200-301 Question 260
Single answerA network administrator is troubleshooting connectivity between two routers, R1 and R2, which are directly connected via a serial interface. The administrator notices that the routers cannot ping each other. An initial check shows that both interfaces are configured with IP addresses in the same subnet and are administratively up. What should the administrator check next to resolve the issue?
- A
Verify the Layer 2 encapsulation type configured on both interfaces
- B
Ensure that both routers are running the same routing protocol
- C
Check for access control lists (ACLs) blocking ICMP traffic
- D
Inspect for mismatched clock rates on the serial interfaces
Show answer and explanation
Correct answer: A
Explanation
When troubleshooting Layer 3 connectivity issues between directly connected devices, verifying the Layer 2 encapsulation type is critical. A mismatch in the encapsulation type (e.g., one side using HDLC and the other using PPP) will prevent the link from transmitting data, even if the IP configuration and administrative status appear correct. This is a common issue in serial interface setups.
- A. Correct.
The Layer 2 encapsulation type (such as HDLC or PPP) must match on both ends of a serial link for communication to occur. A mismatch will prevent the link from functioning properly.
- B. Incorrect.
Routing protocols are not relevant in this scenario since the two routers are directly connected and in the same subnet. Pinging between directly connected devices does not require a routing protocol.
- C. Incorrect.
While ACLs could block ICMP traffic, this is not the first thing to check. The issue likely lies in the Layer 2 configuration since the problem involves direct connectivity.
- D. Incorrect.
Mismatched clock rates can cause issues on serial links, but they would typically prevent the interface from coming up at all. Since the interfaces are administratively up, this is less likely to be the root cause.