From: Howard C. Berkowitz (hcb@xxxxxxxxxxxx)
Date: Wed Apr 24 2002 - 08:36:43 GMT-3
At 11:33 AM +0200 4/24/02, GOLBERY Irhne wrote:
>Thanks Yakout,
>
>Your are explained me what I am able to read and understand in the trace I
>sent to you.
>But my question is very precise.
>I am looking for a documentation where it is explained in details
>Irhne.
Is there a problem you are trying to solve?
What additional detail are you looking for?
1. How it is formatted in the BGP Update message?
See RFC1771, or, probably more accurately, the latest revision draft,
http://www.ietf.org/internet-drafts/draft-ietf-idr-bgp4-17.txt It is
a 16 bit integer attribute of AS-wide scope, which means it will only
be seen in iBGP (although there are some implementations that send it
externally, this is nonstandard. MED is meant for that purpose)>
2. How it is used in the BGP route selection algorithm?
Look in the archives or my WAN Survival Guide book; I've written it
up several times. You can also go to www.nanog.org, and search for
the BGP tutorials by Avi Friedman, Paul Ferguson, and myself?
3. What is its purpose, in the sense of how it is used in real life?
You'll find some of that in the tutorials above. Halabi touches on
it lightly. You can read about preference in the various Routing
Policy Specification Language documents and then see the applicability
of a preference of AS-wide scope. I discuss some detailed applications
in my Building Service Provider Networks book, but that won't be
available until June. I've also posted some applications in the archives.
>-----Message d'origine-----
>De : yakout esmat [mailto:yesmat@iprimus.com.au]
>Envoyi : mercredi 24 avril 2002 11:01
>@ : GOLBERY Irhne; ccielab@groupstudy.com
>Objet : RE: BGP and Local Preference
>
>Irene,
>Easy, Local Pref is a value given to routes as they enter the AS and stay
>with these routes as they propagate within this AS.
>In your example:
>1) Route 10.0.0.0 is assigned LocalPref of 200 as it enters AS1 by RA. it
>shows in your output 'show ip bgp'.
>2) Router A passes the route to RB with the same value, so it also shows in
>'show ip bgp'in RB.
>If you haven't changed the localpref value for the route (10.0.0.0) coming
>from RC as you did, it would have had the default which is 100.
>Rule: You will only see a value (100 or otherwise) next to local routes only
>learned through IBGP.
>Since Localpref value doesn't travel with the route outside the AS, you will
>see this value empty next to routes learned through EBGP as you have in your
>output.
>HTH
>Ya
>
>
>
>
>-----Original Message-----
>From: nobody@groupstudy.com [mailto:nobody@groupstudy.com]On Behalf Of
>GOLBERY Irhne
>Sent: Wednesday, April 24, 2002 6:03 PM
>To: ccielab@groupstudy.com
>Subject: BGP and Local Preference
>
>
>Hello,
>
>I am looking for some URL or documentation " EXPLAINING IN DETAILS " how
>does local preference is represented in the SHOW IP BGP command ?
>
>
>10.0.0.0/8
>------------------------------
> | |
>RC (AS 2) RD
> | |
> | |
> | |
>RA (AS 1) RB
>
>
>I mean,
>* RA(lo0:3.3.3.1) iBGP peer with RB (lo0:3.3.3.2) in AS 1
>* RA(lo0:3.3.3.1) eBGP peer with RC (lo0:3.3.3.3) in AS 2
>* RB(lo0:3.3.3.2) eBGP peer with RD(lo0:3.3.3.4) in AS 2
>
>RA set a Local preference of 200 for network 10. 0.0.0 announced by RC
>RB doesn 't change the defaul local preference
>
>When looking at the sh ip bgp,
>RB#sh ip bgp
>Network Next Hop Metric LocPrf Weight
>Path
>* 10.0.0.0/8 3.3.3.4 0 2 i
>*>i 3.3.3.1 200 0 2 i
>RA#sh ip bgp
> Network Next Hop Metric LocPrf
>Weight Path
>*> 10.0.0.0/8 3.3.3.3 200 0 2 i
>
>How does the " show ip bgp command " interpret the information of the local
>preference ?
>I need to find details informations.
>Thanks for your help
>Irene.
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