From: Michael Snyder (msnyder@xxxxxxx)
Date: Sun Aug 18 2002 - 21:43:47 GMT-3
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