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[MUSIC PLAYING]

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Hello, and welcome to
Explaining Route Filtering.

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In this skill, we're going
to be jumping in and looking

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at how to filter routes
out of our routing table,

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particularly with OSPF and ISIS.

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Now, we spend most of
our time in routing,

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trying to get routes
into our routing table.

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So to a certain extent,
this is a little bit

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intuitive to what we
usually do with routing.

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However, there are
several reasons

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we may want to filter
out routes that we

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are learning through our
dynamic routing protocols.

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One of those reasons could
possibly be for security.

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Now, that would most likely
be a situation involving BGP.

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Hopefully we're not going
to be getting routes

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from our IGP that's causing
us a security issue.

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But it is still a possibility.

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So we should be aware
of how to filter that.

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Oftentimes, the
more common reason

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is to do this to
influence path selection.

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Now, we can usually influence
path selection simply

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by changing a metric, or
something along those lines.

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But maybe this is
between something

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like business
partners, and maybe we

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don't want to get
to certain subnets,

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or we don't want
certain of their subnets

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to be able to get to us
through a certain path.

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So if we want a redundant path,
of course we would use metrics.

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But if we don't want
to be able to get

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to a certain destination
at all, or that destination

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to be able to get to us at
all, then route filtering

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becomes the way to do that.

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So in this skill, we're going
to be focusing on IGP route

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filtering, and we're going to
be focusing on the building

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blocks of that filtering.

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What I mean by that is access
lists and prefix lists.

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Now, access lists,
in all honesty,

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aren't used that often.

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They do have some
drawbacks when it

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comes to doing route filtering.

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They're not necessarily
the best tool.

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However, they can
be used, and they

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do have one little
corner case where

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they sort of have an advantage.

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And we'll talk about that.

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Now, prefix lists are created
to make up those shortcomings

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that we have in ACL.

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So our main focus will
be on prefix lists.

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And as we get into more
advanced filtering that we

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will do in future
skills, these are

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going to be the building
blocks that we're

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going to be using in other
route filtering options,

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such as routing policy
language and route maps that we

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have in IOS XE and IOS XR.

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So this is a good
place to start.

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Even though we may not
use access lists or prefix

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lists by themselves, like
we'll be doing in this skill,

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we'll still use
them in conjunction

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with some of those
more advanced features.

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So either way, we need to
know these building blocks ,

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even before we move into
the more advanced filtering.

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So that said, we have a
lot of cool stuff to cover.

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We're of course, going to
look at how these things work,

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jump into the command line
and see some examples.

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So we're going to
have a lot of fun.

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Join me in the next video,
and I'll see you there.

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