- [Instructor] Let's consider how timers work with EIGRP.
There are two main timers I want you to know about.
The first one is the hello interval,
and by default on a local area network,
where we're mainly focused here in our enterprise studies,
the default hello interval is five seconds.
And the other timer I want you to know about
is the hold time, and the default hold time
on a local area network is 15 seconds.
Here's what those mean.
The hello interval says,
how often am I going to be sending out a hello message
to my neighbors?
This is how neighbors discover one another,
and this is also sort of a keepalive.
If I stop hearing hellos from my neighbor
for a certain period of time,
I'm going to assume that they're not there any longer,
and I'm going to tear down that neighborship.
And that's what the hold time is all about.
The hold time says when this expires,
then the neighborship gets torn down,
but that's not as obvious as you might think.
Here's specifically what's going on.
If I'm router R1 right now,
I'm going to be sending those hello messages out
every five seconds.
It's going to look something like this.
We're going to send a hello message,
and we start with a default hold time of 15 seconds.
And then it counts down to 14, 13, 12, 11, 10.
By the way, when you look at this in the live interface,
you'll never see it be 15 seconds.
It's like it resets to 15 seconds and then instantaneously,
it shows 14 seconds.
So what you'll see in the interface
is it counts from 14 seconds down to 10 seconds,
then another hello message is sent,
and it resets and it counts down again.
But back to the hold time,
the hello time again,
R1 is sending every five seconds to R2.
Now, a lot of people get this mixed up with the hold time.
They assume that the hold time says
if I don't hear from my neighbor, in our example,
if I don't hear from R2 within 15 seconds,
I'm going to consider R2 to be down.
That's not what it means with EIGRP.
With EIGRP, we're telling our neighbor to wait for us
this amount of time.
So I'm on R1.
I've got a hold time of 15 seconds.
I tell R2 to wait for me for 15 seconds
before considering me to be down.
We're not saying I'm going to wait for R2's hello message
for 15 seconds.
Now, normally they are the same,
but technically R2 could have a hold time of 10 seconds
and R2 could send us a message saying,
wait for me for 10 seconds,
and if you don't hear from me within 10 seconds,
consider me to be down.
So the big point is, in fact,
let's compare this with OSPF,
with OSPF, the hello interval is how often we send
hello messages, and with OSPF, if we have a dead interval,
and that says how long we are going to wait
to hear from our neighbor.
It's different with the EIGRP though.
With EIGRP, the hold interval still is how often
we're going to be sending those hellos,
but the difference is the hold time.
With EIGRP, the hold time says,
I'm going to tell my neighbor this number.
I'm going to tell my neighbor to wait for me
this amount of time,
and if my neighbor does not hear from me
within this amount of time,
they should consider me to be down.
And I'm going to receive instructions from my neighbor
telling me how long I should wait for them.
So with OSPF, you have to have
identical hello and dead timers on our neighbors.
And even though that's a good practice for EIGRP,
it's not absolutely necessary
because of the way the hold time works.
Now let's go out to a live interface
and take a look at that hold time as it counts down.
We're sitting here on R1, and let's do
a show ip eigrp neighbors command.
And we can see that our neighbor of R2 has a hold time
currently of 10 seconds.
In other words, if I don't hear from this neighbor
within 10 seconds when I pressed enter here,
then I'm going to consider it to be down.
But if I just repeat this command and press enter,
let's do that a bunch of times,
we'll be able to see that this hold timer gets reset.
Now, we'll never catch it at 15 seconds.
We'll see it at 14 seconds.
We should see it count down from 14, 13, 12, 11, 10,
and bam, we go back to 14 seconds.
So let's try to give this command in rapid succession.
Let's see, we've got 10, 14, 13, 12, 11, 10,
actually another 11,
there's a 10, and back to 14 again.
And if one of those hello messages from our neighbor
were to get dropped for whatever reason,
that would not mean we would immediately tear down
the neighborship because even if that happened,
I'm still waiting for another 10 seconds.
There's still another opportunity or two
for me to receive that hello message from my neighbor
before I tear it down.
And that's a closer look at the operation of EIGRP timers.