was the only Radha on the segment.
It was waiting for other routers.
None appeared.
So it became the D.R. when the other routers were added to the segment, and I added them in the order
of router two, then router three, then router four.
Router one was already the designated router.
Once a designated rod is elected and a writer comes onto the segment with a higher priority or higher
rod ID, it's not going to usurp or preempt the designated rider.
The designated rider remains the designated router and hence the local router with a lower router ID,
which we can see by using the command show IP OSPF interface.
Is elected as the DA.
On gigabit zero zero.
Because it came online first.
Notice it has an adjacency or neighbor relationship with three routers.
Router two is the backup designated rata router three and root of four are druthers.
Show IP, OSPF and neighbor router two is a BTR router three out of four or D or others on gigabit zero
zero.
So what happens if we go into gigabit zero zero?
And we top ip ospf priority zero.
Show ip ospf.
Interface.
What you'll notice is.
The rata state on gigabyte zero zero has changed to dire other.
It's now formed a full relationship to rather to so show IP OSPF neighbor.
Rather one has a full relationship to brother two as well as rather for the reason why rather to became
the D.R is.
It was already the PDR and then.
Out of Route three and Route A four.
Route a four became the BTR because it had a higher router ID.
So the theory is as follows It with the highest priority becomes designated rudder on a segment.
That's not entirely true.
If one rider comes onto the segment first before other riders, that rider will be the designated rider.
But if they all come online at the same time, it's based on highest priority, then highest rate ID,
then highest loopback interface IP address, then highest configured physical interface address.
And in this case, Rider four became the backup designated rider because it had the highest IP address.
So what I'm going to do now is reboot the switch so that you can see the full process of how the exam
explains how rider IDs are elected.
So first thing I'll do is change.
The priority.
Overall a one back to one.
On router two.
Let's confirm the relationships.
Show ip ospf neighbor.
Rather one is a.
The root of three is a draw.
The root of four is a backup designated rider.
And the local Rada is a designated Rada.
In the output chair we can see that we're out of four is the backup designated rider router three and
rider one or druthers.
So let's see what happens when we shut down interfaces on.
The switch, so shut down a range of interfaces, gigabit 0 to 3.
So I've shut those interfaces down.
On router one show ip ospf neighbor.
Now it may take it a while in Genesis three to realize that the interfaces have gone down.
So what I'm going to do at this point is pause the video, and what we should see is that the states
change.
So after a while, the date timer expires for the neighbor relationships.
Previously we had these neighbor relationships.
And now those neighbor relationships have disappeared.
The only neighbor relationship that writer one has is to write a for using this Ethernet link.
So to ensure that it doesn't get confusing, I'll shut that link down as well.
So we have no OSPF neighbor relationships on router one at all.
The switch has all of its interfaces shut down once again.
So let's have a look on router to show ip ospf neighbor.
No neighbor relationships and router to.
Rather three show ip ospf neighbor no neighbor relationships rather for show ip ospf neighbor.
It only has a neighbor relationship on the serial interface, not on the ethernet interface.
So router four has a neighbor relationship to right of five, but no other neighbor relationships.
So on the switch.
No, shut.
We've enabled all the interfaces on the switch.
Some rather one.
After a period of time, we should see that neighbor relationships are formed.
We can do a ping to check if the interfaces have come up.
Interfaces are starting to come up on the switch.
Pings are now succeeding.
And now we see neighbor relationships.
Three neighbor relationships have been formed on rather one such show.
IP ospf neighbor.
Rather one has a two way relationship to router two.
Router four has become the designated router and router three has become the backup designated router.
The reason why router four became the designated router is because it has the highest IP address on
any physical interface.
This interface has an IP address of 10.11.
This interface on the other two is ten one one, two.
This interface on router three is ten 113.
This to face and write a four is ten and 114.
But in addition it has an IP address on the serial interface of ten 1 to 1.
So because it has the highest IP address of any physical interface.
It becomes the D.R..
With this IP address.
The reason why it didn't use this IP address is because this interface was added after the OSPF process
had run, so the router ID had already been selected.
These two interfaces were up when OSPF started.
This interface wasn't up when OSPF was started.
So the router ID that was chosen was ten 1 to 1.
So that proves that the router with the highest IP address of any physical interface will become the
designated rider on a segment when all the devices come up.
Let's see if we can change that by manipulating priorities.
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