The document discusses various techniques for testing and troubleshooting network issues: - The show ip route command displays the IP routing table containing known networks, subnetworks, and how routing information was learned. The "gateway of last resort" refers to the default route, network, or gateway. - Typical layer 1 errors include broken or disconnected cables, cables connected to the wrong ports, intermittent connections, or using the wrong cable type. Layer 2 errors include improperly configured interfaces, improper encapsulation, or NIC problems. Layer 3 errors involve incorrect routing protocol configuration, IP addresses, or subnet masks. - The show interfaces command can help troubleshoot layer 1 by examining carrier transitions and resets, input errors, and hardware issues indicated by

1. NETWORK TESTING & TROUBLESHOOTING TS, PHẠM VĂN TÍNH PART15

2. EXAMINING THE ROUTING TABLE

3. The show ip route Command The show ip route command displays the contents of the IP routing table. This table contains entries for all known networks and subnetworks, and a code that indicates how that information was learned .

4. Determining the Gateway of Last Resort The term “ gateway of last resort ” includes the default route , default network , and default gateway .

5. Determining Administrative Distance

6. Determining the route metric Routing protocols use metrics to determine the best route to a destination.

7. NETWORK TESTING

8. Five Steps Troubleshooting

9. Typical Layer 1 Errors Broken cables Disconnected cables Cables connected to the wrong ports Intermittent cable connection Wrong cables used for the task at hand (must use rollovers, crossover cables, and straight-through cables correctly) Transceiver problems DCE cable problems DTE cable problems Devices turned of

10. Typical Layer 2 Errors Improperly configured serial interfaces Improperly configured Ethernet interfaces Improper encapsulation set (HDLC is default for serial interfaces) Improper clockrate settings on serial interfaces Network interface card (NIC) problems

11. Typical Layer 3 Errors Routing protocol not enabled Wrong routing protocol enabled Incorrect IP addresses Incorrect subnet masks

12. Troubleshooting Layer 1 using show interfaces An increasing number of carrier transitions counts on a serial link may indicate one or more of the following problems: Line interruptions due to problems in the service provider network. Faulty switch, DSU, or router hardware. The following Layer 1 problems could be a cause of interface resets : Bad line causing carrier transitions Possible hardware problem at the CSU, DSU, or switch If carrier transitions and interface resets are increasing or if input errors are high while interface resets are increasing, the problem is likely to be a bad link or defective CSU or DSU .

13. Troubleshooting Layer 2 using show interfaces If the interface is up and the line protocol is down, a Layer 2 problem exists. Among the possible causes are: No keepalives No clock rate Mismatch in encapsulation type

14. Troubleshooting routing issues Show ip route Show ip protocols

15. Troubleshooting using show controllers serial