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So the last thing we're going to be looking at here
is an OSPF SHAM link.
Now, this is a control plane tunnel
that we can use between our PEs to build an OSPF control
plane to allow the routes to come from one PE to another
through our BGP core.
And still come out the other side as internal OSPF routes.
Now, the way it's doing that is, of course,
building this control plane tunnel.
It's literally passing the OSPF packets through the SHAM link
and forming an actual OSPF neighborship between the PEs.
Now, you might be asking, why do we need all this?
The main reason is if our customers have a backup link,
so some sort of connection outside of MPLS,
and they want to use that as a backup, here's the problem.
We've seen this already that the OSPF routes coming
through our MPLS layer three environment
are coming through as OIA routes,
even if they're in the same area.
Well, the problem is the other routers
connecting across that direct link, whatever
it may be, those routes would be coming in probably as O routes
or just straight out intra-area routes.
And there's no way to have OSPF prefer an inter-area route
over an intra-area route.
Which means the customer would always
be using that back-up link.
They would have no way of preferring the primary link
going through MPLS.
And that's what SHAM links allow us to do.
Now truth be told, this is something
we need to be familiar with.
However, I am not really familiar with any service
providers that will offer this as a service
for their customers.
As we go through here, I think you'll probably see why.
There's quite a bit that needs to be set up.
And all the work is on the service provider side.
So this is a lot of things that we're
going to have to configure, set up, and get working,
and maintain on an ongoing basis just
to support this one thing that a customer may want to do.
The better thing to do would just
be not to run OSPF with that customer.
Let them run BGP instead.
And use normal routing admin distances to prefer the path
through MPLS.
So there are other, you could argue,
better ways of doing this.
However, we still want to be familiar with SHAM links
and how they work.
So all of that said, let's go ahead, and jump in,
and take a look at SHAM links.
Of course, we'll start with our diagram
so we know where we're going to build it.
And then we'll jump into the command line
and get a SHAM link up and running.
So for this demonstration, we're going
to be switching to MPLS topology four.
The main difference here is we have
this gigabit three interface available on routers six
and eight that we will be using for our backup link.
This is currently not configured.
So we'll be configuring this as we go through.
Now, the intention, of course, is for this
to become a backup link.
However, as we stated during the introduction,
the routes learned over this link will be O routes.
Therefore, router six and router eight
is going to prefer this path.
So our goal is going to be to build
a SHAM link between router one and router five.
So this SHAM link will allow the routes coming through MPLS
to also show up as O routes.
And at that point, we can just use
OSPF metrics to get OSPF to prefer the path through MPLS.
So that's what we're going to be setting up.
Let's jump into the command line and get started.
So let's just start by looking at the current OSPF routing
situation on router six.
And as we see, of course, all of the OSPF routes
for the remote sites are learned as OIA or inter-area routes
through OSPF.
So let's see what happens when we bring up
our backdoor or backup link.
So simply go into interface gigabit three.
For the IP address, we're going to use 192.168.68.
And this side will be dot 6.
We'll keep it simple.
We'll just use a /24.
And we'll bring that interface up.
That interface should be automatically included
in the OSPF process because of the way we implemented OSPF.
So now, let's do the same thing on router eight.
Again, we'll set the IP address, 192.168.68.
This side, of course, will be 8.
We'll do a no shut.
And if we wait just a few seconds here,
not only should the interface come up, but we should also see
OSPF come up.
So after a few seconds here, the OSPF neighborship
did, in fact, come up.
So let's go ahead and take a look
at that route to the other side.
We'll do a show IP route.
Let's look at the 10.100.1.6.
What we see here is that it's currently going directly
to router six as we suspected it would as an intra-area route
and only a metric of two.
So let's do the typical OSPF thing.
And let's crank up that metric on the interface
that we don't want to use.
So we'll go back into configuration,
into interface gigabit three.
And let's change that IP OSPF cost to something really high.
So OSPF will definitely prefer to go through MPLS.
And let's go ahead configure that.
We'll exit out, and let's look at that route again.
What we see here is the metric has now raised to 1001.
But we're still using the intra-area route
going to router six.
And if we said, show IP route OSPF,
notice we're not using any OIA routes.
So we're not going through MPLS at all.
In fact, if you look at the interfaces,
we're only using gigabit two and gigabit three for our routing.
Which means we're going right around MPLS.
So as we said during the introduction,
if we want to do this through MPLS using
this circuit as a backup, we're going
to have to create a SHAM link.
And that is built entirely on the service provider routers.
