From: Rich Collins (nilsi2002@gmail.com)
Date: Wed Mar 25 2009 - 17:24:34 ART
Ruhann's regex is valid but I always find it easier to remember in this format:
^100_(100_)*$
On Wed, Mar 25, 2009 at 3:29 AM, Ruhann <groupstudy@ru.co.za> wrote:
> just a correction, :/
> ^(100+)(_\1)*$ would match as-100 and allow prepending
>
> BB3# sh ip bgp regex ^(100+)(_\1)*$
> BGP table version is 12, local router ID is 31.3.0.1
> Status codes: s suppressed, d damped, h history, * valid, > best, i -
> internal,
> r RIB-failure, S Stale
> Origin codes: i - IGP, e - EGP, ? - incomplete
> Network Next Hop Metric LocPrf Weight Path
> *> 174.1.0.0 100.12.1.5 0 0 100 100 100 100
> 100 100 100 100 ?
>
>
> On Wed, Mar 25, 2009 at 9:22 AM, Ruhann <groupstudy@ru.co.za> wrote:
>
>> The two filters do a similar thing but are different:
>>
>> ^100$ - Directly connected to AS 100 (begins and ends in AS 100) 1 ASN
>> hop.
>> _100& - Originated in AS 100, but could have passed through other AS's.
>>
>> You will use the first if you want to match only your directly connect AS'
>> routes, ie to match routes originated in a connected peering AS,
>> the only problem with this approach you are not allowing the peer AS to
>> use prepending of his own AS-number, as you only matching a AS-path length
>> with 1 hop being 100,
>> so in that regard the following would be the better filer: ^(100)+$
>> The second filter _100$, also list routes that AS-100 originated, but the
>> difference is AS-100 could be directly connected or it could be 10 AS hops
>> away, having passed through several AS's.
>> This would also allow AS-100 to use prepending. This is not a nice filter
>> since it does allow quite alot.
>>
>> Which to use would entirely depends on what you are trying to match of
>> filter.
>> imh more strict is always better, but that more real world than exam :)
>>
>> hth
>>
>>
>> On Sun, Mar 22, 2009 at 7:40 PM, Cisco Fanatic <ebay_products@hotmail.com>wrote:
>>
>>> Hi All,
>>>
>>> If we have to learn only those prefixes that Originate in some AS ASXX. We
>>> can
>>> use ^$ and _XX$. In some workbooks I have seen one or the other being
>>> used. I
>>> have tested both and both are working (the way it should). Which one is
>>> better
>>> over the other.
>>>
>>> For example,
>>> !
>>> router bgp XX
>>> neighbor x.x.x.x filter-list 10 in
>>> !
>>> ip as-path access-list 10 deny _ASXX$
>>> ip as-path access-list 10 permit .*
>>> !
>>> *******************************************
>>> Or/And,
>>>
>>> !
>>>
>>> router bgp XX
>>>
>>> neighbor x.x.x.x filter-list 10 in
>>>
>>> !
>>>
>>> ip as-path access-list 10 deny ^ASXX$
>>> !
>>> *******************************************
>>>
>>>
>>> Yuri
>>>
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>>
>>
>> --
>> <ruhann>
>>
>>
>
>
> --
> <ruhann>
>
>
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