In this video, we are troubleshooting network connectivity problems.
We've been told in this scenario that router 1 which is a VIRL router running in GNS3 is not able
to ping the loopback of VIRL router 2 with the loopback of quadruple 4
and that's true.
Show IP route shows us that we only have a connected and local network in the routing table, show IP
protocols shows us that we are running the OSPF routing protocol and it's enabled on all interfaces
on the router. Show IP OSPF neighbor shows us that we have a full relationship with a neighboring router
with IP address 10 121 which is this router 1 router. So on router 1, show IP OSPF
neighbor we only have a neighbor relationship to 10.1.1.1 the router on the left
on interface fast
Ethernet 00. Show IP protocols OSPF is enabled on this router and is enabled on all interfaces, show IP
OSPF interface brief. We have OSPF enabled on the serial interface and the FastEthernet interface, can
you see the problem?
Notice the state of the serial interface is down.
So show IP interface brief we can see that the serial interface is up, down.
Something wrong with the serial interface.
Show run interface serial 2/0,
shows us that an IP address is configured on the serial interface but the interface is no shut.
In other words it's administratively up
nothing else is shown.
So show interface serial 2/0
we can see that the interfaces up down, we can see the encapsulation keepalive is set to 10 seconds.
Let's have a look at router 2, show IP OSPF interface brief we can see once again that OSPF is enabled on the serial interface
but the neighbor relationship is down, show IP interface brief interface is up, down.
So this interface has not been administratively shut down, show interface serial 2/0.
Can you see the problem?
What's wrong in this output?
Notice here, encapsulation HDLC on this side encapsulation PPP, show run interface serial 2/0.
We can see that this side is being set to use PPP encapsulation.
So we need to either set it to use HDLC which is the default on Cisco serial interfaces or we need
to set the encapsulation to PPP on both sides. So set the encapsulation to HDLC show IP interface
brief
interfaces up, down but notice that it's just come up and notice the OSPF relationship has gone to full.
So on router 1 show IP OSPF neighbor notice we now have two neighbor relationships, one to router 2 across
the several interface and one to this IOS V router on the left using the FastEthernet interface.
So that looks a lot better.
Let's have a look at the routing table of router 1 which is VIRL
IOS V router.
We now see the loopback quadruple 4 in the routing table.
So can we ping that loopback?
Yes, we can.
We're gonna disable IP domain lookup to speed up tracing, so trace quadruple 4.
The trace shows us the path that goes to this router 1 with serial interface to router 2 with
a serial interface and then on to the IOS V router with the loopback quadruple 4. Now to prove
that I could do a debug, so debug IP ICMP do a ping and notice we see the output on the screen of this router.
So be careful when asked to troubleshoot a routing protocol such as OSPF in the CCNA exam.
The issue could be unrelated to OSPF the issue could be an interface that's been misconfigured. In this
example the encapsulation was incorrect.
You have to have either HDLC on both sides or PPP on both sides of a serial link.
A mismatch caused the interface to go up down.
In other words the interface was administratively up but the line protocol was down which caused an
issue with route exchange and advertisement which meant that we couldn't send traffic from the left
end side of the network to the right end side of the network.
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I wish you all the very best.