This is one of multiple troubleshooting videos.
We've been told that switch 1 on the left in this typology is not able to ping switch 2 on the right
in this typology. Now in this topology, I'm using physical devices rather than GNS3.
I'm only using GNS3 to show a diagram of the topology.
So let's verify what we've been told, on switch 1 can we ping switch 2?
It looks like we can't.
The pings are failing.
So let's do some troubleshooting, show IP interface brief.
Can you see the problem here?
What's wrong in this output?
So here's the command to show IP interface brief, what's wrong?
Notice fastEthernet 03 is down down show interface f
03, we're told that the interface is down the line protocol is down,
It's not connected.
Now at this point, you may need to check the cabling to see if there's a problem.
but before you do that let's have a look at the connection on the other side.
So on router 1 show IP interface brief, on this side
the interface is up down show interface f0
in this output, we can see that FastEthernet 0 is up-down, on this side
FastEthernet 03 is down down.
Now based on this output, what's the problem?
Can you see an issue between the switch and the router based on the output?
So before walking and checking the physical cabling or getting someone else to check the physical cabling,
What can you see that's wrong in this output?
Okay, so notice this full duplex 100 megabits per second, on this side
notice auto duplex 10 megabits per second.
It looks like there's a problem with the speed and duplex configuration, so on this side a Mac address and
an IP address have been configured on the FastEthernet interface and the speed of the interface is
10 meg. On this side in the output, we can see that it's using full-duplex 100 meg per second.
So if we look at the interface configuration notice the speed and duplex have been hardcoded.
Typically today you wouldn't do that but let's say for example that we set the speed and duplex to 100
meg.
So on this side we set the speed to 100 meg.
Notice you can see that the interface came up and then went down and we are seeing another problem duplex
mismatch
and we can see that on both sides. We're told that FastEthernet0 is using half-duplex and on our
side we're not using half-duplex. So show run interface f0 speed is 100
what you probably wanna do is set this to do auto negotiation rather than hardcoded so that they
can negotiate properly and on this side
we probably wanna do the same, so show run interface f0/3 rather.
So let's set the speed to auto and the duplex to auto.
So that's automatically negotiated with the other side.
Here we're still seeing problems with speed and duplex show run interface f0
everything is set to auto and on this side showrun interface f0/3.
Everything is set to auto so show interface f0/3
we can now see that the interface has come up up,
It's connected.
Show interface
f0 interface is up up, so that looks a lot better.
So it looks like we've solved a problem between switch 1 and router 1.
We didn't have to check the physical cabling.
The problem here was that the speed and the duplex were wrong.
Okay, so can switch 1 ping switch 2?
We're still having a problem.
We see U's in the output,
in other words unreachable. Show running config
which doesn't have a lot of config on it
but what it does have is an IP address configured locally and the default gateway. So can we ping
the default gateway?
Yes, we can. So I'll disable IP domain lookup to speed up trace route and if we trace to 10 132, notice we
get to 10 111 but we don't get any further.
So it looks like the traffic gets to router 1 but we're receiving an unreachable message because router
1 doesn't know what to do with a traffic.
So here's router 1 show IP route,
can you see a problem here?
We only have a route for Ethernet0
there's no routes to get to such 2, show IP protocol
we are running EIGRP on the router and it's enabled on all interfaces.
So show run section EIGRP, here's the EIGRP configuration that looks good.
Show IP EIGRP interface, EIGRP is enabled on FastEthernet0 as well as serial 0, show CDP
neighbors.
We see a CDP neighbor relationship to switch 1 but we don't see a CDP neighbor relationship to switch
2. CDP allows us to check layer 2 connectivity, if CDP is working
it means that layer 1 and layer 2 are working but at the moment it looks like layer 1 and layer 2 are
not working.
So let's check show IP interface brief.
Can you see the problem?
Notice serial 0
is up down.
There's a problem with the physical link between router 1 and router 2, on router 2 show IP interface
brief.
We have the same problem here
serial 0 is up down.
So now would be a good time to check the cabling.
Fortunately, with serial interfaces, we can use the command show controllers serial and specify the
serial interface number to check whether a cable is connected to the router.
In this case interface serial 0 has a DTE cable connected.
It's a V35 cable
but notice the problem the clocks are stopped on this side show controllers
serial 0.
What's the problem here?
What's wrong and how do you fix it?
Okay notice here we've got a V35 DCE cable and there's no clocking on the cable, show run interface
serial 0 clocking hasn't been configured on the cable.
On a DTE cable
you don't use the clock rate command.
So I could specify a clock rate but that command is gonna have no effect because this is a DTE cable
but on this side, if we enter the clock rate command and notice there are different clock rates available.
I'm just gonna use 64K show run interface serial 0 notice the command is retained in the running
config and that's because the interface here is a DCE interface. So show IP interface brief, interface
is now up up, on this side we saw a neighbor relationship established in EIGRP,
we also saw the interface come up.
So show IP interface brief.
The interface is now up up show controllers
serial 0, on this side
we now see that transmit and receive clocks are detected. On this side show controllers serial 0 the clocking
is set to 64 kilobits per second because this side is the DCE side of the cable, a back to back cable is
connected between router 1 and router 2
and router 2 is the DCE side.
So it needs to provide the clocking for the connection.
So show CDP neighbors we now see router 2 as a neighbor on router 1 ping 10 122
we can ping router 2 show IP EIGRP
neighbor we can see router 2 as a neighbor and hopefully switch 1 will now be able to ping
switch 2 which it can. To confirm that I'll use the command debug IP ICMP on switch 2 and ping from
switch 1 to switch 2 and we can see that an echo reply was sent
switch 2 can also ping switch 1.
So we've successfully resolved two issues in this network.
Be careful when troubleshooting
it's a good idea to know your OSI model and split the troubleshooting ping trace route in CDP are useful
for troubleshooting networks.
It's important that you learn your show commands don't just type a show run on a device, learn how to
use show commands to see the state of interfaces. So as an example here we can see that the interface is up
up will be able to see other information on the interface such as the load packets sent and received
and so forth.
If I removed the clock rate on this interface by using the command no clock rate what we should see on router
1 is that the interface goes down, it stays up for a while because of the keep alive which is set to
10 seconds but after a while as you can see there EIGRP neighbor relationship went down and what we
should see is that the physical interface goes down after a while
like that.
So the interface has just gone down,
show interface serial 0
notice interface is up down.
So learn to use your show command to see what's going on on the network, so show controllers serial 0
here we can see that a DTE cable is connected but we're not receiving clocking,
clocking is set.
This will change as you can see here transmit and receive
clocks are detected interfaces come up show interface serial zero.
We can now see that the interface is up up, one of the most important skills for you as a network engineer
is to learn how to troubleshoot networks.
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I wish you all the very best.