Show vlan brief
The function of the command, ‘show vlan brief’, is to display only a brief summary of all the VLANs within the switch’s VLAN database. The VLAN database is the location on the switch where all VLAN information is stored.
Show vlan id
Verification of VLANs existing in the VLAN database is a crucial part of troubleshooting network connectivity. In scenarios where there are hundreds or thousands of VLANs configured on the switch, ‘show vlan id’ is a helpful command to use to save time. The function of the command, ‘show vlan id’, is to display the configuration for a specified VLAN on a switch.
Show interface status
The function of the command, ‘show interface status’, is to display the interface line status and configuration settings for the switch’s interfaces. ‘Show int status’ gives the network administrator a perfect bird’s eye view, or top level view of the switch’s configuration.
Show run interface
The function of the command, ‘show run interface’, is to display the running configuration of a defined interface. The ‘show run interface’ command is faster and more efficient to view than ‘show run’ because it only shows a specific portion of the running configuration.
show run access-list <id> shows the running configuration on the access list specified
Show interface trunk
The function of the command, ‘show interface trunk’, is to display information regarding all the trunking interfaces on a switch. You have the ability to define a specific Ethernet slot, port-channel, module number, or interface VLAN. If an interface is not specified, the system will display information for all trunk interfaces. The ‘Port’ field in the output of this command is indicating what interface is trunked. The ‘mode’ field is an indicator of what type of mode is presently configured on that trunk. The options are ‘on’, ‘desirable’, or ‘auto’. The mode of ‘on’ will always be present if the command ‘switchport mode trunk’ is configured. ‘Desirable’ or ‘auto’ will be present if Dynamic Trunking Protocol (DTP) is currently negotiating a trunking interface.
The function of the command, ‘show interface’, is to display information about Ethernet interfaces. This command is a helpful tool for troubleshooting. The output of this command is verbose and includes some granular statistics.
Show cdp neighbors
The function of the command, ‘show cdp neighbors’, is to display information of directly connected Cisco devices. Show CDP neighbors only functions on Cisco platforms because CDP is a Cisco proprietary protocol. CDP messages travel over layer 2 as frames.
Show IP route
The function of the command, ‘show ip route’, is to display the current state of the device’s routing table. This command must be entered from a configuration mode of EXEC or privileged EXEC. At the beginning of the ‘show ip route’ command, Cisco displays a code chart used to identify the type of routes in the output.
The function of the command, ‘show arp’, is to display the entries of the Address Resolution Protocol (ARP) table. The first field in the output of this command is ‘Protocol’, such as Internet, or IP. The ‘Address’ field represents the layer 3 network address that corresponds to the layer 2 hardware address. The ‘Age’ field is defined in minutes and the Cisco default ARP cache timeout for an ARP entry is 4 hours, or 240 minutes. The ‘Hardware Addr’ field represents the layer 2 MAC address that corresponds to the layer 3 IP address. The ‘Type’ field represents the encapsulation type the Cisco IOS is utilizing for the entry. The ‘Interface’ field identifies the interface associated with this ARP entry. To manually remove and ARP entry from the ARP table, ‘clear ip arp x.x.x.x’ can be executed in privileged EXEC mode. This is required when you replace a physical device in events such as upgrades or device failures. This must be cleared due to the new MAC address communicating as the same IP address, and the new MAC address not matching the ARP entry in the router.
Show mac address-table
The function of the command, ‘show mac address-table’, is to display the contents of the MAC address table. The MAC address table data is stored in the CAM table, or Content Address Memory, and is used for fast lookups in layer 2 forwarding decisions. The ‘Vlan’ field in the output represents the VLAN associated to the MAC address entry. The ‘Mac Address’ field identifies the layer 2 hardware address of the unique entry.
The function of the command, ‘show monitor’, is to display the status of the Switched Port Analyzer (SPAN) sessions. The purpose of a SPAN session is to capture traffic ingressing, egressing, or bi-directionally traveling across a port and replicate it to another local switchport. A SPAN session can also be configured to monitor VLANs, which is a best practice since it requires less management in the event of switchports being moved on the source device. A use case example of a device that requires this is a device such as an IDS
Show spanning-tree vlan
The function of the command, ‘show spanning-tree vlan’, is to display spanning-tree information for the specified spanning-tree instance. In this command, the VLAN ID range is from 1 to 4094. Depending on the code version, if the VLAN ID value is omitted, the command will generate data on all VLAN spanning-tree instances.
The function of the command, ‘show version’, is to display the versions of system software and hardware that are currently running on the switch or router.
Show vtp status
The function of the command, ‘show vtp status’, is to display general information about the VLAN management domain.
Show etherchannel summary
The function of the command, ‘show etherchannel summary’, is to display a one-line summary per channel-group.
The ‘Flags’ field documents the various flags that are utilized in the output of this command. This command also gives the total number of channel-groups in use on the switch.
A channel-group, or port-channel, is an aggregation, or bonding, of multiple physical interfaces to make one logical interface. Port-channels are used to increase bandwidth and redundancy. The last field of this command shows a listing of each channel-group in a one-line summary format. Each of these summary listings document the port-channel ID, the protocol used in the port-channel, and the individual ports that create the port-channel.
Show standby brief
The function of the command, ‘show standby brief’, is to display a single line summary for each standby group. The ‘Interface’ field in this command identifies the interface or Switch Virtual Interface (SVI) that has Hot Standby Router Protocol (HSRP) operating on it. The ‘Grp’ field identifies the standby group associated. The Rackspace standard is to name IPv4 groups standby 1, and IPv6 groups standby 6. The ‘Pri’ field indicates the priority for the HSRP instance. With HSRP, the highest priority takes the active role, while the lower priority is standby. The Cisco default priority value is 100.
The function of the command, ‘show port-security’, is to display port security settings for all switchports on the switch. The ‘Secure Port’ field identifies the switchports configured to have port-security. The output of this command documents the count of max secure addresses, current addresses, and security violations. The ‘Security Action’ field identifies the action the switch will take when a violation occurs.
The function of the command, ‘show history’, is to display a list of the commands you have entered in the current session. The buffer for this command is limited to the previous 20 commands. Alternatively, you can also press the up arrow on the keyboard to recall the commands instead of retyping a command recently entered.