Let's recall why backup paths are so important.
We know that if we are learning about a particular subnet,
like subnet A,
then the controller is going to send down the best path.
Notice that is a singular word,
the best path to the remote edges.
Now, yes, this could be a tie.
We could have up to four different paths
that we send by default, and we can change that.
We can edit how many, or configure rather,
how many paths that we want to send down from the controller.
So if we have a tie, we could have multiple,
but there is still the reality
that we only send a single path,
or for that matter, we don't send all the paths
that we think we might be sending.
Furthermore, let's say that we have a situation like this,
where our edges on the right are all connected
to all of the different transport mechanisms,
but then we have a remote site here
that's only internet connected.
We only have a single WAN connection.
We don't have a connection going to the other transports.
And this could actually make a lot of sense.
This could be a remote site in the real world,
and we only have internet connectivity out there.
We can't get MPLS, or Metro Ethernet,
or anything else out there.
It's pretty far away.
Maybe it's even in another country.
And so, we have decided that we're only going
to connect this via the internet.
Now, the thing to keep in mind here
is that this information is not known by the controller.
The controller doesn't know whether it has internet,
or MPLS, or maybe it only has one, or both.
It doesn't, the controller isn't interested
in that type of information.
We might think that it would be,
but it doesn't have knowledge of the fact
that we have an internet connection,
but we have no MPLS connection here on edge 11.
So, here's the concerning type of scenario
that might develop.
We might see that routers 13 and 14
both have a path to subnet A.
And furthermore, we've got four different T-locks here
that we'd expect to be advertised.
But what if the, let's say edge 13 here,
what if edge 13 has a better path?
Maybe it has an OSPF cost of 10,
whereas edge 14 has an OSPF cost of 100.
So, we are learning this route via OSPF.
Well, at this point, when we're comparing the paths,
remember, we've got this whole path selection,
we're going to see that I have an edge 13 path
through the internet,
and then an edge 13 path through MPLS,
and those are going to be superior
compared to the edge 14 internet
and the edge 14 MPLS path.
Which means that I'm only advertising
from a tiebreaker perspective,
edge 13 internet and edge 14 MPLS.
Again, the controller doesn't know
that this MPLS path is worthless to edge 11,
which only has the internet.
It's sending this down as the best path,
because again, these are tied
and these paths down here are inferior.
We're not going to send the inferior paths.
So, what ends up happening is edge 11 has two paths
to subnet A, and the good news is
it can use one of those paths.
So, in the routing table,
it's going to include edge 13's internet path.
Now, the danger here is that edge 13
loses its internet connection,
and as a result, this route is no longer valid.
We might look at that and say,
okay, that's fine because we have three other T-locks here,
two of which are on MPLS, which is unfortunate
because we don't have an MPLS circuit,
so those aren't very useful,
but we do still have an internet circuit path via edge 14.
The problem with that is it's not getting advertised.
So, what do we do?
We activate backup paths here on the controller,
and this is going to allow it
to send one additional best path,
which means that if we've got
a bunch of other paths down here,
we're still just going to take the next best,
and yeah, we'll use tie breakers
to help us figure that out.
In this case, we have these inferior paths,
but because they're tied,
we are now going to send those down.
So, I send edge 14 internet and edge 14 MPLS down.
Those don't get installed into the routing table
because they're viewed as inferior.
However, in the event that this outage
just happened with edge 13,
now in my OMP table, I do have another route
that I can install into the routing table,
whereas before, I didn't have this.
So, this entry right here only exists
if backup paths are enabled.
What we want to do now is we want to deploy
this setup into the lab.
This is just a quick refresher
of why it's so important to deploy backup paths
depending on the scenario.
So, what we're going to do in our own lab environment
is we're going to set this up,
except we're going to use EIGRP here in our lab,
and we have to configure EIGRP on edge 14.
We already have it installed on edge 13
from previous skills efforts
when we took a look at configuring EIGRP,
we did so on edge 13,
and then we're also going to make sure
that our bandwidth statements reflect this concept
where we have a superior path through edge 13.
Now, one caveat in all of this
is we don't actually have this full mesh of connectivity
between edges 13 and 14 and internet and MPLS transports.
Instead, we have T-lock extensions.
So, in order to emulate this outage,
we're actually going to perform a shutdown
on the tunnel interface.
Is this likely to happen in a real world scenario?
Not as likely as losing a circuit,
but naturally the same concepts apply here
because this logical topology we have in place
via T-lock extensions,
we just don't have the physical connections,
which is why we have to shut down the tunnel
to represent what's happening here.
The other thing we have to do is,
we know in our lab environment,
we actually do have an MPLS connection to edge 11,
and so we're going to have to shut that down
as part of the process.
We'll do that via SD-WAN manager
since edges 11 and 12 are managed via manager.
So, let's go ahead and jump into the lab
and get all of this configured.