This is one of multiple LAN or local area network troubleshooting videos.
In this video, we have four switches running in GNS3 switch 1 and switch 2 are configured
as layer 3 switches, switch 3 and switch 4 are configured as layer 2 switches.
So I could change the symbol as an example
to indicate that that's a layer 2 switch.
However, these four switches are running IOS VLAN 2
and what I've done is simply disable IP routing on switch 3 and switch 4.
So we have four switches in the topology and two IOS V routers, router 1 and router 2 acting as PCs.
This is a very simple topology where we have a core and an access Layer.
Typically you would connect the core to the access layer using cross-connects.
I'm not gonna do that in this trouble shooting video.
Have a look in my CCNA course which is linked below for a more complex topology running HSRP optimized
spanning-tree redundancy between the core and the access layer and multiple other options that this
network will suffice for our basic troubleshooting.
So now let's assume that a new hire has made some configuration changes on the network and users are
complaining about a network connectivity issues.
You've been told that this user 10 121 in VLAN 2 is unable to ping this user that has a PC
with IP address 10.1.3.2 in VLAN 3 you've been told that switch 3
is connected to switch 1 as follows.
Switch 1 is connected to switch 2 and switch 2 is connected to switch 4 but as Ronald Reagan
said "Trust but verify"
and what I like to say is don't trust anything that you've been told about a network until you've checked
it yourself.
So in our topology router 1 is acting as PC 1, show IP interface brief shows us the IP address on
the gigabit interface that looks right, the IP address looks right, subnet mask looks right.
Let's see if we can ping 10.1.3.2 which is the IP address of router 2 acting as PC 2.
Okay so the ping is failing.
We don't seem to be able to ping that device.
So that piece of information is true.
Show IP route shows us no routes in the routing table but it shows us that the default gateway is set
to 10.1.2.254.
Now that's because IP routing has been disabled on this router. In this topology, we've got router 1 acting as
a PC.
So when IP routing is disabled this is what the routing table looks like.
The default gateway has been configured on the PC so that's good.
Let's check if we can ping the default gateway, Yes we can.
Now where is the default gateway.
We could try and telnet to the default gateway
and let's try and telnet to the right IP address.
So the device is there but we're told that a password is required but none is set.
So hopefully the default gateway and this typologies either switch one or switch to which all of the
core layer 3 switches. So here's switch 1 show IP interface brief we can see that VLAN 1, VLAN 2 and VLAN
3 have an IP address configure
and that's the IP address on VLAN 2.
So these should be the device that the PC is pinging.
I'll do a debug IP ICMP on that switch and get the PC to ping the default gateway.
Now you need to be careful with debugging in a real world environment.
Here because we're studying for the CCNA we can simply enable any debug command because that'll help you
learn the debugs that are available.
But in the real world be careful with simply enabling a debug especially on a core device.
You may get so much output on the console that you can't read anything or in a very bad situation
the device may fall over so we don't want that.
So in the real world be careful with debugging.
Okay, so the pings do arrive on this default gateway so that's good.
I'm gonna turn off IP domain look up in the lab here to make things happen quicker
and then what we'll do is trace 10.1.3.2 which should be PC 2 in our topology.
Let's see how far it gets.
So it gets to the default gateway which we've now determined the switch 1.
So what we should do is update the documentation so we could say that these are the IP addresses on
the VLANs of that switch.
That once again can be seen on switch 1 by using the show IP interface brief command
and there they are once again.
The trace route shows us that the traffic gets to the default gateway but then doesn't go anywhere else.
Let's check if PC 2 can ping its default gateway.
Now a test I like to do is to make sure that the local device can ping the other VLAN of the default
gateway.
So in other words this PC is in VLAN 2 it's in subnet 10 120 and I'm checking that it could ping
the subnet of the device that we're trying to go to but this is the IP address of the default gateway in
that VLAN.
In other words, we've proven that inter VLAN routing is working on the layer 3 switch but now let's
check the connectivity of PC 2, can it get to its default gateway?
So this is router 2 acting as PC 2 show IP interface brief IP address looks right per the diagram subnet
mask looks right now that information is once again not shown in the diagram.
So we'd wanna add that somewhere and you may wanna add that on individual devices or make a note
somewhere that the subnets have a /24 mask, show IP route shows us the default gateway of PC 2.
So can it ping its default gateway?
No, it can't.
So there's something wrong either here or here
or here. Let's have a look at switch 4, switch 4 is the local access switch, show IP interface brief.
It has an IP address of 10 1114, can it ping switch 1 in VLAN 1?
Yes, it can, show IP route default gateway is 10.1.1.254, can it ping 10.
1.3.254? Yes it can, can it ping the PC in VLAN 3?
No it can't.
So this switch could ping the default gateway inter-VLAN routing is working on the default gateway but
it can't ping this PC. Traffic from the switch to this PC you'd have to traverse to here to get to the default
gateway and then it would have to come back again.
Let's confirm the ports on switch 4, so firstly show IP interface brief, let's confirm that interfaces
are up.
All interfaces look up including gigabit 01
that interface is up gigabit 00 is up.
Which is the link to the core network pings did work previously so,
so we have an indication that those interfaces are already up and working.
Interface trunk is a trunk to the core on gigabit 0 that looks good.
Let's have a look at the gigabit 01 interface, so show interface gigabit 01 switch port this interface
should be in VLAN 3. Can you see the problem in this out
put? Interface name is gigabit
01 it's enabled us to switch port administer of mode is dynamic auto.
It's currently acting as a static access port so dynamic trunk protocol or DTP did not negotiate to
form a trunk.
So DTP is on but there wasn't a switch to negotiate trunking with.
Notice the problem access mode VLAN is 1, show run interface gigabit 01 this port is
not in VLAN 3.
Interface gigabit
01 is currently in VLAN 1.
So switch port access VLAN 3 show interface gigabit 01 switch port, port is now an access port in
VLAN 3 show run interface gigabit 01,
there's our configuration.
Let's check if the local switch can ping that PC, at the moment it still can't.
It may take a while for spanning tree and other protocols to converge.
So you may just need to wait a bit before you assume that there's a problem but spanning tree at this point
looks good.
So show CDP neighbors.
We can see that router 2 is connected to gigabit 01 using gigabit 00, show interface gigabit
01 switch port confirms that this port is in VLAN 3.
So let's go back onto the PC and check if it can ping its default gateway.
It can now.
So once again you may just have to wait a little bit before you assume that your change hasn't made any
difference.
Can it ping router 1 acting as PC 1?
Yes, it can.
So it looks like we've solved the problem.
We could trace to the PC which didn't work before that now works.
So previously when we traced to 10 132 it failed at the default gateway
but now it's working and we could do a ping to prove it
and do a debug on this side to prove that the traffic arrives.
So that was an example of how to troubleshoot a local area network.
It can sometimes be complicated to troubleshoot a layer 2 issues but remember to check your interfaces
check your VLANs, check your encapsulations. In other videos
I will show you all the problems that you may encounter.
I need to troubleshoot in this topology.
I hope you enjoyed this video.
If you did please like it and please subscribe to my YouTube channel.
I wish you all the very best.