This is one of multiple Ethernet troubleshooting videos. In this topology
we've been told that router 1 on the top left is not able to ping some of the other routers in the
topology. So let's verify that on router 1 show IP interface brief, IP address of router 1 is 10
.1.1.1 router 2s IP address is 10.1.1.2.
So, from router 1 can we ping router 2? pings are failing. Let's confirm the IP address information
so show IP interface gigabit 00 IP address is 10 112/24.
Let's do the same over here
make sure that the subnet mask is correct, IP address is 10 111/24 this side it's 10 112/
24.
So that looks good,
doesn't look like they'll be any other reason why we can't ping to router 2.
What we'll do on router 1 is do a debug IP packet.
Be careful doing these commands in the real world.
You may have to use an access list on that debug.
Otherwise, you could kill your router with the amount of output that's generated.
Okay, so we see messages such as sending full packet.
So it looks like router 1 is forwarding the packet out of the ethernet interface onto the network, debug IP
packet on this side.
What I'll do now is repeat one ping only so only send one packet onto the network.
It doesn't look like anything's arriving on this side.
If you can use Wireshark to help you capture traffic. So I'm gonna capture traffic between the switch
and router 2 and I'll do that ping again
but I don't see any ICMP traffic in Wireshark.
We see spanning-tree information but we don't see any other information in Wireshark we see for instance
a loop from a device with a MAC address
we see some spanning tree we don't see any other traffic.
So let's see if the traffic is actually getting to the switch.
I'll do that ping again.
Okay, so there's the ICMP message sent from router 1 to router 2 the fact that we see ICMP means that
the router has learnt the MAC address of router 2, source MAC address is zeros ending in one destination
MAC address is zeros ending in a two.
So it looks like ARP worked
but the pings are failing.
Let's confirm the MAC address on router 1 MAC address is set to this, burnt in MAC address is this but
the MAC address has been changed to that value.
What about on router 2? MAC address is this burnt-in MAC address is that.
So it looks like the traffic from router 1 to router 2 gets sent on this link but doesn't arrive at
router 2. What I'll do is do a broadcast
again you need to be careful with doing this kind of stuff in the real world but
because this is a lab I'm gonna show you various options.
So notice when I send a broadcast traffic it does arrive at router 2.
So let me un all that and only do a debug IP ICMP not a debug packet because that generates a lot of traffic.
I'll un all over here.
In other words, turn off all debugging and only send one broadcast.
So the broadcast gets to router 2, so here we got a reply to the broadcast it looks like but it doesn't
look like unicast traffic works in the network.
So broadcast is sent it gets to router 2
but a unicast doesn't seem to get to router 2.
So let's have a look at that switch and see if there's a problem on the switch.
So on the switch show MAC address table.
Okay,
Can you see a problem in the output? What looks wrong in the output?
Notice here we've got a static MAC address for MAC address 2 going out of gigabit
2 and gigabit 3
but switch 2, show interface gigabit
0 shows us that has a MAC address of two.
So this static MAC address is a problem show run pipe include MAC.
We've got this entry in the running-config which shouldn't be there.
So no, I'll paste that in.
So no MAC address show MAC Address table.
Now we only have dynamic MAC addresses. I'll do a ping from router 1 to router 2 that succeeds
now because the MAC address has been learnt properly.
Be careful with static MAC addresses in your MAC address table that will override your dynamic entries
and could send traffic to the wrong port in your topology.
Okay, so can we ping router 3 which has an IP address of 10 113?
No we can't, show IP interface gigabit 00.
So this is the IP address of router 3 debug IP ICMP.
Let's do a ping again.
It doesn't look like the traffic's arriving at router 3.
Let's have a look at the MAC address table.
I don't see a MAC address for router 3, let's ping router 1 from router 3.
Look at this output and see if you see a problem.
So show MAC address table, showed us previously that the MAC address ending in one was connected to Gigabit
00 on the switch.
In other words over here but now it looks like that MAC address is on gigabit 02 this port here.
So I'll do a ping from router 1 again and let's have a look at the MAC address table.
Notice that MAC address is now shown on gigabit 00, ping is finished so let's do a ping from this side.
Notice the MAC address has moved to gigabit
02.
So we're getting MAC addresses flipping on the switch.
Let's confirm what the MAC addresses are on these routers.
So show interface gigabit 00.
That's the MAC address of router 1.
What does it look like on router 3?
So show interface
gigabit 00 this is the routers interface.
Notice the MAC address.
The routers have duplicate MAC addresses.
That's not gonna work in an Ethernet environment, so let's configure the MAC address as follows.
And let's see if router 1 can ping router 3 now, notice the ping succeeds and that's because the
devices have different MAC addresses now available on different ports.
Which is correct per our diagram ethernet breaks if you have the same MAC addresses on the same layer
2 segment. MAC addresses are meant to be unique
but in the real world, you can encounter duplicate MAC addresses. Check your MAC address tables to see
if MAC addresses are flipping from one port to another.
Hope you enjoy this video.
If it's been of benefit to you please like it and please subscribe to my YouTube channel.
I wish you all the very best.