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Hello, and welcome to Explaining Route Filtering.
In this skill, we're going to be jumping in and looking
at how to filter routes out of our routing table,
particularly with OSPF and ISIS.
Now, we spend most of our time in routing,
trying to get routes into our routing table.
So to a certain extent, this is a little bit
intuitive to what we usually do with routing.
However, there are several reasons
we may want to filter out routes that we
are learning through our dynamic routing protocols.
One of those reasons could possibly be for security.
Now, that would most likely be a situation involving BGP.
Hopefully we're not going to be getting routes
from our IGP that's causing us a security issue.
But it is still a possibility.
So we should be aware of how to filter that.
Oftentimes, the more common reason
is to do this to influence path selection.
Now, we can usually influence path selection simply
by changing a metric, or something along those lines.
But maybe this is between something
like business partners, and maybe we
don't want to get to certain subnets,
or we don't want certain of their subnets
to be able to get to us through a certain path.
So if we want a redundant path, of course we would use metrics.
But if we don't want to be able to get
to a certain destination at all, or that destination
to be able to get to us at all, then route filtering
becomes the way to do that.
So in this skill, we're going to be focusing on IGP route
filtering, and we're going to be focusing on the building
blocks of that filtering.
What I mean by that is access lists and prefix lists.
Now, access lists, in all honesty,
aren't used that often.
They do have some drawbacks when it
comes to doing route filtering.
They're not necessarily the best tool.
However, they can be used, and they
do have one little corner case where
they sort of have an advantage.
And we'll talk about that.
Now, prefix lists are created to make up those shortcomings
that we have in ACL.
So our main focus will be on prefix lists.
And as we get into more advanced filtering that we
will do in future skills, these are
going to be the building blocks that we're
going to be using in other route filtering options,
such as routing policy language and route maps that we
have in IOS XE and IOS XR.
So this is a good place to start.
Even though we may not use access lists or prefix
lists by themselves, like we'll be doing in this skill,
we'll still use them in conjunction
with some of those more advanced features.
So either way, we need to know these building blocks ,
even before we move into the more advanced filtering.
So that said, we have a lot of cool stuff to cover.
We're of course, going to look at how these things work,
jump into the command line and see some examples.
So we're going to have a lot of fun.
Join me in the next video, and I'll see you there.