All right, it is time to jump into the concept of path selection with OMP.
We want to be very comfortable with how and why those decisions are made,
including if it comes down to some of those arbitrary tiebreakers.
So, scenario number one here.
By default, which four routes are sent to edge three over here for subnet A,
which is a connected route connecting to edges two and three?
Well, it's right there in the question that we're going to select four routes,
but always remember that that is the default.
The default is four.
That would include the routes that are sent to a receiving edge,
as well as how many routes get installed.
So, we're not asking to change the default here.
We're actually just using that default value.
We're going to send four routes and install four routes.
The problem is we're going to receive one, two, three, four, five, and six routes.
So, we've got to get this down to four.
Now, when we look at the path selection criteria,
we are not told that the TLOC preference has changed,
so we don't need to worry about that.
We are not told that the OMP preference has changed.
We don't need to worry about that.
And as far as origin protocol and metric are concerned,
this is a connected route for both edges.
That means that all of these routes are equal.
So, we are truly talking about these arbitrary tiebreakers
and trying to determine which of these routes is going to be favored over the others.
So, when we look at the tiebreakers, it starts with that path source.
So, this is number one, path source.
Remember, we always prefer a route learned from an edge
rather than or over a route learned from a controller.
Now, in our case, yeah, we've got some relationships happening here,
but all six of these routes are coming from edges,
so we don't need to worry about this.
This is not going to help us out with determining which of these six routes
should be chosen as far as the four that get to be sent.
So, next tiebreaker is the system IP address.
This is useful because we have two different system IPs.
We have a 1.1.1.1 and we have a 2.2.2.2.
So, the question is, do we prefer the lower or the higher?
Well, as I said during the quiz, we always prefer lower
unless it has to do with preference.
These are going to be higher. Everything else we want to see lower.
So, the system IP address, we're going to prefer 1.1.1.1 paths,
which means we can officially select all three of these,
although we haven't established the order in there.
So, right now, we could say that we have now sorted it out
so that we've got edge1, edge1, and edge1 routes all tied for first,
and tied for second, we have the three edge2 routes.
So, we need to do two things here.
First of all, we need to sort these three out.
We need to understand how that would be done.
And we need to sort these three out because we have to eliminate two of them.
But which two do we eliminate?
Well, now we're coming down to the third tiebreaker,
which is the lowest private IP address.
Now, at first glance, we only have one private IP address, which is MPLS.
and for that matter, edge1 also, because we're still considering those paths as well,
direct Internet paths don't get a private IP address,
which means that when we list out in the TLOC,
we're going to list the public address under this field.
So, I think that's an important distinction.
This is a field, or really an attribute, of the TLOC route.
And so, if it's an attribute, we can put anything in that field,
which means that if we do have a private address, it will be compared over the public address.
But if we don't have a private address, then this becomes a public,
and the same IP address gets listed for both private and public values.
That's a very important concept.
So, at this point, again, we want the lower value.
So, what we're really looking at here is we have these actually in order.
So, this is the lowest, this is the middle, and this IP address would be the highest.
So, we're going to prefer the first Internet path,
and then we will take the second Internet path, and then we will take MPLS.
So, what is the final verdict here?
What four routes are we going to choose?
Well, let's in fact go ahead and call this Internet 1 and Internet 2, just for distinction purposes.
So, the first path we're going to pick is Edge 1, Internet 1.
That has the lowest IP address compared to the other Edge 1 routes,
which includes Internet 2 second, and Edge 1, not Internet 3.
Actually, there's no Internet 3, just MPLS.
And then the fourth route that we're going to choose would be one of these Edge 2 paths.
And just like with Edge 1s, we're going to favor the lowest IP address, which is Internet 1.
So, again, let's not get confused.
The private IP address, kind of we can use air quotes there, it's an attribute,
and whatever gets put into that attribute is what we're comparing.
And if we're not using that, that means that we have the same IP address listed in private and public fields, or attributes.
So, yes, we're going to favor Internet 1 because it has a lower IP address,
which gets inserted into the private field,
rather than looking at, strictly speaking, whether an IP address is public or private.
So, this here, these four paths in this order, this would be the answer to scenario number one.