- [Lecturer] In our previous video,
we discussed that for a couple of routers to form
an OSPF neighborship, they have to have matching timers.
And we have a couple of main timers
who we want to talk about in this video.
The first timer is the hello timer,
and this works similar to how an EIGRP hello timer works.
With EIGRP, that hello interval as we called it,
that was the number of seconds that we would wait
between sending hello messages out to our neighbors.
Now with EIGRP, we sent those with IPV4,
we sent those to a multicast address of 224.0.0.10.
Here we're going to 224.0.0.5.
That's all OSPF speaking routers.
And by default, even though we can adjust this,
by default, our dead timer
is going to be four times the hello timer.
And that's how long we're going to wait
to receive a hello message from our neighbor
before considering that neighbor to be down.
Now, this is quite different than the whole interval we had
with the EIGRP.
With EIGRP, we had this hello interval
and we had a hold interval.
And the hold interval was a number of seconds
that we advertised to our neighbor.
We were telling our neighbor,
"Wait this long on us.
If you've not heard a hello message from me
within this amount of seconds,
then you, my neighbor consider me to be down."
That's not what's happening here.
With OSPF, we're not telling our neighbor
how long to wait for us,
we're just saying, I'm going to wait this long for my neighbor.
That's the reason we have to have matching timers.
And again, the default is, for the dead timer
to be four times the hello timer.
And we're going to prove in this video,
out of the live interface,
that we must have matching timers.
Now let's go out to a live interface
and checkout OSPF timers.
As we're going to be seeing in an upcoming video,
OSPF has different network types.
Now in this topology,
all of our links are gigabit ethernet links.
That means by fault, the OSPF network type
is the broadcast network type.
We want to confirm that though,
and let's see what the default hello and dead timers are
for these gig interfaces.
Let's take a look at the bottom interface
on R1 going out to R2.
That's interface gigabit 0/2.
We'll say show IP ospf interface gig 0/2.
And we can see that this is a broadcast network type.
And we see we've got a default hello interval of 10 seconds
and the dead interval or the dead timer,
it's four times the hello interval,
which is 10 times four, it's 40 seconds.
And by the way, we haven't talked about the wait timer
or the retransmit timer.
Here's what's going on there.
The wait timer says, how long is a router going to wait
for a DR or BDR to be advertised inside of a hello packet
before it begins a DOR and a BDR election?
And the retransmit timer says how long a router
is going to wait to retransmit an OSPF packet
that's not yet been acknowledged.
Now let's confirm that the timers must match
between our neighbors in order for that neighborship
to be established or to stay up.
Right now, we have a neighborship.
I can say show IP ospf neighbor.
And I have a neighborship with router R2.
By the way, notice R2s neighbor ID, that's its router ID,
because I did not set a router ID
in OSP of router configuration mode.
And because we don't have any loopback interfaces
on router R2.
We chose the highest IP address of an interface that was up.
And the highest IP address was interface gigabit 0/2 on R2
which is 203.0.113.1.
That became its router ID.
And by the way, we can also see in this output
that our neighbor is the DR.
Notice that the designated router,
each router ID, that's R2.
And the BDR on this segment, that's us.
That's our router ID, 192.168.1.1.
That's the highest IP address assigned to an interface
that is up.
So that becomes our router ID.
And R2 has a higher ratter ID than we do.
So we say that it has a higher priority
because I didn't configure a priority.
That's the reason it's elected as a DR.
But let's go on and change the timer value
and see if this neighborship goes down.
Let's go into interface configuration mode
for gigabit 0/2.
And I can say, IP OSPF.
And let's give some context sensitive help,
and we can see how we can set these timers.
If we want to set the dead interval,
we have an option for that.
We want to set the hello interval though.
It defaulted to 10 seconds on this broadcast network.
Let's set it to, let's say three seconds.
We'll say IP OSPF hello interval three.
Now here we might have to wait up to 40 seconds.
And after that time, the neighborship will be torn down.
But while we're waiting,
let's check out what the timers are now.
Let's back out and once again,
do a show IP ospf interface gigabit 0/2.
Now, notice that the hello interval is three seconds.
We set it to that.
We did not set the dead time or the dead interval,
but it defaults to four times a hello timer,
so four times three, that is our 12 seconds.
Now, in this case,
we actually explicitly set the hello timer.
What will be another situation
where we might have a timer mismatch
if we did not set the hello timer?
Oh, by the way, did you notice,
I guess that was about 40 seconds.
Our neighborship went down with R2.
That's because we have these mismatched timers.
But to answer the question of,
when might we have mismatched timers,
even if we didn't explicitly set those timers?
That can happen if we have mismatched network types.
We've got a video coming up in this section,
which is going to get into the default, hello times
for different OSPF network types.
And you might have a situation where you want your neighbors
to have interfaces with different OSPF network types.
If they have different default timers,
then they can still be neighbors
with different network types,
but you'll need to go in
and manually give them matching timers.
But in this video, we've seen what the default network type
and the default timers are for an ethernet segment.
Here we had a gigabit ethernet segment,
and we confirmed that in order to have a neighborship,
those neighbors, they have to have matching timers.