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 riders and backup designated riders in this apology I have six riders
connected to the same Ethernet segment so assume the D6 route is connected to a switch or a hub all
sharing the same Ethernet segment designated riders or dealers 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 Rata let's assume that this network 10 1 1 0 is connected
to auto 1 and the 6 riders are connected to this Ethan it segment and assume that this network goes
down so let's assume for the moment that there is no designated rider on this Ethan it segment and hopefully
you'll quickly see why there's a requirement for a designated rata with a designated Rado all of these
riders would have a full adjacency in a full adjacency LSD are exchanged between riders so in this example
or one needs to notify the other riders using a link state update that there's been a change in the
network topology so or one will send an update two or three send an update or two to our four to our
five to our six notifying all of the riders that there's been a change in the topology or two when receiving
that update from our one in this case because there's no designated rider has a full relationship with
all other riders 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 our one so it notifies all of its
neighbors that there's been a change in the topology or fall 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 writer is selected on this specific segment.
So let's assume that R2 was elected as a designated Rada designated writers are selected on two criteria.
The first one is highest priority.
You can specify the priority on an interface.
The default priority is 1 0 excludes a writer from becoming a designated Rada or backup designate Rada.
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 Rada with the highest Rada I.D. is elected as the designated
Rada for that segment.
So in this example we have elected or to use the designated Rada and let us assume again that this network
goes down but what happens now is or one sends an update only to the designated writers designate or
others are listening on this multicast address.
2 2 4 0 0 6 other writers are not listening to that multicast address.
So from an OSB point of view they do not receive or C that update only the designated Rada receives
that multicast update now multicore thing isn't covered in this course but briefly this infrastructure
was a hub so the writers 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 writers have
subscribed to that multicast.
In this case only designated writers are listening for and accepting multicast to address 2 2 4 0 0
6.
So the other writers Adelaide 2 will drop this update OSP if residing at layer 4 knows our model will
not see this update on the other writers.
So writers 3 4 5 and 6 from an ISP point of view at lower 4 will not receive the update.
Only writer 2 will receive the update.
So logically what happens.
The link goes down writer 1 is updating rider to the designated writer but sending a multicast to this
address only write it to the designated writers receiving the multicast write it to then sends an update
to all the other writers on this multicast address.
2 2 4 0 0 5.
All OSB if writers are listening to this multicast address so they will receive the update.
Writer 1 would receive the update but wouldn't process it because it's always topology table is already
up to date so logically what happens is the update goes from our 1 2 or two or two sends that update
to all the other writers.
They process the update and therefore their topology databases 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 rather than to allow full adjacencies
between all writers and have all those duplicate updates.
It's important to realize that only the designator order and backup designated writer will have full
relationships to all the other writers.
So for instance four out of four and a 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 writers.
So in other words or for an hour five will not update each other neither will are five or six and so
forth and so on.
All writers will only update the designated writer and backup designated writer with changes in the
topology so they have a full relationship to the designated writer.
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 this example I've only got a designated writer.
The issue with only having a designated Rada is that if this writer goes down updates will not be sent
and received properly.
So on a segment a designated rider will be elected and normally a backup designated rider would also
be elected.
So you'd have both a designated rider and backup designated Rada the BTR will become the D.R. if the
D.R. fails.