- [Instructor] In this lab,
we want to look at troubleshooting BGP.
We have a typology in which we're told
that routing does not appear to be working correctly,
throughout the network.
Specifically, we only have unidirectional traffic,
from R1 to R3.
From router R3,
we are not able to ping the loop back interface, of R1.
So go ahead and pause this video,
import the attached yaml file into Cisco Modeling Labs,
so that you can use this based apology
and configuration that I've provided,
and see if you can resolve the BGP issue.
Once you do that,
you can resume the video
and we'll walk through the solution together.
(upbeat music)
Starting on R1, let's just verify our issue.
Let's try to ping 3.3.3.3,
which is R3 loop back interface.
And indeed, we do have a success there.
Can we ping R2?
Yes. We can also ping that.
Let's try,
from R3.
Let's try to ping first R2.
So we know we can get there. That's good.
What about pinging R1?
We can see that, those pings are failing.
So this is our issue.
So let's jump back over, to router one.
Let's just do a quick,
show IP,
interface brief.
And we can see our interface is all configured
as stated in the topology.
They're all in the upstate,
so that looks good.
Our loop back interface here,
is configured as well, at 1.1.1.1.
So let's check R2.
Let's say show IP interface brief.
And we see the same thing.
All interfaces are correctly configured.
They are in the upstate,
including our 2.2.2.2 loop back address.
Can we ping router one, from router two?
And you'll see that does not work.
Even though we verified
that our interface states and addresses are all correct,
still not working.
So with all of that verified,
with all of our basics covered,
let's start looking at our actual BGP configuration.
Let's go back to router one.
And let's say show IP, BGP summary.
So here we can see our BGP autonomous system number,
which is 65100,
we see that here, our local AS,
that is the correct autonomous system number we want.
We see, we have a neighbor adjacency listed,
at 10.1.1.2, which is of course router two.
And we see that has the correct autonomous system number.
We see we are in the upstate
and we're receiving prefixes from router two.
So all of that looks to be good.
Let's jump over to router two now,
and let's do the same thing.
I'll say show IP, BGP summary.
We can see here that we have two neighbor adjacencies,
10.1.1.1,
which is of course going out to router one
and another one going to router three,
at 20.1.1.2.
That looks good.
Again, both of these neighbor adjacencies
are in the upstate.
They are established.
Now one thing you might notice,
that might be a bit odd is,
we're only receiving one prefix,
from router one,
although we're receiving two prefixes,
from router three.
So that could indicate a potential issue.
Maybe we're not receiving, all of our advertised prefixes,
from R1.
Again, we know, we cannot ping,
the loop back address of R1.
We know that fails. Let's break out.
Let's try to ping the actual,
interface IP address that we're connected to.
And that does work.
So it seems as though,
we don't know how to get to that loop back interface.
Let's look at our BGP table
and verify that.
Or say show IP, BGP.
And you'll notice,
we do not see a route in the BGP routing table,
41.1.1.1.
We see router three's loop back.
We see our own loop bag being advertised,
but we don't see router one.
Let's also say show IP route,
and we'll see the same situation here.
We do not see a route,
a known route to the 1.1.1.1 network.
So it seems there's a problem where R1,
isn't sending this network prefix to R2 for some reason.
Let's jump back to R1 now,
and let's say show IP, BGP.
And here we can see that R1 has learned about,
R2, the loop back address on that router.
It's also learned about the prefix from R3.
It's loop back address.
So that's exactly what we would expect.
However, you'll notice,
that our own 1.1.1.1 network,
is missing from the BGP table.
Even though we know that interface,
is in the upstate.
So it appears as though,
maybe we failed to indicate
that we want this network to be advertised.
Let's take a look at our BGP configuration.
We'll say show run pipe two section, BGP.
And you'll notice that,
actually we do have a network command,
in here for the 1.0.0.0 network.
We also, are advertising our local network,
that we're connected to router two,
which is 10.1.1.0,
with a 24 bit subnet mask.
Now you might see the error right away.
It's in this particular network statement.
If we do not indicate, a network mask,
when we enter a network command,
BGP is going to default to a class full mask,
which for our loop back address,
would be a /8 mask, or 255.0.0.0.
That of course is incorrect.
If you remember our configuration,
we had a /32 mask on that.
We can verify that by saying show IP interface,
loop back zero.
You can see right here our address,
is 1.1.1.1/32.
So it appears as though,
we have set the wrong network command,
the incorrect mask and our BGP configuration.
So let's go under global configuration mode.
Under router BGP, 65100,
our autonomous system number.
And let's remove the incorrect command.
We'll say no network 1.0.0.0.
So we'll take that out.
And now we'll put that incorrectly.
Network 1.1.1.1.
We want to use the mask keyword,
and use a 32 bit mask.
Let's hit enter.
And now let's break out of here
and check our BGP table.
Let's say show IP BGP.
And now you'll see,
our loop back prefix being advertised, into BGP.
It's in our BGP table.
So that should now appear,
on router two,
as we would expect.
Let's again say,
show IP BGP on router two.
This time we do see,
the 1.1.1.1 network,
being advertised here.
So that's good.
We have knowledge of that,
in our BGP table.
Our prefix is in there.
Chances are,
we should be able to ping that loop back interface now.
So let's try that.
Try to ping 1.1.1.1.
And yes, we do have success there.
So we have corrected the issue,
at least between routers one and two.
Let's go over to router three,
where the initial reports of our problem was,
and let's from here also,
try to ping,
the loop back of router one,
and yes, we are able to reach that router now.
So that's a look at troubleshooting a BGP configuration.