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.
Now let's talk about designated routers and backup designated routers.
In this topology.
I have six routers connected to the same Ethernet segment.
So assume that these six routers are connected to a switch or a hub, all sharing the same Ethernet
segment.
Designated Rodders orders are used on broadcast multi axis environments such as Ethernet and certain
when implementations such as non broadcast multi access environments in frame relay.
So to explain why we have a designated router.
Let's assume.
That this network ten 110 is connected to right one.
And the six routers are connected to this Ethernet segment and assume that this network goes down.
So let's assume for the moment that there's no designated rider on this Ethernet segment, and hopefully
you'll quickly see why there's a requirement for a designated rudder.
Without a designated rado, all of these riders would have a full adjacency in a full adjacency also
exchanged between writers.
So in this example are one needs to notify the other routers using a link state update that there's
been a change in the network topology.
So our one will send an update to our three sent an update to our two to or four to or five to or six
notifying all of the routers that there's been a change in the topology or to when receiving that update
from R one in this case, because there's no designated router has a full relationship with all other
routers.
So it sends an update to all of its neighbors to notify them that there's a problem.
The same will happen on our three or three received an update from R one.
So it notifies all of its neighbors that there's been a change in the topology or for we'll do the same
thing.
It received an update from our one.
So it sends an update to all of its neighbors.
And I'm sure you're getting the picture now.
Our five received that update from our one, so it sends an update to all of its neighbors.
And lastly, our six sends an update to all of its neighbors.
So there's a lot of duplicate traffic.
When a single network goes down and these six routers have a full adjacency with one another.
So rather than doing that, a designated router is selected on this specific segment.
So let's assume that our two was elected as a designated writer.
Designated routers are selected on two criteria.
The first one is highest priority.
You can specify the priority on an interface.
The default priority is one zero.
Excludes a router from becoming a designated router or backup designated router.
The values for the priority are from 1 to 255.
So the first criteria is highest priority.
If the priorities are the same, then the rider with the highest rider ID is elected as the designated
rider for that segment.
So in this example, we've elected R2 as the designated rider.
And let's assume again that this network goes down.
But what happens now is our one sends an update only to the designated writers.
Designated Rodders are listening on this multicast address to 24006.
Other riders are not listening to that multicast address.
So from an OSPF point of view, they do not receive or see that update.
Only the designated rider receives that multicast update.
Now multicast isn't covered in this course.
But briefly, if this infrastructure was a hub.
So the riders were connected via hub at layer one, that multicast would go to all the routers.
However, only certain routers are listening or accepting that multicast, so only certain routers have
subscribed to that multicast.
In this case only designated rodders are listening for and accepting multi costs to address 224006.
So the other orders at layer two will drop this update OSPF residing at layer four and the OSA model
will not see this update on the other routers.
So routers three, four, five and six from an OSPF point of view at layer four will not receive the
update.
Only router two will receive the update.
So logically what happens?
The link goes down.
Rather one is updating rather to the designated router, but sending a multicast to this address only
router to the designated routers.
Receiving the multicast router two then sends an update to all the other routers on this multicast address
224005.
All OSPF routers are listening to this multicast address so they will receive the update router.
One would receive the update but wouldn't process it because its OSPF topology table is already up to
date.
So logically what happens is the update goes from R1 to R2 or T sends that update to all the other routers.
They process the update and therefore the topology database is updated with the new information that
this network has gone down.
As you can see here, it's much more efficient to use a designated router than to allow full adjacencies
between all routers and have all those duplicate updates.
It's important to realize that only the designated rider and backup designated rider will have full
relationships to all the other artists.
So for instance, root of four and Root of five will only have a state known as two way.
In two way they know about each other, but no updates will be exchanged between the riders.
So in other words, our phone or five will not update each other.
Neither will or five and or six and so forth and so on.
All riders will only update the designated rider and back up designated rider with changes in the topology
so they have a full relationship to the designated rider.
This allows for the saving of updates and duplicate traffic on a single segment.
Once again, it's important to realize that writers on the segment will only form full relationships
with designated writers and backup designated writers.
Now, in this example, I've only got a designated writer.
The issue with only having a designated writer is that if this writer goes down, updates will not be
sent and received properly.
So on a segment, a designated writer will be elected, and normally a backup designated writer would
also be elected.
So you'd have both a designated writer and a backup designated writer.
The PDR will become the D.R. if the D.R. fails.