From: Howard C. Berkowitz (hcb@xxxxxxxxxxxx)
Date: Wed Apr 24 2002 - 15:11:06 GMT-3
At 7:34 PM +0200 4/24/02, GOLBERY Irhne wrote:
>Thanks a lot Howard for all the sources of information you gave me.
>
>"Is there a problem you are trying to solve?"
>--> Yes or not exactly
>--> probably not so interesting...
>
>In fact I just want to be shure that the test I have done works properly
>and then it's the reason why I am searching for How does the " show ip bgp"
>command interpret the information of the local preference
>
>Details :
>
>R1(lo0:3.3.3.9), R2(lo0:3.3.3.8),R3(lo0:3.3.3.3) and R4(lo0:3.3.3.1) are
>full mesh iBGP peers in AS 64513
>
>R1, R2 and R3 have :
>- an ip route : ip route 199.105.176.0 255.255.255.248 null0
>- and the network command : network 199.105.176.0 mask 255.255.248.0
>
>R1 default localpref is 200
>R2 default localpref is 150
>R3 default localpref is 100 (the default)
I'm guessing a bit because I don't see your full configuration. But I
notice that all the routers except R4 have a default route with a
maximum weight. First, have you tried clear ip bgp * after each
change?
Second, have you tried this without defaults?
>
>
>This is the result of the "SHOW IP BGP" COMMAND
>
>Remarks:
>Look at R4, it sees all the routers announcing the network 199.105.176.0/21
>even the one with the local preference of 100
>Look at R1 , it represents only the network announced by a peer of which the
>localpref is different than 100
>
>
>R1#sh ip bgp
>>Network Next Hop Metric LocPrf Weight
>Path
>* i199.105.176.0/21 3.3.3.8 0 150 0
>i
>*> 0.0.0.0 0 32768 i
>
>R2#sh ip bgp
>>Network Next Hop Metric LocPrf Weight
>Path
>* i199.105.176.0/21 3.3.3.9 0 200 0
>i
>*> 0.0.0.0 0 32768 i
>
>R3#sh ip bgp
>>Network Next Hop Metric LocPrf Weight
>Path
>* i199.105.176.0/21 3.3.3.8 0 150 0
>i
>* i 3.3.3.9 0 200 0
>i
>*> 0.0.0.0 0 32768 i
>
>R4#sh ip bgp
>>Network Next Hop Metric LocPrf Weight
>Path
>* i199.105.176.0/21 3.3.3.3 0 100 0
>i
>*>i 3.3.3.9 0 200 0
>i
>* i 3.3.3.8 0 150 0
>i
>
>
>Connectivity works, I just want to be shure that the result of the "show ip
>bgp" command of all the routers is OK.
>
>Sincerely,
>
>Irene.
>
>
>
>
> -----Message d'origine-----
>De : Howard C. Berkowitz [mailto:hcb@gettcomm.com]
>Envoyi : mercredi 24 avril 2002 13:37
>@ : 'ccielab@groupstudy.com'
>Objet : Re: TR: BGP and Local Preference
>
>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.
This archive was generated by hypermail 2.1.4 : Thu Jun 13 2002 - 10:58:18 GMT-3