GroupStudy.com GroupStudy.com - A virtual community of network engineers
 Home  BookStore  StudyNotes  Links  Archives  StudyRooms  HelpWanted  Discounts  Login
RE: Filtering BGP updates using ip as-path access-lists posted 08/19/2002
[Chronological Index] [Thread Index] [Top] [Date Prev][Date Next] [Thread Prev][Thread Next]


guys ;

i want to participate in this 

regular experssions are actually work with the Finite state machine concept ..it means that when you want to have a match ..the conditios must come in the provided sequence of the regular expression 


to explain this lets take Michael question ..is 4 is the same ^4$ ..the answer is absolutely NOT.

the first one (^4$) tells the router that the strings that will match me (the expression ) should be like that
"when they start with begining of the string ,
it should follow that the number 4 , then it should follow with end of the line "

now only AS path of 4 will match ,no other strings can fit or match here 

i think that one is clear .

now about the second regular expression ..it actually tells the router nothing more than 
" if the number 4 appears in the string , this is a match !! "

so strings like 49 , 354 , 12 4 5 
will all match 


this will explain also why expression like _2_ means to match AS paths that contains 2 
the expression says 
" when space(or begining of line) followed by 2 followed by space (or end of line )appears ..this is a match "

so strings like 33 2 44 , 2 , 13 2 will all match .


now to some of the expressions mentioned in the previous posts 

[0-9]* will match the null string 
(* means zero or more of the occurence)

so the regular expression 
^4 [0-9]*$ will match AS 4 and any other AS attached to it ...the only thing which i have to test is the space if it match the space of not ..i will come back with that soon ..



