Imagine if we had two different paths
that we could take to go to a theater, or to dinner,
or whatever it is.
And on these two paths, we'd get notified that, oh, this path is
going to take 10 minutes, and this path
is going to take an hour.
We are going to avoid the path that takes an hour.
Now, we can influence a router to think
a path is really bad by increasing the metric using
a method called an offset list, which
is what we're going to both implement and verify
in this hands-on lab. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
in this hands-on lab.
So here's our game plan.
Let's toss in eigrp on this topology.
And then we'll take a look at some routes from R8.
And we'll see-- we'll pick a route.
Pick a route, any route.
And we'll identify which path that R8
is using to get to that network-- whether it's
choosing door number one, door number two, or door
number three.
And then we can take that preferred path,
and what we can do is create an offset list.
And here's how an offset list works.
We can say, Dear Mr. Router 8, when you receive-- WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
We can say, Dear Mr. Router 8, when you receive--
and we can actually identify a single route
or a list of routes by using an access control
list to point to the exact routes
that we want to manipulate the offset for, the metric for,
or we can use 0.
So if you 0 instead of an actual access control list,
it'll say, OK, that means all routes.
And then, we can also associate with an interface.
So if-- let's just say for a moment that 3 slash 2
is the preferred path to get to a network.
We can specify in eigrp that we're
going to use an offset list.
We can increase the metric by, say, 100 million. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
We can increase the metric by, say, 100 million.
Well, we could also look at the topology table
and see what we would have to be worse than for another route
to be used and simply increase it.
So the offset list increases the metric
by whatever value we specify, and that can thereby
influence the router to no longer take that path that
takes like an hour.
It's going to use a faster path or a quicker path.
But that's yet another tool.
So let's do this.
In the lab environment, click on the link for the virtual lab.
We'll implement eigrp on this topology. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
We'll implement eigrp on this topology.
That will take just a few moments.
And then we'll identify the best path R8's taking.
And then we'll manipulate that with an offset list
locally on R8.
And for eigrp everywhere, it's just config t.
Router eigrp.
We'll use autonomous system one.
And also, we have other videos on name mode
and numbered mode for eigrp, so enjoy those videos as well.
And we'll just dump everything in.
So we'll do a Control-A to select all that, right-click,
Copy, and then we'll just go ahead and plug it in on each
of these routers--
just clicking on the tab, right-clicking to paste, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
just clicking on the tab, right-clicking to paste,
and it is done.
One thing about eigrp, man, is it's so darn fast.
All right, so we have full connectivity right now.
So iproute eigrp.
It looks like we're using Serial3/2 a lot.
Let's go ahead and pick 5.5.5.5.
That'll be a nice one we can pick out and identify.
So to get to 5.5.5.5, it's using Serial3/2.
So if we look at our topology, that's right here.
So for 5.5.5.5, it's going to use Serial3/2
as its egress interface. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
as its egress interface.
And let's go ahead, and we could do an access control list
to specifically call out 5.5.5.5, or we could just say,
hey, any routes that we [INAUDIBLE] in on Serial3/2,
we want to increase the cost, set the offset
to a greater number.
Or we could say on Serial3/2, any updates we receive there,
just go ahead and let's offset that metric
by some number that'll make that path no longer desirable.
And we can also verify that before we do it
by looking at the eigrp topology table, which WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
by looking at the eigrp topology table, which
will be a good exercise, too, in this lab.
So let's go to R8, and let's take a look at it.
So here on R8, we'll do a show ip eigrp topology.
[LAUGHS] All right, and let's go ahead and put in 5.5.5.5/32.
So as we decode this for this specific route,
there is our current metric, which is--
I'm going to put commas in here so I can read it--
it is 2,298,112, and that is this bad boy. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
it is 2,298,112, and that is this bad boy.
So this is our successful route.
There's the reported distance from our neighbor.
And here is our feasible successor right here.
Well, at the moment it's a feasible successor
because the advertised distance is
lower than the current feasible distance.
So currently, it's a feasible successor.
Great, and wow, look at this.
[LAUGHS] Its current metric, its current distance to get to 555,
is--
it's going to put the commas in here--
is 40,672,000. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
is 40,672,000.
40 million versus 2 million--
holy schneikers.
The reason I point that out is because when we do the offset,
we need to grow it by about 2 million versus 40.
So if we grow it by 38 million, will that be enough?
Actually, let's just grow it by 39 million.
All right, so what we'll do is we'll set offset.
We'll create an offset list, and we'll increase the metric WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
We'll create an offset list, and we'll increase the metric
by 39 million.
So it's going to make this route, this path,
look worse than this guy.
And this guy will take over as the successor route.
