350-401 Question 151
Single answerYou are tasked with configuring eBGP between two directly connected routers, R1 and R2. Both routers have been configured with the correct neighbor IP addresses and Autonomous System (AS) numbers. However, the BGP session does not establish. Upon investigation, you notice that the TTL (Time to Live) of the eBGP packets being sent by R1 does not match the expected value. Which configuration command should you apply on R1 to resolve this issue?
- A
neighbor
ebgp-multihop 2 - B
neighbor
update-source loopback0 - C
neighbor
ttl-security hops 1 - D
neighbor
remote-as
Show answer and explanation
Correct answer: C
Explanation
In eBGP, the default TTL value for BGP packets is 1, which means the session can only be established with directly connected neighbors. If the TTL value is not explicitly matched or enforced, the session may fail to establish. The 'ttl-security hops' command ensures the TTL is correctly handled, and setting it to 1 enforces a session with only directly connected peers, resolving the issue in this scenario.
- A. Incorrect.
This command is used to allow eBGP sessions to be established with neighbors that are multiple hops away. However, in this scenario, R1 and R2 are directly connected, so this is not the correct solution.
- B. Incorrect.
This command specifies the loopback interface as the source of the BGP packets. However, this is not relevant in this scenario since the issue is related to TTL and not the source of the packets.
- C. Correct.
The 'ttl-security hops' command ensures that BGP packets' TTL matches the expected value, which directly addresses the issue. Setting it to 1 ensures the session is established only with directly connected neighbors.
- D. Incorrect.
This command sets the remote AS of the neighbor. While this is required for eBGP configuration, it is not relevant to resolving the TTL-related issue in this scenario.