i hope this  help 





 
--

On Sun, 18 Aug 2002 19:43:47  
 Michael Snyder wrote:
>I wouldn't say that the expression nullifies routes from as4.  Routes
>originating from AS4 would only have the string ^4$.  The expression in
>question ^4 [0-9]* [0-9]*$ requires that only paths that start with as4
>and have two more wildcard paths will match.
>
>I don't believe [0-9]* will match a null string. Therefore there's a
>problem with my string I didn't see before, the problem is that isn't
>inclusive of paths that are only one or two AS's long.
>
>If my requirement was to get the directly connected neighbors of AS4,
>and those directly connected to the neighbors, then I think I would need
>three expressions.
>
>^4$			; Need a path to AS4
>^4 [0-9]*$		; Need AS4's neighbors		
>^4 [0-9]* [0-9]*$ ; Need AS4's neighbor's neighbors
> 
>About when to us a space char (ascii 32) vs using an underscore _ (acsii
>95), I just use my own experience.  After reading the BGP tables for a
>few months during labs, I know what string I want to match.
>
>Your last question about confederations, they use braces.  Acsii 123,125
>
>Don't think I've seen a comma in an AS path yet.  
>
>Your question, has brought another question to my mind.  Wonder if ^4$
>and 4 would do the same thing.  I'll lab it this week.
>
>
>-----Original Message-----
>From: Omer Ansari [mailto:omer@xxxxxxxxxx] 
>Sent: Sunday, August 18, 2002 12:08 PM
>To: Michael Snyder
>Cc: steven.j.nelson@xxxxxx; ccielab@xxxxxxxxxxxxxx
>Subject: RE: Filtering BGP updates using ip as-path access-lists
>
>Mike,
>the spaces in your regexp automatically nullify any routes originating
>from AS4 right?
>
>a little digression: anyone care to explain the difference between a
>comma
>(,) and a space ( ) in a regexp?
>
>are commas there for confederations?
>
>
>
>On Thu, 15 Aug 2002, Michael Snyder wrote:
>
>> _ (Matches a comma (,), left brace ({), right brace (}), the beginning
>> of the input string, the end of the input string, or a space.
>>
>> Steve I just wanted to match a space. Thats why I didn't use _ ,
>Cisco
>> shows examples of not using _ to match spaces.  In other words, a
>space
>> char will match a space character.
>>
>> I stand by my expression of ^4 [0-9]* [0-9]*$
>>
>>
>> I think another legal expression would be ^4 12 16 3$ notice that _
>> isn't needed.
>>
>> -----Original Message-----
>> From: steven.j.nelson@xxxxxx [mailto:steven.j.nelson@xxxxxx]
>> Sent: Thursday, August 15, 2002 10:14 AM
>> To: msnyder@xxxxxxx
>> Subject: RE: Filtering BGP updates using ip as-path access-lists
>>
>> Michael
>>
>> You have missed out the _
>>
>>
>> ^4_[0-9]*_[0-9]*$
>>
>> Steve
>>
>> -----Original Message-----
>> From: Michael Snyder [mailto:msnyder@xxxxxxx]
>> Sent: 15 August 2002 15:53
>> To: 'Chaim Gev'
>> Cc: ccielab
>> Subject: RE: Filtering BGP updates using ip as-path access-lists
>>
>>
>> ^4 [0-9]* [0-9]*$
>>
>> Would match any AS that was directly connected to an AS that was
>> directly connected to AS 4.  Is this correct?
>>
>> Also I think the expression you cited ^_4_[0-9]*$ is different than
>> ^4_[0-9]*$. If they are the same, why not use the shorter one?
>>
>> Please Advise
>>
>>
>> -----Original Message-----
>> From: nobody@xxxxxxxxxxxxxx [mailto:nobody@xxxxxxxxxxxxxx] On Behalf
>Of
>> Chaim Gev
>> Sent: Thursday, August 15, 2002 8:13 AM
>> To: msnyder@xxxxxxx; raj.bahad@xxxxxxxxxxxxxx
>> Cc: ccielab@xxxxxxxxxxxxxx
>> Subject: RE: Filtering BGP updates using ip as-path access-lists
>>
>> The "*" means "match 0 or more sequences of the pattern" which is null
>> in
>> your suggestion.
>> ^_4_[0-9]*$ is more accurate if you are asked to filter exactly AS4
>(and
>> not
>> AS14).
>>
>>
>>
>>
>> >From: "Michael Snyder" <msnyder@xxxxxxx>
>> >Reply-To: "Michael Snyder" <msnyder@xxxxxxx>
>> >To: "'Raj'" <raj.bahad@xxxxxxxxxxxxxx>
>> >CC: <ccielab@xxxxxxxxxxxxxx>
>> >Subject: RE: Filtering BGP updates using ip as-path access-lists
>> >Date: Thu, 15 Aug 2002 07:52:31 -0500
>> >
>> >I'm no expert on as-path access-lists.
>> >
>> >But, I believe the $ is the main difference.
>> >
>> >^4_  anything beginning with as path 4.
>> >
>> >^4_[0-9]*$ anything beginning with as path 4 and ENDS with one
>> >additional AS.
>> >
>> >Which brings up a question of mine,
>> >
>> >Why wouldn't ^4_*$ do the same thing?
>> >
>> >
>> >
>> >-----Original Message-----
>> >From: nobody@xxxxxxxxxxxxxx [mailto:nobody@xxxxxxxxxxxxxx] On Behalf
>Of
>> >Raj
>> >Sent: Thursday, August 15, 2002 7:02 AM
>> >To: ccielab@xxxxxxxxxxxxxx
>> >Subject: Filtering BGP updates using ip as-path access-lists
>> >
>> >Hi all,
>> >
>> >I've come across a scenario for which I require some clarification.
>> >
>> >I want to permit only networks originated from AS4, and AS's directly
>> >attached to AS4. I use the following config to meet the requirements,
>> >however I have come across documentation that has achieved the same
>> >results
>> >using a slightly different version. I've listed both of them below:
>> >
>> >ip as-path access-list 1 permit ^4_
>> >
>> >and the other being:
>> >
>> >ip as-path access-list 1 permit ^4_[0-9]*$
>> >
>> >Could someone please help clarify why one method would be preferred
>> over
>> >the
>> >other?
>> >
>> >Thanks,
>> >
>> >Raj.
>> >_________________________________________________________________
>> >Commercial lab list: http://www.groupstudy.com/list/commercial.html
>> >Please discuss commercial lab solutions on this list.
>> >_________________________________________________________________
>> >Commercial lab list: http://www.groupstudy.com/list/commercial.html
>> >Please discuss commercial lab solutions on this list.
>> _________________________________________________________________
>> Chat with friends online, try MSN Messenger: http://messenger.msn.com
>> _________________________________________________________________
>> Commercial lab list: http://www.groupstudy.com/list/commercial.html
>> Please discuss commercial lab solutions on this list.
>> _________________________________________________________________
>> Commercial lab list: http://www.groupstudy.com/list/commercial.html
>> Please discuss commercial lab solutions on this list.
>> _________________________________________________________________
>> Commercial lab list: http://www.groupstudy.com/list/commercial.html
>> Please discuss commercial lab solutions on this list.
>_________________________________________________________________
>Commercial lab list: http://www.groupstudy.com/list/commercial.html
>Please discuss commercial lab solutions on this list.
__________________________________________________________
Outgrown your current e-mail service? Get a 25MB Inbox, POP3 Access,
No Ads and No Taglines with LYCOS MAIL PLUS.
http://login.mail.lycos.com/brandPage.shtml?pageId=plus
_________________________________________________________________
Commercial lab list: http://www.groupstudy.com/list/commercial.html
Please discuss commercial lab solutions on this list.
__________________________________________________________________
To unsubscribe from the CCIELAB list, send a message to
majordomo@xxxxxxxxxxxxxx with the body containing:
unsubscribe ccielab