So let's go over to router one.
So the first thing we'll do here is go into configuration mode.
We're going to need a new loopback interface.
Let's make this loopback 101.
It has to be in the customer's VRF.
And of course, it's going to need an IP address.
So let's do something like 100.100.100.101.
And of course, we'll make this a /32.
We'll commit that change.
Make sure there's no issues.
And let's create an identical loopback on router five.
So on router five, we'll just paste the configuration
in we've seen creating a loopback interface before.
So there's a loopback 105 interface.
And we can see it has a corresponding address there
of 105.
Also, in the customer's VRF.
The next thing we need to do is bring this into BGP.
So let's go back over to router one.
We'll go into router BGP 100, into the customer's VRF,
under the address family IPv4.
And we'll simply use the network statement to inject
the 100.100.100.101.
We'll commit those changes.
And now, we'll go bring the loopback in on router five.
Again, we'll just paste in the configuration.
We've seen this a few times, how to inject networks.
We'll give BGP just a moment to converge.
And then we'll verify the routing table.
So let's just do a show IP route for the VRF customer 1A.
And of course, what we're looking for is this entry right
here that we have learned the 100.100.100.101 through BGP
in the customer's VRF.
Let's verify on router one as well just
to make sure we have the route on both sides.
And now, we see that we have the 100.100.100.105 here on router
one.
So it looks like we have our loopbacks
to build our SHAM link between.
So let's go ahead and build the SHAM link.
This is done under the OSPF process in the customer's VRF,
of course, area zero.
And we're going to build another SHAM link between
the 100.100.100.101 and the 100.100.100.105.
We'll commit those changes.
And at this point, we should be able to exit configuration.
Now of course, we have to do the same thing on router five
before the SHAM link will come up.
Again, it's a similar configuration.
Except we have to go into the customer-specific OSPF process.
And then we just say, area zero SHAM link.
And it's still then source and destination.
And we can see here on the console,
that we've just formed an OSPF neighborship over the OSPF SHAM
link zero.
So this looks like it's working fine.
Let's do some verification.
Since we're here on router five, let's just do a show
IP OSPF SHAM links.
We can see here that the SHAM link is up.
It's being run as a demand circuit.
Meaning, we don't have to send hellos.
We can actually see down here that the adjacency state
is full.
And it even says, Hello suppressed.
So this looks like it's up and running.
Let's verify on router one as well.
Now, of course, on router one here, it's
already giving us a console message
that the neighborship is up.
So we have pretty high confidence that it's working.
But just so we can see the same command here
it's show OSPF VRF customer A SHAM links.
We see the output is almost identical.
But we see that the SHAM link, of course, is up.
Once again, it's being run as a demand circuit.
And the adjacency state is full with the hellos suppressed.
So if we've done our job correctly,
if we go back over to router eight now,
and we look at this same output again,
yes, we have these two external links being brought in by BGP
through our redistribution.
We're not going to worry about those.
Normally, of course, you would probably
want to filter those out.
But it won't hurt anything for the customer to see those.
It's just not something we would normally want to do.
The important thing we're here for
is to see that, for example, this 10.6.6.6 network is now
going out through gigabit one in addition to several
of these other routes.
Meaning, that we're now preferring the MPLS path
over the backup link, since we had set that metric to 1,000.
And we can see that these metrics are more like four.
So in this video, we saw how to use an OSPF SHAM
link to use MPLS as our primary path,
if we have a backup circuit that we, well, just
don't want to prefer.
So as you can see, this is quite a bit of work
on the service provider.
What do we have to do?
We had to build a new loopback and put it
in the customer's VRF.
We had to then advertise that route into BGP.
We then had to use OSPF to build the SHAM
link between those loopbacks.
As I'm sure, you can probably see here,
this is not going to scale well.
So again, I'm not aware of any service providers.
Certainly, not that we'll do this to scale.
Maybe, for a larger customer, they
might be able to get the service provider to do it.
But my argument there would be, if it's a larger customer,
they're probably already running BGP with the service provider
anyway, as they're CEPE not using OSPF.
So this would probably be more of a small company
or small customer type solution anyway.
So again, I would find it pretty unlikely
that you'll find these out there too often in a service provider
environment.
But if you ever do, at least now you know how they work
and what they're for.
That said, that covers everything
that we need to cover in this skill.
So that leaves us just with one more thing to do.
And that is our review in the form of a quiz.
And that's exactly what we're going to do next.
Until then, I hope this has been informative for you.
And I'd like to thank you for viewing.