Troubleshooting
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Questions derived from the 640-801 - Cisco Certified Network Associate (single-exam option) Cisco Self-Test Software Practice Test.
Objective: Troubleshooting
SubObjective: Use the OSI model as a guide for systematic network troubleshooting
Item Number: 640-801.3.1.3
Single Answer, Multiple Choice
You have two Cisco routers (Router A and Router B) that are connected via a serial link. The link is experiencing problems, and you are trying to determine the cause. You are at Router A, and you are trying to check for connectivity to Router B. You cannot ping Router B, but you can see information about Router B through CDP. Which of the following options might be the problem?
- Incorrect IP address or subnet mask
- Mismatched encapsulations on the serial link
- No clock rate set
- Serial interface not enabled
- No encapsulation set on the serial link
Answer:
A. Incorrect IP address or subnet mask
Tutorial:
An incorrect IP address or subnet mask on one of the routers could cause this problem. If this were the case, then the two routers would not be able to ping each other. CDP is a Layer 2 protocol and does not rely on Layer 3 functionality to gain connectivity. It only requires that Layers 1 and 2 be operational. Therefore, if the IP address is not set or is incorrect, CDP will still operate.
If the Serial interface were not enabled, then Layer 1 is not enabled and CDP would not operate. The same goes for no clock rate and mismatched encapsulations.
Even if you do not set an encapsulation on a Cisco router serial interface, it does default to HDLC. Assuming everything else was configured properly, IP would be functional.
Reference:
http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/osi_prot.htm
