Alright, it is time to jump into this topology and make sure we understand the nature of routing as well as to explore that black hole scenario.
We do need to make sure we understand the purpose of backup paths so that when we're managing an SD-WAN environment that we can properly deploy it.
We don't just want to deploy it as a just-in-case. We want to be able to look at a topology and decide whether or not it's going to be required.
So let's just go ahead and walk through the questions as they're presented.
We're told to assume that OMP preference is the same on all of these devices.
What we should see here is that we have subnet A attached to edge 100 and importantly it only has a single transport link to the internet.
Subnet B has connectivity through edge 200. It is directly connected like subnet A is directly connected to edge 100.
And it has connectivity to both transports.
Meanwhile we have this double edge site over here with edges 300 and 301.
And importantly it's running OSPF downstream to the distribution layer.
So question number one. How many paths are distributed for subnets A, B, and C?
So we need to take a look at each one of these subnets individually.
For subnet A we're talking about paths. What we're really talking about is the route and the T-lock.
Because the T-lock serves as the next hop.
So I could have one route, one subnet essentially with multiple T-locks depending on the situation.
When we're looking at subnet A, well subnet A is itself a subnet and then there's really only one T-lock here.
And so interestingly subnet A here only has one route or one path that's going to be advertised as the T-lock on edge 100.
Meanwhile B, this is going to advertise two paths because edge 200 has two T-locks.
So it's essentially going to say that I have accessibility to subnet B and it's accessible via T-lock number one and T-lock number two.
Meanwhile subnet C is where things get a little bit more interesting.
Because we are going to see that edge 300 has two T-locks.
Edge 301 has two T-locks. However, edge 300 has a better path to the subnet.
Well we're just asking about how many paths are distributed.
And so the question is when we have our controller up here, what exactly is it pushing down to the rest of the network?
Well when we look at this we're going to see that all four of these routes or paths get advertised.
So there's four paths available. However, it's only going to push two of these paths down.
And the reason for that is because it's going to order the list.
And our order is going to be based on the fact that two of these routes have a origin metric or an OSPF cost of 10.
And then the other two have 100.
Which means it's going to see these two as the best paths and it's going to forward those down.
So really when we're talking about distributing, we're only going to distribute two paths.
So next question. When edge 100 receives a route to subnet C over here, what does it see as the next hop or the next hops?
And how many routes are installed into the routing table?
Well as we see we're going to advertise two subnets down to edge 100 and edge 200 for that matter.
Both of them are receiving both these routes.
Just keeping in mind that one of these routes is an internet T-lock and the other one is an MPLS T-lock.
And so edge A is going to run that evaluation that we did before where I've got an internet and an MPLS path.
Where the MPLS path is the same cost as the internet path.
So the order might be different here.
But either way, when I check for reachability, the MPLS is not reachable by this edge device.
It doesn't have an MPLS T-lock and because there's no connectivity here between the two,
well that means that I can't actually get down to, I guess it would be right here, the MPLS T-lock for edge 13.
So we are going to receive two routes, but we're only going to install one.
So the answer to number two here is it's going to be edge 300's internet T-lock.
That would be the next hop and it really is only going to install one route or one path.
So that is the answer to question number two.
Now question number three, how is this different when edge 200 receives the route to subnet C?
How many routes are installed in its table?
Well again, it's going to get this exact same scenario.
It's going to receive both of these routes and it's going to look at it as saying I've got a path to the internet,
or that via the internet T-lock and a path via the MPLS T-lock and hey, check it out.
I can reach the internet T-lock and I can reach the MPLS T-lock.
So edge 200 is going to receive the same two routes, but instead of crossing one of them out,
it's going to say that both of them are okay.
So it's going to install both of these paths into the routing table for subnet C.
So last question, if backup paths are disabled, which is the default by the way,
what failure scenario would cause edge 100 to be unable to reach subnet C?
So one of the biggest takeaways of this whole conversation is understanding the role of backup paths.
If we understand the role of backup paths, then that not only helps us know when to deploy backup paths,
but it also makes sure that or really emphasizes that we understand routing in the OMP space.
Because this whole distribution via the controller can get a little confusing.
And if we don't fully understand how the routes are distributed and how T-locks are used as the next hop,
then we're not going to understand backup paths.
So if we struggle to answer this question, we still struggle to understand backup paths.
It might be worth going back and reviewing the routing section of this skill,
because the routing section is what's really going to drive home the value of backup paths in our own minds.
So here's the deal.
We are looking at the fact that the controller's advertising only two T-locks to edge 100, let's say.
And edge 100, because it only has one T-lock and only has accessibility to the internet T-lock that's being advertised,
is only going to install a single T-lock into its routing table.
Well, if this link goes down right here, the internet link for edge 300,
then edge 100 is not able to access subnet C anymore, even though subnet C is accessible on three different paths.
Importantly, one of which is layer three accessible to edge 100.
The problem is simply that edge 100 is not going to be aware of that subnet,
of that route, because it won't have it installed in the routing table.
Because the controller is only advertising the best path,
the best path, even after the internet is withdrawn, is still going to be seen as that MPLS link.
And as a result, edge 100 is going to now receive not the internet, but it's going to receive MPLS,
and it will be unable to install the MPLS T-lock into its routing table because that T-lock is unreachable.
So this right here, the internet link going down on edge 300,
is the scenario that would cause edge 100 to be unable to reach subnet C.
So very important for us to understand that.
The simple solution to that is simply to enable the backup path concept,
because the backup path will look at the next best path, not all paths, but the next best path,
and these two T-locks right here would be tied for next best.
So at that point, we'd be advertising the best, but also the next best.
And now edge 100 will have the three different routes to filter through.
It will recognize that I can't reach either of the MPLS T-lock advertisements,
but I do still have that backup internet path,
and that backup internet path right here is going to get me to subnet C.
So that brings us to the end of our conversation around OMP routing.
If we can bring this all together, including explaining the purpose of backup paths,
then we've probably got routing down pretty well.
But if this challenge made us kind of question whether we fully understand the routing propagation,
it's really best to go back and continue to review that to make sure we've got that down
before moving on and continuing to go deeper into OMP and SD-WAN routing.
I hope this has been informative for you, and I'd like to thank you for viewing.