So let's wrap up by looking at topology independent
loop free alternative.
Our main focus here is going to be looking
at the differences between traditional loop
free alternative and topology independent.
What's the difference?
What does that mean?
Keep in mind that the overall idea
behind loop free alternative is that our routing protocol will
have pre-chosen a repair path or a backup path
that it's going to use in the event of a failure.
Now, the problem with loop free alternative, the legacy method,
is it doesn't always find the loop free path.
However, with topology independent, it will.
Now, this is assuming, of course,
that a backup path actually exists.
The other thing we want to do is avoid suboptimal routing.
Traditional LFA in the wrong circumstances
could lead to suboptimal routing.
And one more thing to keep in mind as we jump in here,
we're talking about loop free alternative,
meaning having that backup path ready if there's a failure.
We are not talking about regular routing protocol convergence.
So throughout this video when we say things
like it can't use this path or it won't use that path,
we mean for LFA, in this context.
Of course, if there's a failure, and there's another path,
the routing protocol will find it, it will converge.
We're talking about what paths it will find as a backup
before it has to completely reconverge.
So all of that said, let's go ahead and jump in and take
a more detailed look at topology independent
loop free alternative and how that compares
to traditional or legacy LFA.
So TI-LFA is a resiliency feature
that allows for sub 50 millisecond convergence
because what it does is it calculates
that repair path before a failure actually occurs.
Now, one thing this has over classic or traditional LFA
is this can provide 100% coverage
because it's topology independent.
Classic LFA is, of course topology dependent
and therefore may not always provide a backup path.
Let's take a look at what that means.
So let's bring up a diagram here, and let's
put some costs on these links.
We'll make this one a 10, 10, make this one a 20.
This one will be a 10, and this is
going to be the suboptimal path up here through router 5,
so we'll make that one a cost of 100.
During this whole discussion, we'll
be talking about traffic going from this source
to this destination.
So we're not going to focus on those a lot.
Just keep in mind that that's the traffic
path we're talking about.
We would, of course, normally send this to router 1.
Router 1 would typically send this to router 2
because it's a cost of 10.
And of course, it would then get delivered to the destination.
But the problem, of course, comes
in is what happens when this link goes down.
This is where the question comes in.
Do we have a loop free alternative?
So in this case, let's talk about what
would be happening down here on router 3.
Router 3 would have been going through these two
links with a cost of 10 each for a total cost of 20.
If it had gone across the bottom that, of course, would have
been a cost of 30, and that's not the path
we would have chosen.
The problem is router 3's path would
have been pointing up to router 1 to get to the destination.
So if we tried to use that as an alternative path,
the source would send the traffic to router 1.
Router 1 would send it to router 3.
Router 3 would send it right back up to router 1,
and right there's your routing loop.
So in this example, there's not a loop free alternative
using the classic method.
And that's what we want to address
with topology independent loop free alternative
because we, as humans, can look at this
and see that there's clearly an alternative path where
we could go to 4 and then 2 and then to the destination.
And it's not even that much higher of a cost.
It just would have been a cost of 30.
However, traditional LFA will not address that issue.
That's where TI-LFA can help.
Now TI-LFA can provide protection
for the link, a node, or a shared risk link group,
which is something you as the administrator create to tell
it not to use certain links.
And it does prevent suboptimal routing
because what could have happened here,
we could have possibly chosen to go through this path
through the top through router 5.
That could have possibly been a loop free alternative.
However, that's a really suboptimal path.
So if router 3 did choose to go to router 1,
and router 1 decided to go to router 5,
would the traffic get to the destination?
Yes, most likely, but definitely through a suboptimal path.
This would end up being a cost of 200,
where with TI-LFA using the bottom path,
it only would have been 30.
So even if LFA could have provided this loop free path,
it would have been suboptimal in this topology.
So this is all the things we're trying to avoid.
And the cool thing about this whole thing
is this is a very, very simple operation.
This is all computed by the IGP, so either OSPF or IS-IS.
TI-LFA works by calculating what's
called the post convergence path, the path we're
going to use after the convergence of, well, just
realizing the link is down.
So this is, again, going to be calculated before any failure.
That's the whole point here.
And this path, again, may not actually be an LFA.
Remember that router would have this looping situation going on
over here if this link went down and therefore not really
have an LFA.
But with TI-LFA, the idea is we want
to be able to use this path through the bottom
to get to the destination.
And as we said, with traditional LFA,
this would not be a loop free path.
The way we get around this is we enforce a loop free path
by using the segment list.
