Router-Based Network diagnosis steps and troubleshooting skills

Network diagnosis is an important technical task that manages and makes good use of the network to maximize its role. This article briefly describes the hierarchical diagnosis technology, discusses the diagnosis of various router interfaces, and reviews the troubleshooting of the connectivity fault of the interconnected network.
Network Fault Diagnosis Overview

Network Fault Diagnosis: based on the fault phenomenon, the Network diagnosis tool is used to obtain the diagnosis information, determine the network fault point, find the root cause of the problem, eliminate the fault, and restore the normal operation of the network. Network faults may occur in the physical layer.

Device connection failure or hardware and line problems; interface configuration problems of network devices on the data link layer; Network Layer Protocol configuration or operation errors; transmission Layer equipment performance or communication congestion problems; the upper layer or network application is incorrect. The process of diagnosing network faults should start from the physical layer to the OSI Layer 7 model. First, check the physical layer, then the data link layer, and so on, and try to identify the failure point of the communication until the system communication is normal.
You can use a variety of tools for Network diagnosis: Router diagnostic commands, network management tools, and other fault diagnostic tools including LAN or WAN analyzers. Viewing the route table is a good way to start searching for network faults. ICMP ping, trace, Cisco show, and debug commands are Network Tools for fault diagnosis. How can I monitor the operation details and fault situations of the network under normal conditions and what content? The show interface command can easily obtain information about each interface to be checked. The show buffer command regularly displays the buffer size, usage, and usage. The show proc command and show proc mem command can be used to track the CPU and memory usage. The data can be collected regularly for diagnosis reference when a fault occurs.
Troubleshooting procedure
Step 1: first determine the specific fault phenomenon and analyze the cause type of the fault. For example, the host does not respond to customer requests. The possible causes are host configuration problems, Interface Card faults, or loss of router configuration commands.
Step 2: Collect information needed to help isolate possible causes of failure. Collect useful information from network management systems, protocol analysis tracking, router diagnostic command output reports, or software manuals.
Step 3: determine the possible causes based on the collected information. For example, some data can be used to eliminate hardware faults and focus on software causes.
Step 4: Create a diagnosis plan based on the cause of the failure. You can start to diagnose the fault with only one of the most likely causes, so that the fault can be easily restored to its original state. If multiple fault causes are considered at one time, it is much more difficult to attempt to return to the original fault status.
Step 5: Execute the Diagnosis Plan, carefully test and observe each step, and confirm the results for each parameter change. The analysis results determine whether the problem is resolved. If the problem persists, continue until the fault disappears.
Layered Network diagnosis technology

Physical Layer faults mainly occur when the physical connection mode of the device is correct, the connection cable is correct, and the configurations and operations of devices such as Modem, CSU, and DSU are correct. The best way to determine whether the physical connection of a vro port is intact is to use the show interface command to check the status of each port, explain the screen output information, and view the port status, protocol establishment status, and EIA status.
To find and eliminate data link layer faults, You need to view the router configuration and check whether the connection port shares the encapsulation of the same data link layer. Each pair of interfaces must have the same encapsulation with other devices that communicate with them. Check the encapsulation of A vro by viewing its configuration, or use the show command to view the encapsulation of the corresponding interface.
The basic method for troubleshooting network layer faults is to view the route table of the router along the path from the source to the target, and check the IP address of the router interface. If the route does not appear in the routing table, check whether a proper static route, default route, or dynamic route has been entered. Then manually configure some lost routes, or eliminate some dynamic route selection failures, including RIP or IGRP routing protocol faults. For example, for an IGRP route, select the information to exchange data only between systems with the same autonomous system number AS) to view the matching information of the autonomous system number configured by the router.
Router interface troubleshooting

Serial Port troubleshooting
When a connectivity problem occurs in the serial port, in order to eliminate the serial port fault, the show interface serial command is generally used to analyze its screen output report content and locate the problem. The start of the serial port report provides the interface status and line protocol status. Possible Combinations of interfaces and line protocols are as follows:
1. Serial Port running and line protocol running are all working conditions. The serial port and line protocol have been initialized and are switching protocol survival information.
2. serial Port running and line protocol disabling. This display indicates that the router is connected to the device that provides the carrier detection signal, indicating that the carrier signal appears between the local and remote modem, however, the Protocol survival information at both ends of the connection is not correctly exchanged. Possible faults occur in vro configuration problems, modem Operation Problems, lease line interference or remote router faults, Digital Modem clock problems, two serial ports connected through the link are not on the same subnet, this report will appear.

