From: Scott Morris (swm@emanon.com)
Date: Sat Mar 10 2007 - 01:37:09 ART
Looking at the list of attributes that BGP routes have, the first thing to
look at is what stuff actually is transitive to other AS's... Many of them
are not.
As you noted, AS-Path prepending is certainly a good way to go, but there's
a lot of things for path selection that occur in the list BEFORE that value
is.
http://www.cisco.com/univercd/cc/td/doc/product/software/ios124/124cg/hirp_c
/ch05/brbext.htm#wp1027188
You could also manipulate the MED value (metric), although you'll likely
find that most people ignore that value. There's something political (and
philosophical) about having an autonomous system and yet letting other
people dictate how YOU are going to route traffic!
You'll note though that this is lower on the list of path selection
criteria.
The overall list...
http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094431
.shtml
So take a different tact. There are some things that the typical path
selection process doesn't take into account. Namely the basics of routing.
Most specific path always wins.
So if you have a /24 and want to be sure that return traffic comes in from
one particular link/ISP, then advertise two /25 networks out one direction
and the /24 out another. As long as your ISP will accept that
advertisement, that's a sure way to make a unique route now and avoid any
other policy they have in place overriding your AS Path prepending.
HTH,
Scott Morris, CCIE4 (R&S/ISP-Dial/Security/Service Provider) #4713, JNCIE
#153, CISSP, et al.
CCSI/JNCI-M/JNCI-J
IPexpert VP - Curriculum Development
IPexpert Sr. Technical Instructor
smorris@ipexpert.com
http://www.ipexpert.com
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Chris Riling
Sent: Friday, March 09, 2007 11:14 PM
To: Jeff Mullan
Cc: dennis lin; ccielab@groupstudy.com
Subject: Re: BGP Bestpath selection -- confused with attributes selection
:Weight and Local-Preference
With that being said, I also have a related question. Historically, If I
wanted other networks out there in the wild to prefer a certain path into my
AS, I would pad my advertisement all to hell from the less preferred routes,
so that those advertisements would have a larger, less desireable AS-path.
However, some providers (due to peering agreements, or whatever reason)
prefer paths through other networks, regardless of how much I pad my
advertisement to them... Is there a better way to attack the issue, or is
there nothing I can do about it?
Thanks,
Chris
On 3/9/07, Jeff Mullan <jmullan78@gmail.com> wrote:
>
> Dennis,
> If you have multiple exit points from your AS, and you are getting
> same inbound routes on these, you may want to use Local Pref. Incase
> you have multiple exit points out of your AS on the same router, you
> can use weight.
> Thanks,
> -JM
>
>
> On 3/9/07, dennis lin <74dennis@gmail.com> wrote:
> >
> > Hi Every friend in GS
> >
> > I am having a small question that I am confused.
> >
> > For the BGP bestpath selection, weight and local-preference are used
> > to affect how traffic leaves AS, but how can I choose one of them .
> > As I
> konw
> > ,
> > Weight is only local to the router, but what does it really mean? In
> what
> > scenario I have to use local-preference instead of weight to affect
> > traffic leaves AS ? Any examples? Appreciated with that.
> >
> > Dennis
> >
> > ____________________________________________________________________
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