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Next up, let's talk a little bit here about AS path filtering.

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Again, this is going to be specific to BGP because it is looking at that AS

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path, which

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is one of our attributes in BGP.

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This is simply a list of all the AS numbers that a particular update has passed

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through.

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Just like any other access list, we're going to be doing a permitting or

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denying of the

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routes based on the contents of the AS path.
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routes based on the contents of the AS path.

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But it's a permit or deny condition, and the way we're going to match on the AS

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path

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is using something called a regular expression or a red jacks.

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Now regular expressions use a bunch of special characters to do certain types

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of matching,

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and I certainly don't think that you need to have all of these memorized, but

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you should

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at least be familiar with some of them.

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First off, we have an underscore, which will match any white space characters,
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First off, we have an underscore, which will match any white space characters,

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such as

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a space, a tab, etc.

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In the case of the AS path, it's really only going to be either the space or

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the beginning

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or the ending of a string.

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We have the carrot symbol.

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This is going to anchor to the beginning of the line.

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We have the dollar sign.

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This is going to anchor to the end of the line.

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We have a dot.

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This is going to match any single character, but not a new line and not a blank

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space.

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So it basically indicates the requirement of a character.
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So it basically indicates the requirement of a character.

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So just as a quick example, if I were to say carrot three dot two dollar, this

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would match

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on three two two or three seven two or three nine two, because that dot

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indicates any

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single character, but what it will not match on is three space two.

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It will not match on that because this is not a match for the dot.
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It will not match on that because this is not a match for the dot.

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So that's what we mean by it has to be a single character.

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And of course, the case of a new line isn't really going to exist in an AS path

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, but it

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can't be a space.

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Next we have some wild cards.

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You have an asterisk, which will match zero or more of the previous character.

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We have a plus, which will match one or more of the previous character and a

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question mark,

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which will match zero or more of the previous character.
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which will match zero or more of the previous character.

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You can also do ranges.

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So in this case, they're using letters in this example, ABC or like a through G

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, but it's

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really any character out of that specified range.

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Once again, in a S paths, these will be numbers, but nonetheless, you can do a

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range or a grouping.

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You can use the pipe for an or operation.

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This will combine several different options.

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There's parentheses that we can use to group characters to form sub expressions
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There's parentheses that we can use to group characters to form sub expressions

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and or

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actionable groups.

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And you can use a backspace to escape special characters, making them operate

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opposite of

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how they would in a regular expression.

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So for example, let's say I literally wanted to match on the word, hi question

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mark.

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How could I match on that with a regular expression?
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Because I can't match on the question mark, because the question mark matches

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zero or

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more occurrences of the previous character.

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So the default behavior in a regular expression, this would be saying, well,

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maybe the I is

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there, maybe it's not.

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So in other words, as it is right now, it would match on H I or just H. That's

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zero

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or more occurrences of the I.

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But that's not what you were trying to match on.
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But that's not what you were trying to match on.

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You literally want to match on H I question mark.

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To do this, you would do that.

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You're escaping the special meaning of the question mark.

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And instead of say, no, literally a question mark.

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That's what the backslash does.

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So let's jump in and take a look at a few examples.

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So what I've put together here are some examples of very commonly used AS path
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So what I've put together here are some examples of very commonly used AS path

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filters.

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So the first one here is underscore $500.

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Well, remember the dollar anchors it to the end of the line.

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Then there's a 500 clearly and an underscore meaning there has to be a space.

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So what we're trying to say there is this can't match on 3500 because there's

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no opportunity

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for the three.

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There has to be a space in it.

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Now it could match on three space 500.
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Now it could match on three space 500.

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That would be okay because there's a space and there's nothing to denote where

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the line

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starts.

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So there could be as much stuff in front of this as we want as long as it ends

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in a 500.

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The way we generally express this in terms of AS path filtering is to permit

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any update

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that originated in a 500.

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The reason it would be originated is it would be at the end of the AS path
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The reason it would be originated is it would be at the end of the AS path

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string.

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Similarly, yet the opposite would be our next example because now what we're

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doing is we're

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anchoring to the beginning of the string then saying it has to be 3221 then

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another underscore.

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So for example, this could not match 32227.

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That would be a no but a yes would be 32117.
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That would be a no but a yes would be 32117.

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Once again, as long as there's a space and this one has to be at the beginning

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of the

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string.

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The way of wording this permit any paths that came from neighboring a yes, 32

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221.

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So in other words, anything can be behind it.

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It could be halfway across the internet, but it has to be coming from my

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directly connected

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neighbor, 32221.

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And I just noticed I missed a two in here.
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And I just noticed I missed a two in here.

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So there should be another two in there.

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So 32221 and then the space would be allowed.

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Another example that we use quite commonly is an underscore then an AS number

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in this

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example, I just happened to use 750 and then another underscore.

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So understand this could be at the beginning of the string at the end of the

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string anywhere

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in the middle.
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in the middle.

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But it does have to be 750 by itself.

