Re: BGP inconsistencies in the lab.

Hope I put it in non-confusing words..In case of your example where from
does R4 have a possibility of learning another path to 1.0.0.0 ? From
R3..isn't it. However R3 has already chosen the path from R4 for 1.0.0.0 as
the best path .Since a router will only advertise the best path information,
R3 will not send the information of 1.0.0.0 that it learned from R2 and
hence R4 has only one path for 1.0.0.0 which is from R1.

Same holds true for the other cases.

HTH,
Ravi

On Fri, Dec 4, 2009 at 5:52 AM, Jack Router <pan.router_at_gmail.com> wrote:

> 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 - 07:16:10 ART