5.5Basic show commands on Cisco devices

On top of the general diagnostics so far, reading a Cisco device's own state centers on show commands. CCST does not ask for complex configuration, but being able to read "what the state is now" with the show family is basic support equipment. This section covers, at an introductory level, the roles of the frequently used show ip interface brief, show cdp neighbors, show ip route, show mac address-table, and show version; the privilege levels distinguished by the prompt symbol—user EXEC mode (>) vs. privileged EXEC mode (#); and the ? help and Tab completion that aid operation.

5.5.1Frequently used basic show commands

• show ip interface brief lists each interface's IP address and status (such as up/up) in a table—the go-to for grasping "which ports are up and have an IP" first. show version shows the IOS version, model, and part number.
• show cdp neighbors shows directly connected adjacent Cisco devices (their model and connecting port)—so you can trace "what is next to this device" even without a diagram. show ip route shows the routing table, revealing how to reach which networks.
• show mac address-table shows the table of MAC addresses and their ports learned by the switch—so you can trace "which port a PC is on." Others include show interface status (a port status list) and show inventory (installed modules).

5.5.2Privilege levels and operation aids

• User EXEC mode has the prompt > (e.g., Switch>)—view-oriented, an entry mode with limited capability. Privileged EXEC mode has the prompt # (e.g., Switch#), entered with the enable command, allowing all show and management operations. The symbol tells you which mode you are in.
• The ? (help) lists the available commands or the next arguments right there. Typing show ? lines up the keywords that can follow show, so you can find it even from a vague memory. It is the first resort when you do not know a command.
• Tab completion: pressing Tab partway through a command auto-completes the rest. Typing sh then Tab becomes show; show ip int then Tab extends to show ip interface. It lets you type faster and more accurately with fewer typos.

Suppose you are assigned to investigate why a PC at 192.168.10.20, which should be connected to an access switch, cannot communicate. Entering the switch over SSH, you notice the prompt is Switch>. That > is the mark of user EXEC mode, where what you can view is limited. So you type enable to move to Switch#—privileged EXEC mode—putting yourself in a state to run the needed show commands. First, show ip interface brief lists the interfaces, showing at a glance whether the port for the target PC is up/up or down. If the port is up/up, the physical link is alive, so next you look at show mac address-table to check whether that PC's MAC address is learned on the expected port. If it is not learned, it comes into view that the PC may not be sending traffic, or may be on a different port or VLAN. To check the relationship with a neighboring device, show cdp neighbors traces the directly connected Cisco device, and if you suspect a routing problem, show ip route shows the routing table. A common beginner stumble here is typing a half-remembered command and getting rejected, but then typing show ? lists the candidates that can follow, and sh plus Tab completes to show, so you can proceed even without recalling the exact command. What matters is the sequence: first confirm by the symbol which mode you are in (> or #), choose the show that fits the goal, and gather information reliably with the help of ? and Tab. Before changing any configuration, first read "what the state is now" accurately with show—this is the entrance to Cisco device diagnosis.

5.5.3Section summary

• Basic show: show ip interface brief (interfaces/up/up), show cdp neighbors (adjacency), show ip route (routing), show mac address-table (MAC learning), show version (IOS/model)
• The prompt > = user EXEC (view-oriented) and # = privileged EXEC (entered with enable, full capability)—tell the mode by the symbol
• Using ? (help listing candidates) and Tab completion lets you type commands fast and accurately even from a vague memory