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Regular Expression form www.ciscopress.com

While I was reading this link:

http://www.ciscopress.com/articles/article.asp?p=169556&rl=1

I found this :

((Example 4-2 shows the use of a regular expression to describe an AS_PATH pattern that matches all AS_PATHs that are originated (my confusion here) from the neighboring AS 100.

Example 4-2 Regular Expression Matches AS_PATH Patterns

ip as-path access-list permit ^(100_)+$ ))

is ^(100_)+$ same as ^100$ ?

1 REPLY

Re: Regular Expression form www.ciscopress.com

Hello,

the regular expression you give does not allow AS path prepending, only the AS path with exactly "100" will be accepted.

In the book they add a "+", which means the expression in brackets 100_ can occur one or more times. So f.e. the AS path "100 100" will also be allowed. AS path prepending is used very often in the internet to influence the path of return traffic.

So you are right and the book as well - both expressions will only accept networks originated in AS 100. Just the result is slightly different.

Hope this helps! PLease rate all posts.

Regards, Martin

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