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Again, there could be something in front of it.

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375921 83 all fine.

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As long as 750 is by itself in there somewhere what I mean by itself, it can't

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be 7501.

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That's not going to match because there has to be a space before the seven and

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a space

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after the zero.

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And the way we say this in a path filtering land, we would say we're permitting
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And the way we say this in a path filtering land, we would say we're permitting

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any path

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that has passed through a seven 50.

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Two more very common ones, although the first one's a little bit complicated.

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We have to probably talk about this one just a little bit.

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First, yes, we have our beginning and our end of string.

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This is important because we're sort of using these like book ends.

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We're saying there can't be anything in front of this.
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We're saying there can't be anything in front of this.

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There can't be anything after this.

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So we're containing our as path.

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Next, we have a plus sign.

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Now, I remember that the plus sign indicates one or more instance of the

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character in front

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of it.

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But the character in front of it is a range.

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It's a grouping of zero through nine using parentheses.

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Let me put this in terms that might be easier to understand.

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This will permit three seven nine one.
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This will permit three seven nine one.

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This will permit eight two three.

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This will permit seven nine eight three two seven one nine three eight four.

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You're never going to see that in an as path.

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I'm just saying.

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This will match on any single as number.

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What this will not match on three seven nine one eight two three in one string.
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What this will not match on three seven nine one eight two three in one string.

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When I wrote them up above, those were three separate examples in one string.

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You can't have these together because notice what's not allowed in our range,

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our ranges

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zero through nine and we have to have at least one or more zero through nine's.

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However, we can't have anything in front of it because we have a beginning of

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string.

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We can't have anything after it because we have an end of string and there's
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We can't have anything after it because we have an end of string and there's

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nothing

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in here that allows for a space.

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So the way we refer to this in a as path land permit any single as number.

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This one would be very, very common with service providers when connecting to

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customers.

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They don't want to have to write a separate as path for every single one of

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their 28 million

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customers, but they also don't want their customers to become a transit area.

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So this way we can say you can send me any as you want, but only one.
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So this way we can say you can send me any as you want, but only one.

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The presumption, of course, is they would use their own.

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If they didn't, well, then we probably wouldn't have a neighborhood with them,

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but this is

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a very common example.

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Now another very common example and this one's far easier is permit caret

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dollar.

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Now, as you can plainly see, that's an empty string.

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Essentially what this means is none.
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Essentially what this means is none.

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We're not allowed to have anything in our as path to match this.

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And at first that might seem like a silly thing.

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However, incredibly common for a customer to use this outbound towards a

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service provider,

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because the only updates that you would be trying to send to your service

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provider should

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have nothing in the as path.

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Now ultimately, it's going to have your as in the as path when we send it to
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Now ultimately, it's going to have your as in the as path when we send it to

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the service

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provider, but understand that the as path that we prepend when we send updates

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to our

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neighbors happens after filtering.

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So at the filtering part, when we're choosing what updates to send to our

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neighbors, your

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own as will not be counted.

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Therefore, it would still be an empty string if these are routes that you are
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Therefore, it would still be an empty string if these are routes that you are

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originating.

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That way you can't accidentally learn a route from somebody else and then pass

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it through

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and try to send it back up to your service provider.

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Just to make this a little clearer, you don't want to learn the path to get to

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Cisco.com

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from Verizon and then turn around and advertise that to AT&T and have AT&T

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decide that you

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look like a pretty good way to get to Cisco.

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And all of a sudden, every customer of AT&T going to Cisco.com starts coming
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And all of a sudden, every customer of AT&T going to Cisco.com starts coming

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through

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your network.

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That's called becoming a transit as and that is the last thing that you as a

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customer would

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want to happen.

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By the way, the service providers don't want it either.

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That's why they probably implemented this to prevent you from accidentally

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doing that.

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So this line is the service provider protecting you from becoming a transit as
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So this line is the service provider protecting you from becoming a transit as

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this line is

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generally used by the customer to prevent themselves from becoming a transit as

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.

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So again, when we start running protocols like BGP, we have to be very careful

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with

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our filtering and we can't always do it based on prefix because we may not know

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what the

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prefix is.

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You're not going to sit there and try and deny every single internet route with

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a prefix

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list.

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At that point, AS path filters become a better choice.
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At that point, AS path filters become a better choice.

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So in this video, we spent some time looking at some of the caveats and

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attributes of prefix

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lists.

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And then we also talked about AS path filters, two different very powerful

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methods that we

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have for filtering updates, prefix lists really with any protocol, but as path

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filters, particularly

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with BGP.

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Now as soon as we have more than one protocol in the mix, we talked about OSPF
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Now as soon as we have more than one protocol in the mix, we talked about OSPF

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in the previous

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scale.

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We're talking about BGP in this scale.

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Anytime there's more than one protocol involved, we're almost always with, of

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course, a few

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exceptions going to have to do redistribution.

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So let's talk about some details and caveats to watch out for in redistribution

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in the

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next video.

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