- [Instructor] When we're working with
route redistribution,
one of the things we need to keep in mind
about those protocols that we're redistributing,
is their administrative distance.
So in this video,
let's take a look at administrative distance.
Think of administrative distance
as the believability of a routing source.
Let's say that we're sitting on a router
R5 and we want to get to 10.1.1.0/ 24.
Well, maybe R1 is telling us
Hey, this is the way to 10.1.1.0/24.
R2 says, here's how to reach that network.
R3 says, I'll get you there.
And R4 says,
here's your ticket to 10.1.1.0/24.
And the question is, who do we believe?
What we have different routing sources
with different believability.
Specifically,
some routing sources are more authoritative than others.
And the way we measure the believability
of a routing source
is with something called administrative distance or AD.
Let's take a look at some common routing sources
and their administrative distances.
First, let's consider a directly connected network.
It doesn't get more believable
than being directly connected to a network.
So that has the most believable
of all administrative distances
a zero, because lower is more believable.
But
if we, as administrators go in and configure a static route
odds are
we know what we're doing, and we want that route to be taken
into account before any dynamic routing protocols.
So by default, a static the configured route
has an administrative distance of a one.
Now notice I say by default
because we can play some games
and do something called a floating static route.
We can set a static route
with a higher administrative distance
a higher administrative distance
than the writing protocol we're using.
And that route would kick in only if,
our routing protocol field
or the routing protocol could no longer get us to
our destination network.
Next, we have internal EIGRP
and this is what we typically see in an EIGRP network.
And it has an administrative distance of a nani.
Now I say internal because
we're not redistributing those routes
from an edge European autonomous system
into another autonomous system,
like an OSPF autonomous system, everything's happening
within EIGRP.
So it's internal EIGRP.
And It's got an AD of nani, OSPF is a little less believable
than EIGRP with an AD of 110 behind that.
We have intermediate system to intermediate system
with an AD of 115.
Then we've got rip with an AD
of 120, and now let's talk about external EIGRP.
It's much higher than internal EIGRP.
It's a whopping 170.
Now, where would we see this?
This is when we're redistributing an EIGRP learned route
into another autonomous system.
So if there's a network that EIGRP European knows about,
and that route gets redistributed into let's say
our autonomous system, which is running OSPF
then that EIGRP learned route is going to have an AD
of 170.
So even though OSPF is less believable than internally
EIGRP, it's more believable than external EIGRP.
Now let's go out to a live interface and take a look
at administrative distance in action.
We're sitting here on router R1
and notice that R1 has its gigabit zero slash one interface
running as EIGRP.
And it's got its gigabit 0/2
interface running OSPF, and both of its neighbors.
It's R2 neighbor, running EIGRP
and it's R3 neighbor running OSPF.
They are each telling R1, hey, here's how you get
to the 192.0.2.0/24 network.
That's the network hanging off of router R4.
In fact, let's prove that is indeed the case.
Let's take a look at our EIGRP typology table, to do that,
we say show IPE EIGRP topology.
And if we want to get to the 192.0.2.0/24 network,
it says we should go
to a next hop IP address of 198.51.100.2.
And that's route are R2, let's take a look at OSPF,
routing information base or it's rib.
We'll do a show IP OSPF rib.
Does it know how to get to 192.0.2.0/24?
It sure does.
Here it is.
And it says we should go
to an X top of 198.51.100.6.
That's R3, they're telling R1 to go
in different directions.
So the question is, who does R1 believe?
Well, it all comes down to administrative distance,
and as we just discussed, EIGRP
specifically internal EIGRP,
has a lower administrative distance.
It has nani, it has a lower administrative distance
than OSPs, which is 110.
Let's confirm that
by looking at our IP routing table, show IP route.
And how does it think we get to 192.0.2.0/24?
Well we know it was learned by EIGRP,
because we have that code of D,
that means it was learned via EIGRP.
And it says if you want to get to this network,
we're going to go to a next hop of 198.51.100.2.
That's R2.
And we also see evidence here
of our administrative distances,
for the route that was learned via EIGRP.
You see this nani,
that's our administrative distance for EIGRP
and here's a different route for another network learned
by OSPF, but notice, the one tin, that tells us
that the administrative distance for OSPF is 110.
So since EIGRP had a lower administrative distance
than OSPF,
R1 believed EIGRP.
However, as administrators,
we have the ability to override that route.
And one way to do that is to create a static route
with a lower administrative distance.
Now we're called that by default, a static route has an AD
an administrative distance of a one,
even though we can set it higher
if we want to, if you want to do one
of those floating static routes.
But here let's override that EIGRP learned route
by creating a static route.
To do that, we'll go into global configuration mode
and they'll say IP route
and I'm going to add static entry for the network
192.0.2.0.
And we're going to give a 24 bit subnet mask,
255.255.25.0.
And now I'm going to specify the next top,
I want R1 to use.
Let's say that I want R1 to use R3.
So I'll specify the next top is 198.51.100.6.
That's R3.
And by the way, if I just pressed enter right now
that would set a static route with an AD of one.
However, let's give some context sensitive help.
If I did want to set the AD to a higher value,
here's where I could do that.
I can set that anywhere in the range, one through 255
but we're just going to stick with the default of one,
and let's press enter.
And now that we've done that,
let's check out our IP writing table.
Let's do a show IP route.
Now, how do we get to that network?
Well, notice that
instead of going up via an EIGRP learned route
it says we're going to use a static route.
That's what the S is for, it indicates static.
And we statically said that to get to this network,
we're going to go to 198.51.100.6.
That's R3, the reason is,
this static route has an administrative distance
of a , it's more authoritative than EIGRP,
which has an administrative distance of a nani.
And that's a look at administrative distance.