Thanks Again Carlos!

Chris Zhang · ‎05-16-2016

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

Juan Ibañez · ‎05-16-2016

Hi Chris Zhang,

Please attach the configuration of all router in the topology.

Best Regards

Juan Ibañez

Chris Zhang · ‎05-16-2016

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

Juan Ibañez · ‎05-16-2016

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

Chris Zhang · ‎05-16-2016

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?

Carlos Villagran · ‎05-16-2016

Hi!

Is R2 learning the routes via R3?

Regards!

JC

Chris Zhang · ‎05-16-2016

R2 does not peer with R3, it is only learning routes from R1 and R4.

Carlos Villagran · ‎05-16-2016

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

Chris Zhang · ‎05-16-2016

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?

Carlos Villagran · ‎05-16-2016

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

Chris Zhang · ‎05-16-2016

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?

BGP path section