From: Chris Riling (criling@gmail.com)
Date: Sat Mar 10 2007 - 01:41:17 ART
Thanks for the info... The longest prefix match method always seems to be a
good bet in an IGP, but it was my understanding that most people filtered
anything less than a /24 in the global routing table...
On 3/9/07, Scott Morris <swm@emanon.com> wrote:
>
> 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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