05-16-2016 08:26 AM - edited 03-08-2019 05:46 AM
Hi everyone,
In the attached digram R4 in AS 30 seems to have two paths to 75.1.0.1/16 and 75.2.0.1/16 (on R7), one is through R6 and the other one is through R2 -> R1 -> R3 -> R5.
But the following on R4 only shows there is one path? I am expecting two paths to be shown but the best path being what is shown? Any ideas?
R4#sh ip bgp
Network Next Hop Metric LocPrf Weight Path
*> 75.1.0.0/16 46.0.0.6 0 0 40 50 ?
*> 75.2.0.0/16 46.0.0.6 0 0 40 50 ?
*> 125.1.0.0/16 46.0.0.6 0 0 40 i
*> 125.2.0.0/16 46.0.0.6 0 0 40 i
Thanks!
Chris
05-16-2016 08:55 AM
Hi Chris Zhang,
Please attach the configuration of all router in the topology.
Best Regards
Juan Ibañez
05-16-2016 09:03 AM
Thanks Juan,
Please see below.
R1:
interface FastEthernet0/0
ip address 12.0.0.1 255.255.255.0
speed auto
duplex auto
!
interface FastEthernet0/1
ip address 13.0.0.1 255.255.255.0
speed auto
duplex auto
!
router bgp 10
bgp log-neighbor-changes
neighbor 12.0.0.2 remote-as 10
neighbor 13.0.0.3 remote-as 10
R2:
interface FastEthernet0/0
ip address 12.0.0.2 255.255.255.0
speed auto
duplex auto
!
interface FastEthernet0/1
ip address 24.0.0.2 255.255.255.0
speed auto
duplex auto
!
router bgp 10
bgp log-neighbor-changes
neighbor 12.0.0.1 remote-as 10
neighbor 12.0.0.1 next-hop-self
neighbor 24.0.0.4 remote-as 30
R3:
interface FastEthernet0/0
ip address 13.0.0.3 255.255.255.0
speed auto
duplex auto
!
interface FastEthernet0/1
ip address 35.0.0.3 255.255.255.0
speed auto
duplex auto
!
router bgp 10
bgp log-neighbor-changes
neighbor 13.0.0.1 remote-as 10
neighbor 13.0.0.1 next-hop-self
neighbor 35.0.0.5 remote-as 20
R4:
interface FastEthernet0/0
ip address 24.0.0.4 255.255.255.0
speed auto
duplex auto
!
interface FastEthernet0/1
ip address 46.0.0.4 255.255.255.0
speed auto
duplex auto
!
router bgp 30
bgp log-neighbor-changes
neighbor 24.0.0.2 remote-as 10
neighbor 46.0.0.6 remote-as 40
!
R5:
interface FastEthernet0/0
ip address 57.0.0.5 255.255.255.0
speed auto
duplex auto
!
interface FastEthernet0/1
ip address 35.0.0.5 255.255.255.0
speed auto
duplex auto
!
router bgp 20
bgp log-neighbor-changes
neighbor 35.0.0.3 remote-as 10
neighbor 57.0.0.7 remote-as 50
R6:
interface Loopback10
ip address 125.1.0.1 255.255.0.0
!
interface Loopback20
ip address 125.2.0.1 255.255.0.0
!
interface FastEthernet0/0
ip address 46.0.0.6 255.255.255.0
speed auto
duplex auto
!
interface FastEthernet0/1
ip address 67.0.0.6 255.255.255.0
speed auto
duplex auto
!
router bgp 40
bgp log-neighbor-changes
network 125.1.0.0 mask 255.255.0.0
network 125.2.0.0 mask 255.255.0.0
neighbor 46.0.0.4 remote-as 30
neighbor 67.0.0.7 remote-as 50
R7:
interface Loopback10
ip address 75.1.0.1 255.255.0.0
!
interface Loopback20
ip address 75.2.0.1 255.255.0.0
!
interface FastEthernet0/0
ip address 67.0.0.7 255.255.255.0
speed auto
duplex auto
!
interface FastEthernet0/1
ip address 57.0.0.7 255.255.255.0
speed auto
duplex auto
!
router bgp 50
bgp log-neighbor-changes
redistribute connected route-map rmr7
neighbor 57.0.0.5 remote-as 20
neighbor 67.0.0.6 remote-as 40
!
ip forward-protocol nd
!
