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RE: BGP Path Question posted 08/26/2007
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That makes perfect sense Joel - Thanks!

 

Do you know if there is a debug message that I should of picked up on R1 to
indicate that?

 

 

Thanks again,

Ken

  _____  

From: Joel Amao [mailto:femmy79@xxxxxxxxxxx] 
Sent: August 26, 2007 12:13 AM
To: Ken Young; ccielab@xxxxxxxxxxxxxx
Subject: RE: BGP Path Question

 

 
Only the best path (shown by the ">") is advertised  to the neighbor.
R1 wont advertise the path back to  its neighbor R3, since it is   using the
advertisement from R3 (that same neighbor)  in the first place == Split
Horizon



Joel  
CCIE#18128
 
 
 



> From: CiscoKid@xxxxxxxxxxxxxxx
> To: ccielab@xxxxxxxxxxxxxx
> Subject: BGP Path Question
> Date: Sat, 25 Aug 2007 21:29:01 -0300
> 
> R2 (AS2000)
> ----------fa0/0----------
> | |
> |e0 |fa0/0
> R8 (AS8000) R1(AS1000)
> |e1 |fa0/1
> | |
> ----------fa0/0----------
> R3 (AS3000)
> 
> eBGP Peerings:
> R8--R2
> R8--R3
> R1--R2
> R1--R3
> 
> Situation:
> R8 is advertising 80.0.0.0/8 with all default attributes.
> 
> Result:
> 
> R2-2610XM#show ip bgp
> Network Next Hop Metric LocPrf Weight Path
> * 80.0.0.0 10.10.10.1 0 1000 3000 8000
> i
> *> 10.10.10.8 0 0 8000 i
> 
> 
> 
> R1-2611XM#show ip bgp
> Network Next Hop Metric LocPrf Weight Path
> * 80.0.0.0 10.10.10.8 0 2000 8000 i
> *> 20.20.20.8 0 3000 8000 i
> R1-2611XM#
> 
> 
> R3-2610XM#show ip bgp
> Network Next Hop Metric LocPrf Weight Path
> *> 80.0.0.0 20.20.20.8 0 0 8000 i
> 
> 
> 
> Question: 
> 
> Why is R3 not seeing both paths? Is R1 not advertising the longer 
> path to R3 because it is aware that R3 already has a shorter path?
> 
> Even if that was the case I would have assumed both paths would be 
> seen by R3 but the shortest path via 8000 would be selected as best.
> 
> 
> When I debug "ip bgp updates" R1 does not send an update to R3 for 
> that path unless, R3 sends a withdrawal of its existing path.
> 
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