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(modern digital music)

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- [Instructor] In the last
video, when I introduced you

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to the concept of configuring OSPF

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and showed you most of the
commands you would need,

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I verbally introduced
the concept of OSPF Cost.

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I want to have a special
video just dedicated to that

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because I think it's very important that

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for each routing protocol you learn about,

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that you're able to understand
the metric type that's used

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by that protocol so you can compare

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and contrast them along the way.

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So hopefully by this point,
you've learned about RIP

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and the routing metric
for RIP is hop count.

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And you've learned about
EIRGP and the routing metric

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for EIRGP is called distance.

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Now the routing metric that
we're gonna deal with, OSPF,

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is called OSPF Cost.

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So like Spanning-Tree,

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if you've learned about Spanning-Tree,

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this should be easy.

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Because just like
Spanning-Tree, cost is a value

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that's derived from the
interface bandwidth.

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And just like Spanning-Tree,
the greater the bandwidth,

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the faster the interface,
the lower the cost.

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And just like in Spanning-Tree,
if I have two paths

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to a particular place, the
lower cost is the better path.

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Now, OSPF does not use the
same formula as Spanning-Tree.

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So, you don't have to worry about that

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so that the numbers are
not going to be the same.

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As an example, review question.

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In LAN switching with Spanning-Tree,

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what was the Spanning-Tree Cost

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of a fast ethernet interface,

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so a 100 megabit per second
fast ethernet interface.

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Hopefully you answered the number 19.

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Fast ethernet is 19th in
the world of Spanning-Tree.

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Well, in OSPF, the cost of a
fast ethernet interface is one.

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So, different formula.

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And the cost of an OSPF
interface will default

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to be whatever you know,
derived from the bandwidth,

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but you can change that,
you can explicitly configure

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the cost of an interface or if you modify

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the bandwidth of an
interface, that implicitly

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changes the cost as well.

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So there are two ways
that you can change it.

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You can go on to an interface and use

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the ip ospf cost command
and then explicitly

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set the value to whatever
you want, or like I said,

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by changing the bandwidth,
that will also affect it.

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And you can use the command
show ip ospf interface

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to actually see what the cost
is of any given interface

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and certainly when you
look at an ospf route

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in the routing table with
the command show ip route,

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or show ip route ospf.

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You'll see what the
total cumulative cost is

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to reach any given destination.

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So let's see an example of this.

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So on our whiteboard here, we can see that

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if we look from Switch-3's perspective,

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and we look at his way of
getting to the 3.2.3 network,

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he actually has two equal cost paths.

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He can get to there via Switch-2,

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or you can get there via Router-3.

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So if we actually go into Switch-3,

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we take a look as routing
table, that confirms that.

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We can see right here, 3.2.3.0

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and he's got two equal cost paths.

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They both have a cost of two.

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And that makes sense, because
this is a fast ethernet link

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so that's a cost of one, and
this is a fast ethernet link,

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so that gives us another cost of one.

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So one plus one is two going this way.

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One plus one is two going this way.

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So let me start by saying,
okay, maybe I want Switch-3

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only to use this path.

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So what I'm gonna do, I'm
gonna go on to Switch-2.

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I'm gonna go on to as fast
ethernet, 0/3 interface

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and I'm gonna raise the cost.

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I'm gonna set the cost to
five on that interface.

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So then, this path will
not be as preferable.

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That will be a total cost of six.

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This will be a cost of two.

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And so Switch-3 will
now only have one route

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in his routing table.

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So I'm gonna go to
Switch-2, sorry, Switch-3

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will have a single route
in his writing table

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going this way.

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So now I'm gonna go to Switch-2.

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I'm gonna go on a fast ethernet 0/3,

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I'm gonna officially
increase the cost to five.

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Okay, so, #int fast 0/3,

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#ip ospf cost,

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actually, before I do it,

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#do show ip ospf interface fast 0/3.

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And you can see, by default, it's Cost: 1,

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'cause fast ethernet.

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Now I'm gonna type #ip ospf cost 5.

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And now when I take a look at
the output of that command,

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now we can verify that the cost is five.

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So, at this point if
I go back to Switch-3,

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he should now see a total
cost of six going this way

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and a cost of two going this way.

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So only this path, the path via Router-3

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should now be the only path
to get to the 3.2.3 network.

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(keyboard clicks)

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And that's what we see.

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Now there's only one path
and it's via Router-3.

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Now, let's just change it one more time.

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So now what I want to do is I want to have

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Switch-3 prefer this path.

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I want this to be the better path.

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So what I'm gonna do, is I'm
gonna go under Router-3's

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fast ethernet interface.

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I'm gonna take the bandwidth
and I'm gonna lower it

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to some really small number.

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Make this interface look
like it's incredibly slow

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by giving it a really low bandwidth.

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And by making the bandwidth
number really small,

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that will make the cost really big.

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Because remember, the larger the cost,

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the less preferable it its.

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So let's go on the Router-3 now

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and change it's interface bandwidth.

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(keyboard clicks)

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Your fast paced ethernet 0/0,

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#do sho int fast 0/0 so
we can see right now,

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it's current bandwidth is 100000 kbit/sec.

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So if you actually do the math,
that's one hundred million

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bits per second.

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So now I'm gonna say, #bandwidth.

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How about we say, 2000 kilo
bits per second, really slow.

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Only 2000 bits per second.

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So now if I type sho ip ospf int fast 0/0,

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look at his cost now, 50000.

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It was one before, and now it's 50000.

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So we can see how modifying the bandwidth

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clearly affects the OSPF Cost.

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And lastly, if I go over to Switch-3,

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now we can see that the 3.2.3
route has switched over.

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It's now pointing at Switch-2, 2.3.2.22

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as the next hop, because the cost of six

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is certainly better than
the cost of 50000 and one.

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(modern digital music)