Essentially what's happening here
is router 1 is making the decision
to go through routers 3, 4, 2, and then
to the destination, this path across the bottom.
The way we stop router 3 from engaging in this loop
is by using our segment list.
Remember, we tell router 3 to go to router 4.
This is where that source-based routing kicks in
to force this loop free path.
So the real question then becomes,
how does TI-LFA determine this path to begin with?
If legacy LFA can't do it, then how does TI-LFA actually
pull this off?
One of the key factors here is that TI-LFA
uses the concept of two different spaces, the P
space and the Q space.
Both spaces are calculating using pre-convergence paths.
Remember this is all calculated before there is a failure.
The P space is a set of nodes that
are reachable from the point of local repair or the PLR.
In our example, router 1 would be the PLR.
This is the point of local repair.
So let's think about this for a moment.
Which set of nodes are reachable from the point of local repair
without using the protected link?
So let's remember that we were saying this was a cost of 100.
This was 100.
We had a 10, a 10, a 10, and a 20.
So where can the PLR get without using the protected link?
Remember we're focusing on this link right
here as being the link that we're trying to protect.
So where can router 1 get without using that link?
Well, let's think about this.
It can definitely get to router 3.
So router 3 is definitely going to be in the P space.
It's a cost of 10 to get to router 3,
and there's certainly no better path.
If it tried to go from router 1 to router 2
to router 4 to router 3, that would be a cost of 40
to go around that way.
And of course, going up through the top
would definitely not be a better option.
So router 1 can get to router 3, no problem.
That's in the P space.
Router 4 would also be in the P space
because, again, it's 10 and 10 to go through the bottom, where
it would be 10 and 20 to go through the protected link.
So it wouldn't choose to go through the protected link.
And we could say the same thing about router 5.
Yes, it's a cost of 100 to get to router 5,
but going through router 2 through the protected link
would be 110.
That would be even more.
So these are the routers that are
reachable through the P space.
Of course, router 2 is not because router 2
would literally be trying to use the protected link.
So these are the P space routers.
Again, the routers that can be reached
by the point of local repair without using the protected
link using standard metrics.
Next is the Q space.
These are the nodes that can reach the destination
without using the protected link.
So let's go back down to look at our diagram again.
Of course, router 1 can't because it's
going straight across the protected link
to get to router 2 and therefore the destination.
Router 3, well, router 3 would prefer 10 and 10
over 10 and 20, so router 3 would
be using the protected link.
Therefore router 3 is not in the Q space.
Router 4.
Router 4 would, of course, go the path of 20
rather than 10, 20, 30 to go around the other way.
So router 4 is in the Q space.
And then router 5, of course, is going to take the path directly
to router 2 so that would make router 5 in the Q space
as well.
And of course, router 2 goes directly to the destination.
So it's definitely in the Q space.
So once the Q and the P spaces have been defined,
couple things to keep in mind.
First, we can certainly have PQ routers.
This is a router that's in both the P space and the Q space.
And of course, in our example, we
can see that router 5 and router 4
are both in the P and Q spaces.
And then ultimately, the final decision here
is realizing that the packet can then be released
on any router in the Q space.
So essentially what it's going to do
is router 1 is going to look at this and say, OK
the TI-LFA path I could release this on router 4.
I could release it on router 2 as well or router 5.
But router 4 is going to be the best path because it only
costs me 20 to get to router 4.
So it's going to try to go across the bottom.
So ultimately, what this means is the source
sends the update to router 1.
Router 1 sends it to router 3 with the appropriate segment
list on it so that it doesn't loop.
Router 3 sends it to router 4.
Router 4 sends it up to router 2.
Router 2 sends it to the destination,
and we've provided a topology independent loop free path
to the destination without using the protected link.
And again, remember all of this is calculated
before a failure occurs.
In this video, we took a look at topology independent LFA.
And unlike traditional or classic LFA,
TI-LFA will always provide a backup path,
assuming one actually exists.
Of course, if there's a single path, we can't back that up.
But if there is one, TI-LFA will find it
by using these two separate spaces, the P space and the Q
space and then using our source-based segment routing
to get that traffic through the environment
to the necessary router without creating
a loop, which as we demonstrated,
could clearly happen with classic LFA,
and that's why there's not always
a backup path in traditional LFA.
So that wraps up everything that we
need to discuss in this skill, and that
leaves us with just the review in the form of a quiz.
And that's exactly what we're going to do next.
Till then, I hope this has been informative for you,
and I'd like to thank you for viewing.