All right, let's go ahead and take a look at this scenario.
We want to make sure we can answer some questions based on
path selection criteria and also
our tiebreaker that we're going to have to
use if we have multiple equal cost pathing.
The first question, how many routes to subnet A will be
installed by edge 101 over here?
Assume default configuration.
What we're looking at when we just take a look at
this scenario and this topology
is we've got this subnet A over here.
It's directly connected to two different edges,
edges 201 and 202.
Presumably, these are at the same site.
Each of these edges is advertising out individual T-locks.
Now, we see that we have three different transport mechanisms,
the Internet, MPLS, and Metro Ethernet.
Each one of these edges has three T-locks,
one to each transport mechanism.
We also are given the IP address that belongs to each one of these.
As noted, those are the last octet
in the private IP addresses for those T-locks.
We see that they actually have similar IP addresses in some cases.
But in the real world,
that might not make a whole lot of sense,
but hey, they might be connecting in with
different subnets depending on how
the service provider is doing that.
That is the scenario.
Let's get back to this question.
How many routes will be installed by edge 101?
This doesn't actually have to do with any of
this information other than simply the T-lock count.
We see that we have six T-locks, three on each edge.
Each one of these edges is going to advertise to the controller that,
hey, I can get to subnet A via T-lock 1,
via T-lock 2, and via T-lock 3.
This edge 201 is going to do the same thing.
We're actually going to advertise all six of these as accessible paths,
more or less saying, remember,
from an OMP perspective, yes,
I can get to this subnet.
What is the next hop?
The next hop is the T-lock.
We've got six different next hops going to subnet A,
and it does say assume default configuration.
When we think about the path selection criteria,
we are typically looking at,
first of all, is it valid?
Then we also want to make sure, hey,
are they active or stale?
We don't have to worry about that with either of these edges or any of these advertisements.
Then we start talking about the preference.
Again, if these are the default,
then we don't have to worry about them.
We have OMP preference,
we have T-lock preference,
and then we have origin.
Both of our origins are going to be static,
which means they have the same origin type and origin metric.
All of this is the same.
In the end, all six routes are going to have the same cost.
This is going to lead us to equal cost multi-pathing or ECMP.
We advertise six routes to the controller,
and now the question is,
well, really the question is how many get installed down here?
We might say, well,
how many are going to get installed depends on how many are received,
which depends on how many are sent.
Remember that the default is that we're only going to send four.
The controller is going to order these 1, 2, 3, 4,
5, 6, and it's going to send the top four,
because that's what the default setting is,
it's that send path limit.
That gets sent down to 101,
and in theory, it's going to install these.
The answer to question number 1 is four routes.
Now, question number 2 says,
which paths to subnet A will be installed and assume default configuration?
Well, now we have to pay attention to the TLOC information,
and basically what it's asking is,
how does the controller sort out really all six of these routes,
and how does it decide which four are selected?
Well, again, we look at this,
we realize they're all the same cost,
it's all equal cost multi-pathing,
so now we have to worry about the tiebreakers.
So what are the tiebreakers?
Well, first of all, we prefer things learned from an edge.
Well, these are both coming from edges,
and then we're going to pay attention to the system IP,
and then we're going to pay attention to the private IP address on the TLOC.
So we always want to remember or be able to cite these tiebreakers.
So again, they're both coming from edges,
so we don't have to worry about that.
The system IP does matter, actually.
So the system IP address is going to be the lower system IP address,
which is edge 201,
because we were told in the description that we have 1.1.1.1 on the left,
2.2.2.2 here, and 3.3.3.3 on 202.
So between 201 and 202,
we're going to take these three,
and the order is going to be based on the private IP address.
Remember, lowest to highest.
So this is the first route,
this is the second route,
and this is the third route.
As for the fourth route,
we're going to then look at the remaining three
and also take the lowest private IP address.
So those are the four routes that are going to get installed.
Now, lastly, let's say that we decide to up this to six.
We use that send path limit command,
which we'll take a look at the actual command later on,
but it's the send path limit,
and we set it to six.
And the question is,
hey, we find out that there's only four routes
still being installed over here.
What is wrong and how can it be corrected?
Well, remember, we have two different limits.
We're sending six,
but the ECMP limit over here
is how many equal cost multi-paths that we will install.
So we're still only going to install four.
We're receiving six, that's great.
and we're going to use the same tie breakers
to decide which four that we install.
So it'd be the same four.
So what do we need to do?
We need to increase the ECMP limit
to match the send path limit, which would be six.
So that explains a little bit more
around the tie breaker mechanism,
especially paying attention
to how many routes get advertised,
looking at those limits,
understanding how many routes get sent,
and then how many routes get installed
depending on those limits.
So we want to make sure that we have all of this down
and really understanding the foundation of OMP routing.
If any of these questions tripped you up,
be sure to go back and kind of review that material
to make sure you've got it down
before we go even deeper into path selection criteria.
I hope this has been informative for you,
and I'd like to thank you for viewing.