From: Usankin, Andrew (Andrew.Usankin@twtelecom.com)
Date: Thu Nov 29 2007 - 19:50:09 ART
Very good points indeed. Thanks general :)
Andrew
________________________________
From: Narbik Kocharians [mailto:narbikk@gmail.com]
Sent: Thursday, November 29, 2007 3:20 PM
To: Usankin, Andrew
Cc: Tandou Mohamed; Gupta, Gopal (NWCC); smorris@ipexpert.com;
ccielab@groupstudy.com
Subject: Re: Regular Expression
By the way the _ does not match everything, think of being in a
confederation and what if you wanted to match on all the routes from
your neighbor in 65501 advertised to you, you would need to do the
following:
^\(65501_
Note the \ remove special meaning of character, remember in a
confederation the as-path shows up in parenthises.
As far as prepending, there are two scenarios, if you wanted to allow or
deny your existing and future directly connected neighbors to prepend,
then you need to do the following:
^([0-9]+)(_\1)*$
But if you wanted to allow/deny your existing neighbor in AS 200 to
perform prepending, then you would need to do the following:
^200(_200)*$
On 11/29/07, Usankin, Andrew <Andrew.Usankin@twtelecom.com> wrote:
Dunno how to put it right, but you missing the point here.
Nobody thinks
about confederation routes neither about aggregates with as-set
applied.
For example aggregate route may look like:
Network Next Hop Metric LocPrf Weight Path
*> 172.16.0.0/22 192.168.1.1 0 0 1
{65530,2}
i
And "}" is actually a character and takes place in expression.
Confederates have "(" and ")". So what I'm saying is that "_"
sign is in
fact matches all those characters. Here is a quote from "BGP4
handbook":
_ (underscore) Matches a comma (,), left brace ({), right brace
(}),
left parenthesis ((), right parenthesis ()), the beginning of
the
string, the end of the string, or a space.
(Pay attention here to "beginning of the string and the end of
the
string")
So if you say "_200$" it will actual match as those routes
originated in
AS 200 as well as those passing through this AS. I like to
always make
it as more specific as it possible, so if I want to filter
"routes
originated in AS 200" I make expression as "^200$" not "_200$".
Andrew
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On
Behalf Of
Tandou Mohamed
Sent: Wednesday, November 28, 2007 8:01 AM
To: Gupta, Gopal (NWCC); smorris@ipexpert.com;
ccielab@groupstudy.com
Subject: RE: Regular Expression
Thanks a lot everyone to clear this out for me.
"Gupta, Gopal (NWCC)" <gopal.gupta@hp.com> wrote:
Hi Tandou,
B means Routes originated in 200,
just think it logically and go behind the syntax, go behind the
original
string and then apply the syntax logically.
We have As-path like this (100 300 200) (Original string)
B (_200$) now look at the string we want to match out of the
original
string what original string says is the route has traversed 100
300 and
has originated in 200 coz 200 is the first AS in the original
string.
Now "_" means any AS number in the start of the string and $
means the
end of the string.
In case of B we have $ but with 200 and in the start we have
mentioned
any AS number using "_"
Hope I have tried to make you understand........
HTH
Gops
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On
Behalf Of
Tandou Mohamed
Sent: Wednesday, November 28, 2007 19:41
To: smorris@ipexpert.com; ccielab@groupstudy.com
Subject: RE: Regular Expression
Importance: Low
Now i understand D. what about B? the origin of the routes will
be AS
200 only or can be any adjacent neighbor to AS 200 ?
Thanks,
Mohamed
Scott Morris wrote:
D would be routes LEARNED FROM AS 690 (your peer)
HTH,
Scott Morris, CCIE4 (R&S/ISP-Dial/Security/Service Provider)
#4713,
JNCIE-M #153, JNCIS-ER, CISSP, et al.
CCSI/JNCI-M/JNCI-ER
VP - Technical Training - IPexpert, Inc.
IPexpert Sr. Technical Instructor
A Cisco Learning Partner - We Accept Learning Credits!
Telephone: +1.810.326.1444
Fax: +1.810.454.0130
http://www.ipexpert.com
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On
Behalf Of
Tandou Mohamed
Sent: Wednesday, November 28, 2007 8:34 AM
To: ccielab@groupstudy.com
Subject: Regular Expression
Hello GS,
can someone check this out if i am correct about these
definition also
any comment on B,C,D and E those really confuse me sometimes.
Thanks,
Mohamed
A) ip as-path access-list 1 permit _109_ => routes containing or
transiting AS 109
B) ip as-path access-list 2 permit _200$ => routes originated in
AS 200
C) ip as-path access-list 2 permit ^100$ => routes originated by
a
neighbor in AS 100
D) ip as-path access-list 3 permit ^690_ => routes originating
from AS
690
E) ip as-path access-list 3 permit ^$ => routes originating from
this AS
(my AS)
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