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So let's begin by
jumping in and looking

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at the concept behind the
BGP path selection process.

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We'll start by looking at
what it is, why we need it,

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what it's for.

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But then we'll also take
a little bit of a look

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at how it works,
the logic behind it,

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and how it actually
chooses that best path.

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So let's go ahead and
jump in and take a look

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at some of those details.

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So at first, we have what might
seem to be an obvious statement

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here, that BGP will
quite often learn, well,

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quite a few different paths to
get to any given destination.

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But we have to have
a premise as far as

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why we need to
choose the best path.

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If there is only
one path, that path

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will, of course, be the best.

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But if not, the path
selection criteria

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is used to choose the best path.

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In the case of
BGP, this is going

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to come down to one best path.

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So how do we choose
that best path, then?

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Well, we're going to be
using the BGP attributes

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as our primary factor.

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So of course, our goal here is
to look at these attributes,

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discuss what order
they're considered in.

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And then we'll be using that to
design our own path selection

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criteria.

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When we change attributes
to influence the best

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path, what we're doing is, we're
implementing a routing policy.

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One of BGP's biggest strengths
is our ability to control this.

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Please note, I'm very carefully
calling this routing policy,

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not policy routing or
policy based routing.

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Because PBR is actually
something different.

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Now I have heard people refer
to BGP as policy routing.

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There's not necessarily
anything wrong with that.

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I just didn't want to
have any confusion here.

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So we're going to
be very careful

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and say that we're
implementing a routing policy.

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Now interestingly
enough, unlike a lot

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of other routing protocols, one
of the criteria or attributes

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is actually not bandwidth.

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BGP does not look at bandwidth
as part of its criteria.

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We could, of course, base
our policy on bandwidth

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if we choose to.

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However, these attributes
that we spoke about, well,

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one of them is not bandwidth.

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So that's why we say it's
more based on policy.

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Now the way this works
is, all of the paths--

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so we're talking
about all of these

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that we mentioned
up here at the top,

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that we're going to
learn multiple paths

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to a destination.

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Each of these are considered
against the selection criteria

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until, again, like we
mentioned before, one

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is considered the best.

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So essentially, we can Enter the
top of the BGP route selection

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criteria with, let's say
10 paths to a destination.

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And maybe at rule one,
this eliminates two.

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So we're down to eight paths.

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Maybe the next rule
doesn't eliminate anything.

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And as we go down, we
keep eliminating paths,

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until eventually,
we end up at one.

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Once we get to that
point, of course,

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we'll stop going down the list.

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But our path
selection criteria is

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acting almost as a filter here
to get us down to one route.

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And one last statement here
before we jump into the path

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selection criteria itself--

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there's sort of a prerequisite.

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Before it can even
be considered,

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it has to be a valid path.

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That means that the next
hop has to be reachable.

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There can be no AS loop.

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And the route must
be synchronized.

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As long as we meet
those criteria,

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we'll then Enter the
path selection process

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and start eliminating
down to one path.

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In this video, we just
jumped in and took a look

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at the overall concept
behind the BGP path selection

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process, how it goes down
the list of all of the paths

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that it's learned to a
particular destination,

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and how it chooses the
best path out of that list,

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based on BGP attributes.

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I hope this has been
informative for you,

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and I'd like to thank
you for viewing.

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