Forum Replies

  1. Hey Rene,

    I have found one of Swisscom BGP router from that expressions:

    AS path access list 1
        permit ^$
        permit 650[0-9][0-9][)]$

    permit ^$ : I think this one is written for locally originating routes

    I can’t get a meaning the last one’s " [)] "section ? You have an idea?
    By the way ; these ip as-path access lists , bgp filtering commands just filtering the AS’s that advertising from another BGP routers right?


  2. Hi Rene,
    Just looked at BGP looking glass server. How can one practice regular expression on this site? Thanks!

  3. Hi A,

    Most of the looking glass servers support regular expressions so that would be the best option to get some practice. If you want to practice this “locally” then I would configure some BGP routers and use route-maps for things like AS path prepending, this can be used as a nice simulation of the Internet.


  4. Is it abnormal not really to be real fluent with these?

    Don’t get me wrong I see the examples and as I went through lessons later on I see some of the examples come up that can be useful.

    I am good with ones like:

    ^$ which can be useful for applying to everything (you use this one when dealing with Transit issues when multi-homing and need to filter) or ^63100$ apply to an AS specifically.

    I also get and like the ones like


    which would give every AS that

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  5. Hi Brian,

    It’s normal I think…as network engineers, we don’t use regular expressions much. If you are into programming, you’ll use them quite a lot to match strings/numbers/etc… If you want to practice these, try a site like:


    Paste in the output of a BGP table there and test it…it’s easier and quicker than testing regex on your router.

    I wouldn’t worry about this too much though…when you need to use them for BGP, you can always look them up…test it, then apply it to your router. No need to memorize all the different options. When you need it

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