All right, I wouldn't have guessed
it was going to be that high, but I'm glad that we looked.
Also, it's a great way to help troubleshoot
by looking at the details of the metrics involved.
All right, let's go ahead and do it.
So here on R8, let's config t.
Instead of using an access control list
to say which specific network, I'm WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
going to go ahead and say all networks coming in
on interface Serial3/2 right there.
All right, so we'll go and do a router, eigrp,
autonomous-system 1, and press Enter.
And then we'll do an offset list.
And this is where we can specify an ACL to identify which routes
are going to have this offset.
I'm going to use a 0, which is a shortcut for Select All
Networks and it says right here.
And then we'll go ahead and put it in our offset.
So this is going to be for traffic or updates that WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So this is going to be for traffic or updates that
are coming in, and we're going to specify the offset of, what
did we say 39 million, 39?
1, 2, 3, 1, 2, 3.
All right, and that should do.
[LAUGHS]
We have an update here.
We have our eigrp neighbors, which is fine.
And let's scroll up to find the original route.
So we're looking at 5.5.5.5.
This is before.
It was using Serial3/2.
But now that metric of 2,298,000 is no longer WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
But now that metric of 2,298,000 is no longer
going to be that, because we bumped it up
by like 39 million.
And so if we do, if we hit the Up Arrow key a couple of times
and do a Show IP Route for eigrp,
and we look at the 5.5.5 network again, and--
oh, my gosh.
[LAUGHS] It's still using Serial3/2.
That's because I did not--
I said, hey!
That's why we had the neighborship messages
with all of our neighbors.
So basically, I increased by 39 million
every single update coming in. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
every single update coming in.
I need to go correct that, because this is not
the results that we wanted.
So we'll do a config t router eigrp, autonomous-system 1.
Hit the Up arrow key a few times.
We'll do a Control-A and say no, offset list 0.
Then we'll go ahead and do that command again.
But this time, we will include the interface,
which is going to be serial 3--
and then let me get the right interface here.
Let me go back here for a moment, our drawing.
It's going to be 3/2.
So only the updates coming in on 3/2
will have that offset applied. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
will have that offset applied.
And let's go ahead, and 3/2--
all right.
So Serial3/2 is not going to be too preferred anymore.
Let's show IP route, and let's go ahead and do for--
this [INAUDIBLE] show IP route eigrp.
And let's look for the five network.
And there we go.
There's the five network.
It's now using egress interface of FastEthernet 4/1
with an [INAUDIBLE] of 0.6 because the other route
looks terrible.
So we can verify that with a show IP eigrp topology. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So we can verify that with a show IP eigrp topology.
And there it is right there.
It shows us that we have a successor,
and there's this metric, and that's out 4/1.
[LAUGHS] And here is 3/1, which is a feasible successor
because the advertised distance is
less than the actual current successor cost.
And [LAUGHS] our route through 3/2
is not even showing up anymore.
Let's do this.
Let's do a show eigrp topology, and let's do an All Links
option and press Enter.
Now let's look for 5.5.5.5 again. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Now let's look for 5.5.5.5 again.
So there it is.
Even though it's not a feasible successor anymore, now,
with the All Links option, it's going to show up right there.
So there's our winner, there is our feasible successor,
and there is the route that we just
toasted based on setting the offset so stinking high
for that interface.
Another command, while we're here, is a show eigrp topology.
This is a cool one, too.
And that is 0--
0 successors.
Show me the eigrp topology database WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Show me the eigrp topology database
where there's no winning eigrp route.
And here we go.
These two networks, 10.38.0.1, and also 10.78.0.8.
And it says the FD is infinity.
Well, the metric right here, it doesn't look like infinite
to me.
It looks like a really large number, like 41 million
or whatever.
But what happens here is that if we have a route with a better
AD, like a directly connected network with an AD of 0,
they're going to beat out these routes.
And that's what's happening.
So this is on R8. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So this is on R8.
It is directly connected to the 10.38.0 network.
It's also directly connected to the 10.78.0 network.
And that's why these routes, with the FD as infinity,
they are not successor routes.
There's no successor route from the eigrp
because their AD isn't good enough
to compete with the directly connected network, which
has an AD of 0.
So in this video, we've had some fun
with playing with another tool that we
can use to manipulate routing, and that is
the concept of an offset list.
We can apply it to an interface.
We can include whether or not it's inbound or outbound
when we modify the offset. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
when we modify the offset.
We can also specify with an access control
list which routes it would apply to, specifically
regarding that offset.
Thank you for joining me in this video regarding offset lists
in a Cisco environment, and I'll see you in the next video.
Meanwhile, I hope this has been informative for you,
and I'd like to thank you for viewing.