Basic troubleshooting methods for Cisco network faults

1. Troubleshooting)
Here we will introduce the hardware and software problems that may occur when using vrouters. During the networking process, such as X.25, FR, DDN, DDR, TCP/IP, problems encountered in different network environments such as interconnection with IBM large hosts and solutions .? Troubleshooting tool? What is the Troubleshooting of TCP/IP connection? What is the problem with the serial port connection? What is the Troubleshooting of the dialing connection? What is the Troubleshooting of Frame Relay connection? X.25 connected Troubleshooting? Troubleshooting connected to an IBM host
　　
2. Troubleshooting Tool
Troubleshooting provides many diagnostic tools, including diagnostic commands for routers, Cisco network management tools (CiscoWorks), and procedure analyzers. The following describes diagnostic commands for routers.
There are four router diagnostic commands:
? Use the show command
? Debug Command
? Ping Command
? Use the trace command
Use the show command
Show is a useful monitoring command and tool for solving system problems. Below are some common show commands:
? Show interface --- display interface statistics.
　　
Some common show interface commands:
? Show interface ethernet
? Show interface tokenring
? Show interface serial
? Show controllers --- display the statistics of the interface card controller.
　　
Some common show controllers commands:
? Show controllers cxbus
? Show controllers e1
? Show running-config --- displays the configuration of the current vro.
? Show startup-config --- displays the NVRAM configuration.
? Show flash --- Flash memory content.
? Show buffers --- displays the bufferpools statistics in the vro.
? Show memory --- the memory usage statistics of the vro, including the idle pool statistics.
? Show processes --- vro activity process information.
? Show version --- displays the system hardware, software version, configuration file, and startup system image.
Debug Command
In superuser mode, the debug command provides port transmission information, error messages generated by nodes, diagnostic protocol packages, and other useful troubleshooting data.
!! Note: when using the debug command, it will occupy system resources and cause some unpredictable phenomena. to stop using the debug command, use the nodebug all command.
The Debug command is displayed on the Console port by default. The logbuffer command can be used to direct the output to buffers. If telnet is used, the Router # terminalmonitor can be used to monitor the console information.
Ping Command
Ping to determine network connectivity.
Use the trace command
Trace command to track router packet transmission.
3. Troubleshooting connected by TCP/IP
Symptom: the Ethernet port from the host to the local router is disconnected.
Suggestion: we can regard the Ethernet port of the router as the ethernet card of a common host, which becomes a local area network connection problem,
1. Run the show interface ethernet number command.
Router # show interface ethernet 0
Ethernet is up, line protocol is down
2. If the Ethernet is down, connect the cable (coaxial cable or twisted pair cable). If the ethernet is down, contact your agent.
3. If Ethernet is adms1_atively down.
Router # conf t
Router (config) # interface ethernet 0
Router (config-if) # no shutdown
Router (config-if) # ^ Z
Router #
4. If Ethernet is up, and line protocol is down.
The host 10 m Nic is connected to the M Ethernet port of the router. It is not adaptive (current version). Otherwise, no problem occurs.
If the coaxial cable is used, check whether the cable, T-header, and terminator are correctly connected.
If it is a twisted pair, check whether the cable is correct and whether it is connected through the HUB in the middle. If it is a direct connection host, use a crossover line.
If the 100BaseTX interface is used, five cat-5e twisted pair wires are required.
If an interface provides two types of physical media, such as the crude AUI and UTPRJ45, the default value is AUI. To use RJ45, you must:
Router # conf t
Router (config) # interface ethernet 0
Router (config-if) # media-type 10 baset
Router (config-if) # ^ Z
Router #
5. If Ethernet is up, line protocol is up; but ping fails.
Check whether the IP address of the Ethernet port of the router is in the same network segment as the host IP address.
6. After the above steps, the problem persists. Please contact your agent.
Symptom: The WAN network port or Ethernet port of the host to the peer router is disconnected.
Suggestion: assume that the Ethernet port from the host to the local router is connected.
1. Check whether two wide area network ports are connected on the vro. If the ports are disconnected, see the following section about the wide area network troubleshooting.
2. If the router is connected between two wide-area network ports.
Run the "netstat-rn" command on the host to find the route. If not, use "routeadd.
Take sco unix as an example:
# Netstat-rn
# Route add destination CIDR Block mask gateway 1
Or
# Vi/etc/gateways
Net destination network segment gateway local router Ethernet port address metric 1 passive
3. If there is a default gateway on the host, check the router routing protocol configuration.
Router # show ip route
Router # show running-config
.
.
.
Router VPN 1
Network...
Network...
Whether the routing protocols are consistent between the two vrouters and whether the CIDR blocks added to the "network" are correct in an autonomous system.
