This is one of multiple videos that demonstrate troubleshooting scenarios in campus environments.
In this typology I have four switches configured switch one and switch to switches switch three and
switch 4 are access switches.
Now to keep the topology simple I haven't got cross-connects configured here or
additional access switches. In my CCNA course, I show you a larger topology but for this troubleshooting
video we'll keep it simple.
We've been told that PC 1 in this network is not able to ping PC 2 in this topology I'm using routers to simulate
PCs.
So let's test that, can router 1 or PC 1 ping 10 132?
So it looks like the pings are failing.
PC 1 is in VLAN 2 and PC 2 is in VLAN 3, show IP route routing is disabled on this router
but it does have a default gateway of 10.1.2.254.
So we should be able to ping this default gateway, so ping 10.1.2.254.
Yes, we can.
Can we ping VLAN 3 on the default gateway?
Yes, we can.
So PC 1 can get to the default gateway.
What about PC 2? See here's router 2 acting as PC 2, show IP interface brief.
We have 10.1.3.2 configured show IP route default gateways 10.1.3.
254.
So can we ping that default gateway?
Looks like we can't.
So there seems to be a problem somewhere between PC 2 and switch 1.
Let's check on switch 4 connected to PC 2.
So this is switch 4 show IP interface brief.
It's got an IP address of 10 114 ping 10.1.1.254
that works.
That is the default gateway of the switch.
So the switch can ping it's default gateway can it ping 10.1.3.254?
Yes, it can.
So we've established that there's basic connectivity between this switch and the default gateway.
Let's have a look at our trunks, so show interface trunk
trunking is enabled on gigabit 00 that's the interface between switch 4 and switch 2 show CDP neighbors.
We have a relationship to switch to but we've already proven that that works by being able to ping the
default gateway VLANs are allowed across the trunk. Now can you spot a problem already in this topology?
What's the problem based on that single command?
Something's wrong, notice gigabit
00 is a trunk but it's only allowing villans 1 to 2 and 4
but the PCs in VLAN 3.
So let's have a look at the configuration of this gigabit interface.
This interface is configured for trunking and we're not restricting any VLANs but we're only permitting
VLANs 1 to 2 and 4 across the trunk show interface gigabit 01 switch port.
What's wrong here?
Can you spot the problem? We're looking at the output of this interface on this switch.
What's wrong?
Port is enabled
It's set as static access it's configured to use access of helium 3 but notice this key word something's
wrong.
Show run interface gigabit 01.
So the port is correctly configured to be in VLAN 3 but show VLAN brief will show us that there's
a problem.
Can you see the issue?
Where is VLAN 3?
So be careful.
You can have a port configured in a VLAN and not have that VLAN in the VLAN database.
If that's true the port is essentially error disabled it won't work.
No traffic is gonna be sent through that port because it belongs to a rogue VLAN.
This is often the case when VTP is used.
If you delete a VLAN on another switch that update is propagated to other switches in the topology, VTP
removes the VLAN from the VLAN database but doesn't take the port out of the VLAN.
This port is manually configured to be part of VLAN 3 but VLAN 3 doesn't exist in the
VLAN database and hence the switch port command is showing us that that's an inactive VLAN on the
port.
That's a problem.
So show VTP status is configured as a server the switch so let's create VLAN 3 show VLAN
brief.
Suddenly now we see port gigabit 01 configured in VLAN 3
and when we look at the switch port we see that it's in VLAN 3.
That looks good.
So Ken wrote it to ping its default gateway.
We may have to wait a while for things to update.
But notice they the pings started succeeding Ken wrote a one now ping right to acting's PC too.
Yes it can.
So we've successfully result of the issue in this typology.
Be careful.
A port is statically configured in a villain but that requires the villain and the villain database.
When using stented villans if the villain is not in the villain database the port is essentially in
a rogue villain and is not used.
Traffic will not be sent through that port.
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I wish you all the very best.