BGP inconsistencies in the lab.

Hello Experts,

I am testing BGP in the following eBGP topology. All routers do not show two
paths to every other network. Some do, some other do not. There is no error
in the connectivity, all adjacencies are OK.

The questions is: why some routers do not show all paths to every other
router with "sh ip bgp" as shown in the output below ?

Thanks in advance...

*******
Here is the topology:
Every router advertises its loopback0 network. Loopbacks are configured in
format: R1: 1.1.1.1/8 R2: 2.2.2.2/8 etc..
All routers are configured basically the same, except AS numbers, ip
addresses and neighbors.
Routers connect with Ethernet interfaces.
Each router has two eBGP neighbors defined.

AS100 AS200
R1-------------------------R2
| |
| |
| |
| |
R4-------------------------R3
AS400 AS300

*******
Here is output of "sh ip bgp". R1 and R2 displays all paths, while R3 and R4
do not. For example, Why R2 sees two paths to 1.0.0.0, while R4 sees only
one path to 1.0.0.0?

R1#sh ip bgp | b Network
   Network Next Hop Metric LocPrf Weight Path
*> 1.0.0.0 0.0.0.0 0 32768 i
*> 2.0.0.0 10.1.12.2 0 0 200 i
* 10.1.14.4 0 400 300 200 i
* 3.0.0.0 10.1.12.2 0 200 300 i
*> 10.1.14.4 0 400 300 i
* 4.0.0.0 10.1.12.2 0 200 300 400 i
*> 10.1.14.4 0 0 400 i
R1#

R2#sh ip bgp | b Network
   Network Next Hop Metric LocPrf Weight Path
*> 1.0.0.0 10.1.12.1 0 0 100 i
* 10.1.23.3 0 300 400 100 i
*> 2.0.0.0 0.0.0.0 0 32768 i
* 3.0.0.0 10.1.12.1 0 100 400 300 i
*> 10.1.23.3 0 0 300 i
* 4.0.0.0 10.1.12.1 0 100 400 i
*> 10.1.23.3 0 300 400 i
R2#

R3#sh ip bgp | b Network
   Network Next Hop Metric LocPrf Weight Path
* 1.0.0.0 10.1.23.2 0 200 100 i
*> 10.1.34.4 0 400 100 i
*> 2.0.0.0 10.1.23.2 0 0 200 i
*> 3.0.0.0 0.0.0.0 0 32768 i
*> 4.0.0.0 10.1.34.4 0 0 400 i
R3#

R4#sh ip bgp | b Network
   Network Next Hop Metric LocPrf Weight Path
*> 1.0.0.0 10.1.14.1 0 0 100 i
* 2.0.0.0 10.1.14.1 0 100 200 i
*> 10.1.34.3 0 300 200 i
*> 3.0.0.0 10.1.34.3 0 0 300 i
*> 4.0.0.0 0.0.0.0 0 32768 i
R4#

Here is bgp config on R1, all other routers are similar:
R1#sh run | s router bgp
router bgp 100
 no synchronization
 bgp log-neighbor-changes
 network 1.0.0.0
 neighbor 10.1.12.2 remote-as 200
 neighbor 10.1.14.4 remote-as 400
 no auto-summary
R1#

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Received on Fri Dec 04 2009 - 00:52:55 ART