Network troubleshooting involves three big things: predicting what might happen, determining what is wrong, and investigating why those faults occurred. Many network administrators troubleshoot network infrastructure by analyzing the Layer 3 path through the network step by step in both directions. This process helps them to segregate the problem; Once they determine which step in the layer path is failing, they can exercise the details.
Several tools can help with network troubleshooting. Let’s go through them and see what issues they can help you investigate.
Table of Contents
Cisco Discovery Protocol (CDP)
Cisco Discovery Protocol, also known as CDP, discovers basic information about neighboring routers and switches without knowing the passwords for those Cisco network devices. This is promising because Cisco routers and switches routinely send CDP messages that advertise statistics about themselves. Therefore, Cisco hardware that maintenances CDP can learn about other devices by attending to these messages. CAS a result, DP discovers several v
- Device ID: The name of the host
- Address list: network and data connection addresses
- Port Identifier: The edge on the remote device that sent the CDP advertisement
- Feature List: Device type (for example, router or switch)
- Platform: The version of IOS running on the device
To View this Information, Use the Show CDP Command:
Show CDP neighbor – Here, it will show all the neighbors connected to their Cisco network devices
This command lists each neighboring network device, one per line. Each line provides important topology information about the neighbor devices: its hostname (device ID), the local device’s interface, and its boundary (under the Port heading). This command also lists the stage, identifying the exact model of the neighboring router or switch.
To get extra details, such as the full name of the model of the switch & the IP address configured on the neighboring network device, add the detail parameter as follows:
Show CDP neighbor detail – Of course, the possibility of determining a lot of information about neighboring devices is a network security risk. Therefore, Cisco commends disabling CDP on all IP address interfaces that do not require it. To turn CDP on and off for an entire network device, use the global commands no CDP run and CDP run. To snap on a specific interface, use the devices’ no CDP enable & CDP enables interface subcommands.
Show Version Cisco Routers and Switches
You can use the Cisco IOS show version command in privileged execution mode to verify the Cisco IOS version and the version number of the IOS software running on Cisco devices. Produces the following information:
- Cisco IOS Software Release: The name and version number of the Cisco software.
- Switch Up Time – The amount of time since the device was last booted
- Switch Platform – Hardware platform information, including memory size and revision
- Processor Board ID – The serial number of the device
Ping
The essential purpose of the ping is to check reachability, round trip time (RTT), and packet loss. To troubleshoot a device with these properties, we need to provide the IP address of the device, e.g., ping 198.168.1.6. This command directs an Internet Control Message Protocol (ICMP) echo demand and displays one of the following flags:
- An ICMP echo response packet was received within the break period (2 seconds by default)
- No response was received within the timeout period.
You can ping from the same interface by adding the source factor with the interface name to the end of the command. E.g., ping 198.168.1.6 source Ethernet 0/0.
Trace Route
Traceroute is a feature that traces the path from one network to an alternative, so it can help diagnose the cause of many problems. First, Traceroute sends the remote host a classification of three UDP datagram with a TTL of one in the IP header; This grounds the datagram to time out upon reaching the first router in the path, causing the router to respond with an “ICMP timeout” message. Traceroute then sends a set of three UDP datagrams with TTL 2 to time out when they reach the second router, causing it to respond with a timeout message. This procedure continues until the packet reaches the final destination and receives an unreachable ICMP port message.
Therefore, you can use a traceroute to test the route packets took to reach their destination.
You can also use an advanced traceroute command to test connectivity from a specific source, for example, traceroute 10.100.70.6 source Loopback0
Telnet
When you use telnet to join a remote device, it uses the default port (23). However, you can use any port number from 1 to 65535 to test if a remote device is listening on the specific port, e.g., Telnet 172.17.5.74 8080.
Show Interface Commands and Interface Rank Codes
Cisco switches use two diverse sets of interface rank codes. Both sets of rank codes can determine if an interface is up.
Show Interfaces and Show Interface Description
These commands will list the line status and protocol status. These mainly indicate whether layer 1 is working (line status) & whether layer two is working (protocol status). Both codes usually have the same value for LAN switch interfaces, either “up” or “down.”
Show Interface Status
Shows the interface status. This single status code corresponds to various line and protocol status combinations, as shown in the following table. For example, the interface state “connected” corresponds to an active/active state for the other two states.
Cisco shutdown command
When you earliest configure an interface in Configure Terminal mode, you must managerially enable it before the router can use it to send or receive packets. Use the Cisco no shutdown command to permit the IOS software to use the interface.
Later, you might need to disable a specific interface to do hardware maintenance on it or a segment of a network. You may also need to disable an interface if there is a problem on a particular network segment and isolate that section from the rest of the network. The shutdown command managerially brings up an interface. To restart the interface, then use the no shutdown command.
Show IP Route
Most routing tables contain a mix of static and dynamic routes. However, the routing table must contain the directly connected networks used to access remote networks before static or dynamic routing can be used. To examine the static routes in the routing table, custom the show IP route command, stating the network address, subnet mask, & IP address of the next-step router or exit interface.
Common problems with Cisco devices
Interface speed and duplex issues
Many UTP-based Ethernet interfaces support multiple speeds (full duplex or half duplex) and IEEE standard auto-negotiation. For example, these interfaces can run at speed {10 | be configured to use a specific speed 100 | 1000} interface subcommand and use a specific duplex with the duplex {half | full} interface subcommand. If both are configured for an interface, the switch or router disables the IEEE standard auto-negotiation process for that interface.
The show Interfaces Status commands will list the speed & duplex settings on an interface, but only the show Interfaces Status command shows how the switch determined the speediness and duplex settings; it Lists all auto-negotiated configurations with the a- prefix. For example, a-full means full duplex with auto-negotiation, while complete means full duplex but manually configured. Although auto-negotiation works fine, the default values allow for the possibility of a problem called duplex mismatch, where devices would consider the link up. Still, one side would use half duplex, and the other would use full duplex.
The total of input errors and the total of CRC errors are two counts in the output of the show interfaces command. The experiment is deciding which counters you need to see, which indicate a difficulty is occurring, and which are normal and safe.
Below Are The List Of Counters To Help You Understand Which Ones Indicate Problems And Which Ones Only Count Normal Events With No Problems:
Runts: Frames that did not meet the minor frame size requirement (64 bytes, including the 18-byte destination MAC, source MAC, and type). Collisions can cause strokes.
Jumbo: Frames that exceed the extreme frame size (1518 bytes, including 18 bytes of destination MAC, source MAC, & type).
Input Errors: The total counters, plus Runts, Giants, No Buffer, CRC, Frame, Overrun, & Ignored Counts.
CRC: Frames acknowledged that failed FCS math; can be affected by collisions.
Frame: Frames acknowledged that have an illegal format (for example, ended with a nibble); Collisions can cause them.
Packet Egress: Total number of packets (frames) forwarded over the interface.
Egress Errors: Total number of packets (frames) that the switch port tried to transmit but met a problem.
Collisions: Counter all collisions that happened when the interface sent a frame.
Late Collisions: The subset mask of all collisions that arise after byte 64 of the frame has been spread. On a correctly working Ethernet LAN, collisions should arise within the first 64 bytes; today’s late collisions regularly indicate a duplex mismatch.
Conclusion
What information about a cisco router can be verified using the show version command? You now know the basic troubleshooting commands to investigate problems network administrators face daily.
