Have you ever come across a Cisco IOS command that made you say to yourself, “I wish I had known about this command years ago!”? And then once you found it, you wondered how you ever could have gotten by without it? Here are a few of my “go-to” IOS commands:
PING <destination ip address> SOURCE <source ip address>
This command allows you to send PING packets using the source IP address of any interface configured on the router. It is useful when testing access-lists or policy-based routing and you don’t have access to a host in the subnet the packets should be sourced from.
SHOW VLAN ID <#>
This command will display which ports a certain VLAN (Virtual Local Area Network) is assigned to whether the port be configured as an access or a trunk port. In comparison, the “show vlan” command itself will only display access ports that the VLAN is assigned to.
SHOW INTERFACE <interface> SWITCHPORT
A port configured to be part of a VLAN that is no longer found in the switch VLAN database becomes orphaned and unusable. This command will signify the VLAN is not in the database by designating the access port as “INACTIVE.”
TELNET <destination ip address> <destination port number>
This command helps in testing the upper layers of the protocol stack. Although your router and switch are not web or email clients, they can simulate one if you telnet with the destination port number of the application you are testing. For example, if you issue the command TELNET 10.1.1.1 80 from the router or switch, and it displays a reply similar to <i>HTTP/1.0 404 R Content-Type: text/html</i>, as if the router or switch were a web browser, you have successfully tested the HTTP application.
What are some of the lesser known Cisco IOS commands that you use often and are effective in managing your network?