Hi Elephant,
Thanks for you valuable time typing out this e-mail and explaining AS-PATH
expression to me. I fully understand what it means now. Thank you very much.
Regards,
Cheeyong
At 04:26 PM 5/21/01 +0200, ElephantChild wrote:
>On Mon, 21 May 2001, ElephantChild wrote:
>
> > On Mon, 21 May 2001, Tay Chee Yong wrote:
> >
> > > Hi all,
> > >
> > > I am suppose to accept the following routes from my peer AS123, which
> has
> > > peering with another AS (AS456). I was told to accept the following
> routes
> > > from them, but I don't really understand the regular expression. Can
> anyone
> > > please help to explain to me the following? And what does the "+" and
> the
> > > parenthese means in this AS-PATH.
> > >
> > > (_123)+(_456)+
> > >
> > > I am confused with the parenthese and the "+" sign. Please advise.
> Thank you.
> >
> > *browsebrowsebrowse*
> >
http://www.cisco.com/univercd/cc/td/doc/product/software/ios113ed/113ed_cr/
> > np1_r/1rbgp.htm#xtocid2382618
>
>(rest snipped)
>
>Sorry, folks, incomplete answer. I meant to add the URL for the actual
>syntax description, then decided as I read it that it was a bit obscure
>as an introduction to regular expressions. Anyway, here's my FMTYWTK
>answer:
>
>_ matches any character that separates individual ASNs in an AS path,
>including the start or end of the AS path. So _123 ensures that the 123
>in the regular expression won't match say, AS 4123. It could, however,
>match AS 1234 if used by itself, but the rest of the expression will
>take care of that.
>
>+ matches at least one occurence, and perhaps more than one, of the
>preceding character or sub-expression between (). So if your peer
>indulges in AS path prepending, (_123)+ will take care of that by
>matching 123 123 or 123 123 123 12345 as well as a single 123 or 1234.
>The rest of the regular expression, and specifically the _ that starts
>(_456)+, takes care of that, since it doesn't match the 4 in 1234 or
>12345, but only the separator after the 3 of the final 123.
>
>Now that I dissected your regular expression, I will say that IMHO, it's
>more complex than what you need, and may still not do what you want in
>some cases. Depending on what you want, you may use one of the
>following:
>
>- To match any AS path that contains ASNs 123 and 456 in that order, and
> may contain any other ASN either before or after, but not between, use
> _123_456_ (Note that this would still match 123 789 456 123 456, but
> that path has a loop in it, and if your peer will feed you that kind
> of paths, you have more serious trouble than just an unwanted 789.)
>
>- To match any AS path that starts with one or more occurences of ASN
> 123, followed with one (at least) occurence of 456, possibly followed
> by other ASNs, use ^(123_)+456_
>
>- To match any AS path that starts with one or more occurences of ASN
> 123, possibly followed by one or more occurences of ASN 456, but
> contains no other ASN, use ^(123_)+(456_)*$
>
>--
>"Someone approached me and asked me to teach a javascript course. I was
>about to decline, saying that my complete ignorance of the subject made
>me unsuitable, then I thought again, that maybe it doesn't, as driving
>people away from it is a desirable outcome." --Me
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=5360&t=5234
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