Refer to the exhibit. Assume all other configurations are correct and the network is otherwise operating normally. Which conclusion can you draw about the neighbor relationship between routers R1 and R2?
A.
The neighbor relationship is up.
B.
The neighbor relationship will be up only if the two devices have activated the correct neighbor relationships under the IPv4 address family.
C.
The neighbor is down because the local-as value for R2 is missing in the R1 neighbor statement.
D.
The neighbor relationship is down because R1 believes R2 is in AS 65012.
I used the GNS3 lab and have the following output proving __ Answer D __ is correct.
*Sep 5 13:02:35.887: %BGP_SESSION-5-ADJCHANGE: neighbor 192.168.1.2 IPv4 Unicast topology base removed from session BGP Notification received
*Sep 5 13:02:38.211: %BGP-3-NOTIFICATION: sent to neighbor 192.168.1.2 passive 2/2 (peer in wrong AS) 2 bytes FE58
I think Cisco is trying to trick people with the router-id. Yes, a duplicate router-id will prevent the neighbor from establishing, but the syntax in the diagram is wrong. So, the router ID will not be 192.168.1.1 if you type these commands exactly as they are.
Local as altera o as no roteador a onde a co figuração é aplicada. O R1 precisa utilizar o as local configurado no R2 para que a sessão seja estabelecida. Caso contrário a sessão estará down.
The BGP session will not come up because a) AS is wrong b) bgp router-id's are identical:
R2#
*Jul 26 16:58:06.570: %BGP-3-NOTIFICATION: received from neighbor 192.168.1.3 active 2/2 (peer in wrong AS) 2 bytes FE58
*Jul 26 16:58:15.559: %BGP-3-NOTIFICATION: sent to neighbor 192.168.1.3 passive 2/3 (BGP identifier wrong) 4 bytes C0A80101
After removing local-as on R2 session will be still down. Only after changing router-id to unique valuas the session will establish.
First, answers B look right - I can say after testing with two CSR routers directly connected.
CSR-1: Gi1 Gi2: CSR-2
( ← ↕ → )─ ─ ─ ─ ─ ─ ─ ( ← ↕ → )
Here are the outputs:
CSR-2(config-router)#bgp router-id 192.168.1.1
*Feb 23 08:19:34.340: %BGP-3-NOTIFICATION: received from neighbor 192.168.1.3 active 2/3 (BGP identifier wrong) 4 bytes C0A80101
CSR-2(config-router)#bgp router-id 192.168.1.5 **[For testing purposes I use another ip in the same SN]**
CSR-2(config-router)#
*Feb 23 08:19:45.609: %BGP-5-ADJCHANGE: neighbor 192.168.1.3 Up
CSR-2(config-router)#
About the command on R2: "neighbor 192.168.1.3 local-as 65112"
It definitely causes a conflict as option D indicates, but issue has already started after configuring the same BGP router ID on the second router.
CSR-2(config-router)#neighbor 192.168.1.3 local-as 65112
CSR-2(config-router)#
*Feb 23 08:34:54.538: %BGP-5-NBR_RESET: Neighbor 192.168.1.3 reset (Local AS change)
*Feb 23 08:34:54.543: %BGP-5-ADJCHANGE: neighbor 192.168.1.3 Down Local AS change
*Feb 23 08:34:54.543: %BGP_SESSION-5-ADJCHANGE: neighbor 192.168.1.3 IPv4 Unicast topology base removed from session Local AS change
....So, as per question statement: "Assume all other configs are correct are correct and network stable" ... I assume the BGP router ID issue "is corrected", then D will be the only option when applying the command: neighbor 192.168.1.3 local-as 65112
It is definitely B the correct answer.
Because when R1 believes R2 is in AS 65012, is precisely when the BGP adjacency is established, of course if correcting the BGP router-id.
Here below is the BGP open message (packet capture) received from R2 and then BGP coming up:
Source: 192.168.1.2 Destination: 192.168.1.3
Type: OPEN Message (1)
Version: 4
My AS: 65012
Hold Time: 180
BGP Identifier: 192.168.1.5 (Testing corrected BGP router ID)
D according to the same link ;)
neighbor 192.168.1.3 local-as 65112
This command makes the R1 to see this R2 router as belonging to AS 65112 instead of AS 65012.
Please disregard my previous comment. R1 in fact does believe R2 is in AS 65012 however connection will fail because the last command local-as 65112 will make R2 advertise that it's 65112.
But the answer to this question is very poor because what R1 thinks R2 does is not the cause of the failure.
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