One of the most important abilities a network administrator can have is the know-how to get information out of his network devices so he can find out what's going on with the network. In most networks, the staple of information gathering has been the "show" commands. Here are my top ten commands to know and love:
show version: Start simple; this command gives uptime, info about your software and hardware and a few other details. show ip interface brief: This command is great for showing up/down status of your IP interfaces, as well as what the IP address is of each interface. It's mostly useful for displaying critical info about a lot of interfaces on one easy to read page. show interface: This is the more popular version of the command that shows detailed output of each interface. You'll usually want to specify a single interface or you'll have to hit 'page down' a lot. This command is useful because it shows traffic counters and also detailed info about duplex and other link-specific goodies. show ip interface: This often overlooked command is great for all the configuration options that are set. These include the switching mode, ACLs, header compression, ICMP redirection, accounting, NAT, policy routing, security level, etc. Basically, this command tells you how the interface is behaving. show ip route: This indispensable command shows your routing table, which is usually the primary purpose of the box. Get to know the options on this command. show arp: Can't ping a neighbor? Make sure you're getting an arp entry. show running-config: This is an easy one. It tells you how the box is configured right now. Also, "show startup-config" will tell you how the router will be configured after the next reboot. show port: Similar to the show interface command on routers, this command gives you the status of ports on a switch. show vlan: With the trend toward having lots of VLANs, check this command to make sure your ports are in the VLANs you think they are. Its output is very well designed. show tech-support: This command is great for collecting a lot of info. It basically runs a whole bunch of other show commands, and spits out dozens of pages of detailed output, designed to be sent to technical support. But, it's also useful for other purposes.