From: Jason Sinclair (sinclairj@xxxxxxxxxxxxxxx)
Date: Mon Aug 19 2002 - 02:45:46 GMT-3
There is another way to represent 4[0-9]*$ - anyone care to take a guess?
Jason Sinclair CCIE #9100
Manager, Network Control Centre
POWERTEL
55 Clarence Street,
SYDNEY NSW 2000
AUSTRALIA
office: + 61 2 8264 3820
mobile: + 61 416 105 858
email: sinclairj@powertel.com.au
-----Original Message-----
From: ccie candidate [mailto:ccie1@lycos.com]
Sent: Monday, 19 August 2002 14:32
To: 'Omer Ansari'; Michael Snyder
Cc: ccielab@groupstudy.com
Subject: RE: Filtering BGP updates using ip as-path access-lists
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@ansari.com] >Sent: Sunday, August 18, 2002 12:08 PM >To: Michael Snyder >Cc: steven.j.nelson@bt.com; ccielab@groupstudy.com >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@bt.com [mailto:steven.j.nelson@bt.com] >> Sent: Thursday, August 15, 2002 10:14 AM >> To: msnyder@ldd.net >> 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@ldd.net] >> 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]*$. If they are the same, why not use the shorter one? >> >> Please Advise >> >> >> -----Original Message----- >> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf >Of >> Chaim Gev >> Sent: Thursday, August 15, 2002 8:13 AM >> To: msnyder@ldd.net; raj.bahad@totalise.co.uk >> Cc: ccielab@groupstudy.com >> Subject: RE: Filtering BGP updates using ip as-path access-lists >> >> The "*" means "match 0 or more sequences of the pattern" which is null >> in >> your suggestion. >> ^_4_[0-9]*$ is more accurate if you are asked to filter exactly AS4 >(and >> not >> AS14). >> >> >> >> >> >From: "Michael Snyder" <msnyder@ldd.net> >> >Reply-To: "Michael Snyder" <msnyder@ldd.net> >> >To: "'Raj'" <raj.bahad@totalise.co.uk> >> >CC: <ccielab@groupstudy.com> >> >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@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf >Of >> >Raj >> >Sent: Thursday, August 15, 2002 7:02 AM >> >To: ccielab@groupstudy.com >> >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.
This archive was generated by hypermail 2.1.4 : Sat Sep 07 2002 - 19:48:28 GMT-3