Symptom: the host cannot connect to the target host.
Suggestion: follow these steps.
1. Check the Ethernet port from the host to the local router.
2. Check two wide-area network ports.
3. Check whether the host is connected to the WAN port of the Peer router.
4. Check the Ethernet port of the host to the peer router.
Use the telnet command to remotely log on to the peer vro, and check the LAN connection of the peer by checking the Ethernet port from the local host to the local vro.
5. Repeat 3 and 4 to check the recipient's local location.
6. After the above steps, the problem persists. Please contact your agent.
4. Troubleshooting
Symptom: During leased line connection, the two wide area network ports directly connected to the router cannot be connected.
Suggestion: We can divide the two router wide-area network ports into three sections ,:
Router A--1---MODEMA----2----MODEMB--3 -- router B
Our task is to check which part of the file cannot be found and solve it.
1. Run the show interface serial number command.
　　
2. If Serial is down, there is no carrier signal CD between the vromodem and the local MODEM.
Connect the serial port and MODEM, and enable MODEM. check whether the sending light TD of the MODEM is on. If the TD light is on, the router sends a signal to the MODEM. if the TD light is not on, check the MODEM, cable (recommended for Cisco), and port. you can use another serial port to try again.
3. If Serial is up, but line protocol is down. There are several possibilities:
A. The local router is not configured.
B. The remote router is not enabled or is not configured.
The same Protocol packaging method needs to be configured at both ends of the vro. For example, if vroa A is packaged with HDLC and vrob B is packaged with PPP, The lineprotocol of the two vrouters is always down. Change the packaging method:
Router # conf t
Router (config) # interface serial 0
Router (config-if) # encapsulation ppp
Router (config-if) # ^ Z
Router #
C. If Newbridge's 26XX and 27XX DTU devices do not send CD signals, set the following on the router:
Router # configure terminal
Router (config) # int serial 0
Router (config-if) # ignored-dcd
Router (config-if) # ^ Z
Router #
D. There is no connection between the MODEM, that is, the leased line is not connected.
Solution: perform a test loop. Ask the Telecommunications Board to help determine the specific line with the problem. If the loop is successful, lineprotocol will change to up (looped ).
4. If Serial is up, but line protocol is up (looped ).
Use show running-config to check whether loopback is configured on the port. If yes, delete it.
Whether the MODEM performs loop testing.
Whether a loop test is performed on the leased line.
5. If Serial is adms1_atively down, line protocol is down.
Router # conf t
Router (config) # interface serial 0
Router (config-if) # no shutdown
Router (config-if) # ^ Z
Router #
5. Troubleshooting for dialing connections
To solve the problem of dialing network connection, you must first:
Confirm that the vromodem and MODEM are connected.
Understand the meaning of show line output
Confirm that the vromodem and MODEM are connected.
We use Reverse Telnet (Reverse Telnet Session) on the vromodem to determine the connection between the vromodem and the MODEM. that is to say, you can use the AT command to configure the MODEM. the procedure is as follows:
1. On the vro console, run the following command:
Telnet ip-address 20yy
Ip-address is the active port address, and yy is the line connecting to the MODEM. For example, the following example uses the ip address 192.169.53.52 to connect to the Auxiliary Port:
Telnet 192.169.53.52 2001
2. If the connection is rejected, other users may connect to the port. Use the showusers EXEC command to determine whether the connection is occupied. If yes, clearline clears the port. If no, retry the anti-Telnet operation.
3. If the connection is still rejected, confirm that the MODEM controls modeminout.
4. confirm that the txspeed and rxspeed of the router are consistent with the number of MODEM settings.
5. After the Telnet logon is successful, run the AT command to confirm the response.
Understand the meaning of show line output
Show line-number EXEC is a very useful trobbleshooting command.
　　
Symptom: No connection is established between the MODEM and the vro. No response is returned during the trial of anti-Logon or the user receives the "ConnectionRefused by Foreign Host" message.
Suggestion:
1. Use show line to check whether the MODEM column is "inout". If not, on the vro:
Router # conf t
Router (config) # line aux 0
Router (config-line) # modem inout
Router (config-line) # ^ Z
Router #
2. Confirm the correct cable.
3. for hardware problems, please contact your agent.
Symptom: MODEM does not dial.
Suggestion: If the MODEM is not dial-up, the hardware and cable may be excluded:
1. the package you are not interested in. use show running-config to check the vro configuration and whether the dialer-list segment has been set to capture the package you want to transfer. If so, reconfigure the access-list table.
2. Chat script configuration error. Open debug information.
Router # debug dialer
% LINEPROTO-5-UPDOWN: Line protoc

Author: "learning and communication"