This is one of multiple LAN or local area network troubleshooting videos.
In this example users are complaining that PC 1 can't ping PC 2, here's router 1 acting as PC 1
ping 10.1.3.2
looks like that pings are failing.
So we don't have connectivity from PC 1 to PC 2, show IP route default gateway of the PC is configured
to 10.1.2.254.
This is a Cisco IOS V router but IP routing has been disabled hence the routing table looks as follows.
IP routing is disabled but a default gateway has been configured
hence we see default gateway as follows.
Can we ping the default gateway?
Yes, we can. Can we ping VLAN 3 on the default gateway PC 2 is in VLAN 3
and notice here is the default gateway switch 1.
Some of the documentation in this topology hasn't been updated
and that's something you constantly gonna find when troubleshooting real network.
So we can ping VLAN 3 on the default gateway.
Can the default gateway ping router 2? So here's switch 1 acting as the default gateway, can it ping
10.1.3.2?
No, it can't.
So it looks like we have a problem between switch 1 and this router.
Let's check if we have connectivity between the access switch and the core switch.
My topology is not showing the IP address of the access switch.
So let's jump onto the access switch and confirm and check what its IP address is.
This is a problem you may encounter if you don't have no IP domain lookup enabled these devices are
IOS V layer 2 switches and routers in GNS3, no IP domain lookup is not enabled by default on
these devices.
So that's probably something we wanna do immediately to save us
frustration.
So the command is no IP domain lookup, so show IP interface brief which has this IP address from VLAN
1. Can we ping the core switch on VLAN 1? Looks like we can't.
Can we ping this core switch
switch 2?
So before going any further let's update the documentation.
That's the IP address of this switch.
Let's check if the switch can ping switch 2, so here's switch 2 show IP interface
brief. Okay there's IP addresses on the three VLANs.
So what I'll do is update the diagram.
It's 253 in all the VLAN, so that's IP addresses of switch 2 can switch 4, the access switch ping
10.
1.1.253?
Doesn't look like it can.
So it looks like there's a problem on this link between switch 2 and switch 4, show CDP neighbor.
We do see a neighbor relationship between the local switch and router 2 as well as switch 2, show IP
interface
brief interfaces look up, show interface trunk.
It looks like this interface is trunking to the core.
Let's check the configuration on the core switch, so show interface trunk can you see the problem.
What I'll do is move the console to the left here, can you see the issue?
What's the problem between switch 2 and switch 4?
Okay, so on switch 4 gigabit 00 has trunking on the encapsulation is 802.1q
native VLAN is 1. On gigabit 01 on switch 2 trunking is on
but the encapsulation is ISL so trunking is enabled with a native VLAN of 1 but the encapsulation
is wrong or different.
So encapsulation is ISL on this port whereas on this port encapsulation is dot1
q, you need to have the same encapsulation of both sides.
So what I'll do is change the encapsulation, show run interface gigabit 01 that looks better.
Can this switch now ping switch 4 with IP address 10 114 it still hasn't come up.
What we may have to do is wait a bit but what I'll do just to force things is shut the interface down
and then no shut it.
You may have a slightly different experience in GNS3 versus a physical switch.
We still only see switch 1 through CDP that may take a while to come up.
Interface is up up show interface trunk, trunking is now set to 802.1q on both sides.
Can this switch now ping switch 2? Yes, it can.
That looks better.
If you have issues you may wanna shut and then no shut the interface so that the encapsulation and other
options are renegotiated.
So can PC 1 ping PC 2? Let's check if that works.
Okay we still have a problem.
PC 1 on the left is not able to ping PC 2 let's do a trace to 10 132, so it gets to the default
gateway and then the trace times out. Can PC 2, router 2 acting as PC 2 ping its default gateway?
Yes, it can
Notice suddenly here we had some kind of output
and I was actually running an ICMP debug here and notice the output.
So let's try and do that ping again
and what I'll do is enable a debug IP ICMP.
So the pings are suddenly working and that may just be something in GNS3.
The problem here is notice this option.
Be careful especially with PCs you may have a problem where everything looks good but the PCs can't
ping each other.
What I should have configured here is an IP default gateway of 10.1.3.254.
So in a real environment that ping may not work.
It depends where the proxy arp is enabled on your
inter VLAN routing devices, proxy ARP will allow a device to proxy the default gateways of other devices.
In many cases that's disabled so you need to have the default gateways enabled on both sides.
Show IP route notice default gateway, show IP route default gateway is configured, so be careful in a real
environment and in the exam that the default gateway's on both edge devices are configured properly.
So we've not fixed the apology.
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