There are situations where the default for a given routing
protocol are going to help protect us,
so that we don't even need to get creative with modifying
the administrative distance, or filtering routes, or tagging
routes, or looking for that tag.
So in this video, I'd like to chat with you about some
of those defaults that are really going to help us right
out of the gate.
Also, remind us of a few of the tools
we can use regarding tagging and filtering,
and also touch on the concept of split horizon, which
in some situations can bite us as well. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
in some situations can bite us as well.
So let's take a look at the topology,
and let's review those concepts together.
So let's imagine we have routing domain A right here,
and routing domain, including this network, B. And then--
or let me include him here also.
And then routing domain C right here.
One of the key things when [CHUCKLES]
first of all, designing a network,
is, you'd never want to design a network intentionally this way,
where you'd have to do redistribution. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
where you'd have to do redistribution.
It'd be much easier and simpler to run one routing domain.
Let's start there.
But as far as redistribution goes,
if we have routes like the 12 network in routing domain A,
and we're redistributing that into routing domain B,
and then B is redistributing into routing
domain C, what we'd want to do is make sure that we're not
redistributing that route, the 12 route,
back into routing domain A.
Or a network in routing domain B to get put into C,
we want to make sure that it doesn't get redistributed back
into B. So we have AD that we can play with. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
into B. So we have AD that we can play with.
Like in a GRP, the administrative distance
for internal is a better AD than it is for externals.
It's 90 and 170.
But we can also manipulate the administrative distance
for a given routing protocol on a specific router,
and we say, hey, dear Mr. R6, you're
going to believe that the administrative distance
for OSPF is 89, and make it better or more believable.
So that's a fairly graceful solution.
But we could also-- if we needed to, we could also do tagging,
and you could do tagging on the redistribution WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
and you could do tagging on the redistribution
with or without a route map.
And so you could tag all the routes that were redistributed,
and then identify those with another route map
before you redistribute it again.
So that way, we can control, on redistribution
for the match, what can be redistributed.
We can look for or deny a certain tag,
and that can also control whether or not
a route will be redistributed.
Along with that route map, of course,
we can set the attributes appropriate for that
routing protocol as well.
So tagging is certainly an option that we can use,
or bringing a route in, tagging it,
and we can look for that tag later, and give WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
and we can look for that tag later, and give
that specific route either another redistribution
or no redistribution, based on the tags that it has.
That does take a little bit more planning, [CHUCKLES]
but it is certainly doable, as far as routes
that are going to be redistributed in,
and then also as we redistribute them, what
tag they are going to be given.
Also, a really good key is to focus on one network at a time.
So if we have the 12 network, for example, up here, or the 78
network, it's important to follow that network's progress
through the network.
And so as you do your redistribution,
and if you identify, whoa, that route 78 is not WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
and if you identify, whoa, that route 78 is not
showing up here at all, then we can identify why.
OK, R4 is doing redistribution from B
to A. That route got into B. Why isn't it picking up?
And each routing protocol behaves a little bit
differently based on the types of route,
and did that route get into the routing table.
And that's why some hands-on practice
in doing the redistribution, looking at the results,
and then having tools like offset lists, and distribute
lists, and route maps, can be very helpful in controlling WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
lists, and route maps, can be very helpful in controlling
the traffic.
Because at the end of the day, we want optimal paths
through the network, and if the routing protocol or a bunch
of routing protocols that are glued together
with redistribution is not causing that to happen,
that's when you start going in and adding
additional techniques, and go ahead
and make certain routes appear better than other routes,
or doing additional filtering so that optimal paths are
taken in the network.
And if we simply need to make a route look worse,
we could use a condition called route poisoning.
And poisoning, from a general concept,
is making it look worse. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
is making it look worse.
So we did that with offset lists,
and with the offset list on R8, I
believe we attached it to serial 3/2,
so that any routes that were being learned on that interface
would automatically have an additional offset or made
to look worse, and that is a form of poisoning.
There is also situations, like in DMVPN, if we are
using Dynamic Multipoint VPN--
which we have another set of videos
here in this course on-- but with DMVPN, we have a hub
and we have spokes.
Here's spoke 1 and spoke 2. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Here's spoke 1 and spoke 2.
And because we're coming into a single multipoint interface,
split horizon, which is built in to the route-- like RIP has it,
and GRP has it on by default. If router 1 advertises the route
and it comes in on this interface,
split horizon says, "Oh, I'm not going
to advertise the route out on the same interface I learned it
on."
So it doesn't send it down here.
However, in DMVPN, we need to turn off that feature.
So that's in the DMVPN videos, where we go to that interface,
and tell it not to use split horizon,
because we do want a route that's WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
because we do want a route that's
advertised by spoke 1, that goes up to the hub,
to be advertised back out that same logical multipoint tunnel
interface, and be sent down to the [CHUCKLES] site 2,
right there, so it can actually learn the route.
So it's important to realize some of those behaviors
and the defaults for the various routing protocols
so that when we're implementing them,
and our routes don't show up like we expect them to,
we can go ahead and correct it by tweaking those defaults,
in this case, by tweaking the split horizon
rule on the multipoint interface at the hub.
And well, another one to think about, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And well, another one to think about,
one other trick that we can do with administrative distances,
if we have some routes up here in this routing domain A,
and we've got some from EIGRP which are external,
and we have others that are EIGRP internal, by default,
the administrative distance is going
to be 170 for the external and 90 for the internal.
And if we want to change those, we can.
[CHUCKLES] We can manipulate the administrative distance
for all external EIGRP-learned routes,
or for a specific route, or we can WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
or for a specific route, or we can
change the administrative distance for a specific route
learned from a specific neighbor.
So we have an administrative distance that we can tweak.
This is an EIGRP example.
But that is yet another way to choose which route would
be used when there's multiple routes presented, simply
by going in and specifying, hey, this administrative distance
is going to be 70 or 60.
And that will make that route show up in the routing table,
and then start to be used if we need it to be used,
for optional padding in our network.
So in this set of videos, we've had the opportunity
to add some additional tools to our tool belt WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
to add some additional tools to our tool belt
as far as manipulating and controlling routing behavior
on a Cisco device.
So I'm glad you joined me.
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.