3. Both the serial port and line protocol are closed, which may be due to line faults, cable faults, or modem faults of the Telecommunications Department.
4. The serial port management is disabled and the line protocol is disabled. In this case, the shutdown command is entered in the interface configuration. Run the no shutdown command to enable management shutdown.
When both interfaces and line protocols are running, although basic communication on the serial port link is established, many potential faults may still occur due to information packet loss and information packet errors. During normal communication, the interface input or output information package should not be lost, or the loss volume is very small and will not increase. If the loss of information packets increases regularly, the traffic transmitted through this interface exceeds the traffic that can be processed by the interface. The solution is to increase the line capacity. Find information packet loss for other reasons, and check that the input and output in the output report of the show interface serial command keep the queue status. When the number of information packets in the keep queue reaches the maximum allowed value, the size of the keep queue settings can be increased.
Ethernet interface troubleshooting

Typical Ethernet interface faults include excessive bandwidth utilization, frequent collision conflicts, and incompatible frame types. You can use the show interface ethernet command to view the throughput, collision conflicts, information packet loss, and content related to the frame type of the interface.
1. Check the throughput of the interface to check the bandwidth utilization of the network. If the percentage of the network broadcast information package is high, the network performance starts to decline. The information package from the optical fiber network to the Ethernet segment may overwhelm the Ethernet port. In this case, the Internet can adopt the optimization interface, that is, the Ethernet interface uses the no ip route-cache command to disable fast conversion, and adjust the buffer zone and keep the queue settings.
2. collision occurs when two interfaces attempt to transfer information to the ethernet cable at the same time. Ethernet requires few conflicts, and different network requirements are different. Generally, the cause of the conflict should be found three or five times per second. Collision conflicts cause congestion. The cause of collision conflicts is usually because the cables are too long, used too much, or "deaf" nodes. Ethernet cables should be considered in Physical Design and Management of Cable Laying systems. Over-standard cables may cause more conflicts. 3. If the interface and line Protocol report the running status, and the node's physical connection is complete, but cannot communicate. The cause may also be that two nodes use incompatible frame types. The solution is to reconfigure and use the same
Frame Type. If two devices in the same network of different frame types need to communicate with each other, you can use the sub-interface on the router interface and specify different encapsulation types for each sub-interface.
Asynchronous Communication Port troubleshooting
During the operation of the Interconnection Network, the asynchronous communication port provides reliable services for users, but is also a fault-prone part. The common external factors of asynchronous communication port faults are: poor performance of dial-up links, connection quality of telephone network switches, and modem settings. Check the modem used at both ends of the link: there are not many questions about connecting to the remote PC port modem, because the modem is usually initialized every time a new dial is generated, most communication programs can send the appropriate setting string before issuing the dialing command. There are many problems with connecting to the vro port. The Modem usually waits for a connection from the remote modem. Before connecting, the specified string is not received. If the modem loses its settings, you should use one method to initialize the remote modem. A simple method is to use a modem that can be configured through the front panel. Another method is to connect the modem to the asynchronous interface of the router, establish reverse telnet, and send the setting command to configure the modem.
The show interface async and show line commands are the most widely used tools for diagnosing asynchronous communication port faults. In the show interface async command output report, the only condition that the interface status report is closed is that the encapsulation type is not set for the interface. The line protocol status is displayed the same as that of the serial port. The show line command displays the interface receiving and transmission speed settings, and the EIA status. The show line command can be considered as an extension of the show interface async interface command. View the EIA signal output by the show line command to determine the network status.
To determine the asynchronous communication port failure, perform the following steps: Check the cable line quality, check the modem parameter settings, and check the modem connection speed; check whether rxspeed and txspeed match the modem configuration. Use the show interface async command and the show line command to check the port communication status. Check the EIA Status display from the Report of the show line command; check interface encapsulation; check information package loss and buffer loss.