So let's jump in and begin with Trouble Ticket 1.
For this ticket, we're going to be using the diagram MPLS
Topology 1 and our core protocol for this ticket is IS-IS.
So now, let's take a look at the ticket itself.
So our ticket says that R6 is not
doing label switching to 10.5.5.5
which is router 5's loopback.
So clearly, it should be since that's
going through our segment routing core, so let's go ahead
and jump in.
As always, we will start with verifying the ticket first.
So let's start on router 6.
So we'll go ahead and do our trace to 10.5.5.5, sourcing it
from our loopback 0 interface.
And what we see from the output of the trace
is that the ticket is in fact correct.
We have IP reachability to router 5 loopback.
We can see, of course here at hop 3
we reached the destination.
However, we can also see that there
are no labels listed anywhere in the output of this trace.
So the first thing to do, why don't we
jump over to router 1 which is our next router in line.
And let's see if it has a label for router 5 loopback.
So let's do a show IS-IS segment routing label table.
And what we see here is we have prefix SIDs for routers 1, 2,
3, and 4 but we don't have one for router 5.
So this is clearly an issue.
So the router directly connected to router 5 is router 3.
Let's go look there.
So once again, let's do a show IS-IS segment
routing label table.
We see that it does not have a prefix SID for router 5 either.
Although if we do a show IS-IS neighbor, of course,
we know that router 5 is a neighbor here
and we knew that already of course,
because we had just routed to it.
So we're learning the route but what we're not
learning is a prefix SID.
And since we're not learning a prefix SID,
if we were to say show IP cef for 10.5.5.5,
we'll see that it's being sent with no label.
We don't know a label for it, therefore
it's not imposing a label here.
All of this is exactly as expected
if we don't have a prefix SID.
So let's go to router 5 then and see why it's not
generating a prefix SID.
So let's start with a show IS-IS segment routing connected SID
and we see that it's empty.
So that's clearly not good.
Let's do a show IS-IS segment routing global block.
We see that we're learning global block information
from routers 1, 2, 3, and 4.
So again, we're receiving information
but this router doesn't have it.
All of this information is really pointing to the fact
that maybe router 5 isn't even configured for segment routing.
So let's take a look.
Let's do a show run and include segment routing.
As we can see here, there's nothing
in the running config on router 5 for segment routing at all.
So this could definitely be the cause of our problem.
Let's go ahead and fix this issue.
So we'll go into configuration mode.
We'll turn on segment routing MPLS.
Now, segment routing MPLS is enabled.
Just as a note here, since it wasn't configured here at all,
we're going to have to enable it for the routing protocol
as well.
So let's go ahead and go into IS-IS
and enable segment routing there also.
So we're going to router IS-IS 1 and do segment routing MPLS.
So let's exit configuration.
And if we repeat some of our last verification commands,
we see that segment routing is now enabled
on the output of our show run.
If we look at the global block again,
we see that now we have all of the routers including
router 5 except, look here under SID index.
Under the SID index, it has none.
So this is a bit of an issue.
Since there's no SID index, this looks
like the advertisement of our prefix SID for our loopback
interface is also missing.
So let's review our show run command and instead of saying
include this time let's say section,
notice this looks the same.
There's nothing configured under the global segment routing
MPLS which is where our prefix SID should be.
So we're going to have to go put that back in as well.
So we'll go back into segment routing MPLS.
We'll go into our connected prefix SID map for the address
family IPv4 and for the prefix 10.5.5.5/32.
We'll configure the absolute value of 16,005 range 1.
Let's exit configuration.
And let's look at our validation commands again.
So first, let's see what the show run should have looked
like when we did section.
You can see here that the connected prefix SID
map with our information for the prefix SID
should have been in there when we did this show run earlier.
And if we look at the global block now,
notice that our SID index is now five.
So this is actually looking much better.
Let's go back to looking at the connected SID again.
And we see that router 5 is listed here with
the 10.5.5.5/32 SID index 5 range 1.
So this is looking much better.
So one other thing we could verify,
we could do a show IS-IS neighbor detail
and what we're looking for here is just the fact
that adjacency SIDs are now being
advertised in the updates.
So these are good indicators that segment routing is now
enabled on router 5.
So let's just go ahead and work our way
backwards through the troubleshooting we had done
and follow it up with verification.
So let's go back to router 3.
So let's do our show IP cef again.
And we see that now, it's imposing
the label implicit null, which of course, is label 3.
That's fine.
It's doing PHP or penultimate hop popping.
The fact of the matter is it is still
doing MPLS label switching which it was not doing a moment ago.
If we look at the label table again,
we see we now have an entry for router 5
so this is looking good back over to router 1.
All we had really looked at on router 1
was the label table to begin with.
So let's just repeat that.
And we see that now, of course, we
have that entry for router 5.
All of this looks like we've solved the ticket.
So let's go back and verify the original ticket is now working.
So that would take us back to router 6.
So back on router 6, we simply do our trace again.
We can tell from this output that on hop 2 there,
we are now doing MPLS label switching
using our prefix SID 16,005.
So this solves trouble ticket 1.
In this video, we saw an example of a ticket
where a router, in this case router 5,
did not have segment routing enabled.
And that of course, breaks the entire label switch path
when trying to get to router 5 loopback.
But once we enabled it, we were able to verify everything.
And now we're doing segment routing
to router 5 loopback I hope this has been informative for you
and I'd like to thank you for viewing.