From: Digital Yemeni (digital-yemeni@hotmail.com)
Date: Sat Mar 10 2007 - 03:08:57 ART
That's absolutely right Mr. Scott! In my compnay which is doing multihoming
BGP it and its ISPs all use the AS-PATH prepending! ;-)
Best Regards,
Digital
------------------------------------------------------------------------------------------------------------------
***********************************************************************************
*.* You'll NEVER succeed as a "CCIE" until you LOVE Cisco MORE than your
sleep! *.*
***********************************************************************************
I've not slept for the past 5 years and I'm expected to be busy for the next
57 years + The 5 CCIEs preparation adds on that a bit. Therefore, please be
concise on your email. Thank you!
>From: "Scott Morris" <swm@emanon.com>
>Reply-To: "Scott Morris" <swm@emanon.com>
>To: "'Chris Riling'" <criling@gmail.com>
>CC: "'Jeff Mullan'" <jmullan78@gmail.com>, "'dennis lin'"
><74dennis@gmail.com>, <ccielab@groupstudy.com>
>Subject: RE: BGP Bestpath selection -- confused with attributes selection
>:Weight and Local-Preference
>Date: Sat, 10 Mar 2007 00:58:45 -0500
>
>heheheh.... If that were true, we wouldn't have nearly as many routes in
>the table!
>
>According to my table:
>
>There are 44 networks at /25
>There are 58 networks at /26
>There are 69 networks at /27
>There are 109 networks at /28
>There are 456 networks at /29
>Fortunately, there are 0 networks at /30!
>
>Go figure... Other people may have different results depending on their
>upstream ISPs filtering (or lack thereof) but I see them with multiple
>AS-Paths, so it's not just customer routes.
>
>:)
>
>Scott
>
>smorris@Emanon-Edge> show route protocol bgp hidden | match /25 | count
>Count: 44 lines
>
>smorris@Emanon-Edge> show route protocol bgp hidden | match /26 | count
>Count: 58 lines
>
>smorris@Emanon-Edge> show route protocol bgp hidden | match /27 | count
>Count: 69 lines
>
>smorris@Emanon-Edge> show route protocol bgp hidden | match /28 | count
>Count: 109 lines
>
>smorris@Emanon-Edge> show route protocol bgp hidden | match /29 | count
>Count: 456 lines
>
>smorris@Emanon-Edge> show route protocol bgp hidden | match /30 | count
>Count: 0 lines
>
>smorris@Emanon-Edge>
>
> _____
>
>From: Chris Riling [mailto:criling@gmail.com]
>Sent: Friday, March 09, 2007 11:41 PM
>To: swm@emanon.com
>Cc: Jeff Mullan; dennis lin; ccielab@groupstudy.com
>Subject: Re: BGP Bestpath selection -- confused with attributes selection
>:Weight and Local-Preference
>
>
>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>
>[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 <mailto: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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> >
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>
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