!
no ip http server
no ip http secure-server
!
access-list 10 permit 75.1.0.0 0.0.255.255
access-list 10 permit 75.2.0.0 0.0.255.255
!
route-map rmr7 permit 10
match ip address 10
05-16-2016 09:53 AM
Hi Chris Zhang,
Where do you see that R4 have two paths to 75.1.0.1/16 and 75.2.0.1/16 (on R7), one is through R6 and the other one is through R2 -> R1 -> R3 -> R5??
Because BGP choose path with less AS-hop, in this case:
To R6 have two AS-hop >> AS40,AS50
To R2 have three AS-hop >> AS10,AS20, AS50
Regards!
Juan Ibañez
05-16-2016 11:39 AM
Thanks Juan. I don't see R4 having two alternative paths, which is why I was asking.
I know the best routes but 'sh ip bpg' should contain all possible paths?
05-16-2016 09:25 AM
Hi!
Is R2 learning the routes via R3?
Regards!
JC
05-16-2016 09:31 AM
R2 does not peer with R3, it is only learning routes from R1 and R4.
05-16-2016 09:55 AM
Hi!
I presume that R2 is not learning 75.1.0.0/16 nor 75.2.0.0/16 routes? If that is the case, R4 will never learn those routes via R2 thus will not be showed in the show ip bgp table as next-hop.
Regards!
JC
05-16-2016 11:38 AM
The following is shown on R2 and R4:
R2#sh ip bgp
Network Next Hop Metric LocPrf Weight Path
*> 75.1.0.0/16 24.0.0.4 0 30 40 50 ?
*> 75.2.0.0/16 24.0.0.4 0 30 40 50 ?
*> 125.1.0.0/16 24.0.0.4 0 30 40 i
*> 125.2.0.0/16 24.0.0.4 0 30 40 i
R4#sh ip bgp
Network Next Hop Metric LocPrf Weight Path
*> 75.1.0.0/16 46.0.0.6 0 40 50 ?
*> 75.2.0.0/16 46.0.0.6 0 40 50 ?
*> 125.1.0.0/16 46.0.0.6 0 0 40 i
*> 125.2.0.0/16 46.0.0.6 0 0 40 i
R2 appears to be learning 75.x routes but it learnt from R4.
Do you know why R2 didn't learn alternative path to 75.x networks through R1 and go anti-clockwise to R3->R5-R7?
05-16-2016 11:45 AM
Hi!
Yes, this happens because R2 does not seem to be peering with R1 or with R3 so R2 is NOT learning routes from those neighbors and therefore does not know any other path for reaching 75.1.0.0/16 other than through R4.
If you want it to have the other path learned via BGP then you will have to peer it with R3. Anyway, the path through R6 will still be the preferred one since the AS_Path is shorter.
Hope it helps, best regards!
JC
05-16-2016 11:53 AM
Thanks Again Carlos!
R1 does appear to be peering with R2.
R2#sh ip bgp summary
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
12.0.0.1 4 10 535 563 15 0 0 07:28:37 0
24.0.0.4 4 30 587 574 15 0 0 08:20:43 4
But it didn't have any prefixes from R1 (12.0.0.1). Is this expected?
Below shows that R1 is peering with R2 (12.0.0.2) and R3 (13.0.0.3) and learnt four prefixes from each.
R1#sh ip bgp summary
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
12.0.0.2 4 10 565 537 55 0 0 07:32:07 4
13.0.0.3 4 10 529 524 55 0 0 07:32:07 4
Is it not expected for R1 to forward routes learnt from R3 (75.x) to R2?
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