350-401 Question 466
Single answerA network engineer is troubleshooting a BGP connectivity issue between two routers in separate autonomous systems (AS). Router R1 in AS 65000 and Router R2 in AS 65001 are configured for eBGP. The following observations are made:
- The BGP session is stuck in the 'Active' state.
- R1 and R2 can ping each other's directly connected interfaces.
- The configured neighbor IPs on R1 and R2 match the directly connected interface IPs.
- The output of 'show ip bgp summary' on R1 shows no received prefixes from R2.
What is the most likely cause of this issue?
- A
The BGP neighbor IP address on R1 is incorrect.
- B
The 'ebgp-multihop' feature is not enabled on both routers.
- C
The TCP port 179 is blocked by an intermediate firewall.
- D
The BGP authentication configuration (MD5) does not match on both routers.
Show answer and explanation
Correct answer: C
Explanation
The BGP session being stuck in the 'Active' state typically indicates that the TCP connection required for BGP is not being established. Since the routers can ping each other, the issue is likely related to higher-layer connectivity rather than a Layer 3 reachability problem. A common cause for this is TCP port 179 being blocked by a firewall or an access control list (ACL) between the two routers.
- A. Incorrect.
The neighbor IP address is configured correctly, as the routers can ping each other's directly connected interfaces and the IPs match.
- B. Incorrect.
The 'ebgp-multihop' feature is not required in this scenario because the BGP neighbors are directly connected.
- C. Correct.
A blocked TCP port 179 could prevent the BGP session from being established, causing it to remain in the 'Active' state despite the routers being able to ping each other.
- D. Incorrect.
There is no mention in the scenario of BGP authentication (MD5) being configured on either router, so this is unlikely to be the issue.