Exam 300-410 topic 1 question 429 discussion

The error message "BGP: 10.0.0.18 passive open failed - 10.0.0.17 is not update-source Loopback0's address (10.10.10.5)" indicates that there was a problem with establishing a BGP session between two routers with IP addresses 10.0.0.18 and 10.0.0.17. The message specifically states that the passive open attempt from the device with IP address 10.0.0.18 has failed because the device with IP address 10.0.0.17 is not using the correct source IP address when sending updates. Instead of using the IP address of the Loopback0 interface, which is 10.10.10.5, it is using a different IP address. In this case, it appears that the device with IP address 10.0.0.17 is using a different source IP address than the one expected by the device with IP address 10.0.0.18. Specifically, the device with IP address 10.0.0.17 should be configured to use the IP address of its Loopback0 interface (10.10.10.5 in this case) as the source address for BGP updates when communicating with the device at IP address 10.0.0.18.

R6(10.0.0.18) is sending BGP open messages to R5(10.0.0.17). They are failing because R5 has its peering source with R6 set to the loopback 10.10.10.5. R5's in its debug output is expecting open messages to it loopback address 10.10.10.5. Solution is remove the neighbor 10.0.0.18 update-source L0 on R5 or configure the neighbor statement using the loopback addresses with ebgp multihop instead of the WAN interface address if you had the option to select 2 choices A and D would be correct, just not one or the other

B is corerct

Reproduced error in CML lab with a simple BGP config: R5 router bgp 65101 neighbor 10.0.0.18 remote-as 65201 network 10.10.10.5 mask 255.255.255.255 !debug ip bgp -> enabled on both routers for viewing the error !When adding this incorrect line the same error comes as in the output: neighbor 10.0.0.18 update-source Loopback0 R6 router bgp 65201 neighbor 10.0.0.17 remote-as 65101 network 10.10.10.6 mask 255.255.255.255

Could someone actually explain this question please? I feel like we don't have enough information from the question itself. For me, B makes sense. Current situation is, that R5 is using 10.10.10.5 address for BGP communication with R6. Also, R6 is using 10.0.0.18 for BGP communication with R5 and it's trying to open a session with R5's address 10.0.0.17. Which means: A - won't help, because then R6 will not respond as it's not using its Lo0 address for communication B - will help, because we remove Lo0 as update source from R5, so R6 can use R5's address 10.0.0.17 and R5 will reply C - won't help, because R6 is clearly not using Lo0 for communication, so this command is redundant D - won't help, because we would make R6 use R5's Lo0 address for communication, but we would also change R6's communication address too, so then there would be a mismatch with R5's neighbor config

B IS CORRECT

I think it's A, it says in the exhibit of R5, 10.0.0.17 is not update source loopback0. We need to to update source loopback and update ebgp multihop to 3.

Correct!