Cisco CCNP Certification / BCMSN Exam Tutorial:

The show interface switchport Command

By Chris Bryant, CCIE #12933

We're all familiar with show interface, but there's a slight variation on this command when it comes to Cisco switches that will give you a great deal of helpful information when it comes to troubleshooting - show interface switchport.   There's actually a very common issue indicated in this output - can you spot it? Better yet, do you know what causes it?

SW1#show interface fast 0/2 switchport
Name: Fa0/2
Switchport: Enabled
Administrative Mode: dynamic desirable
Operational Mode: down
Administrative Trunking Encapsulation: dot1q
Negotiation of Trunking: On
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 1 (default)
Voice VLAN: none
Administrative private-vlan host-association: none
Administrative private-vlan mapping: none
Administrative private-vlan trunk native VLAN: none
Administrative private-vlan trunk encapsulation: dot1q
Administrative private-vlan trunk normal VLANs: none
Administrative private-vlan trunk private VLANs: none
Operational private-vlan: none
Trunking VLANs Enabled: ALL
Pruning VLANs Enabled: 2-1001
Capture Mode Disabled
Capture VLANs Allowed: ALL
Protected: false
Appliance trust: none

From top to bottom, you can see whether the switchport is enabled, what the trunking mode is ("administrative mode"), what trunking encapsulation is in use, whether trunking's being negotiated or not, what the native VLAN is, and so forth.   This is an excellent VLAN and trunking troubleshooting command.

And the problem?  The interface is shut!   :)   Here's what the output looks like when the interface is open.

SW1#show interface fast 0/2 switchport
Name: Fa0/2
Switchport: Enabled
Administrative Mode: dynamic desirable
Operational Mode: static access
Administrative Trunking Encapsulation: dot1q
Operational Trunking Encapsulation: native
Negotiation of Trunking: On
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 1 (default)
Voice VLAN: none
Administrative private-vlan host-association: none
Administrative private-vlan mapping: none
Administrative private-vlan trunk native VLAN: none
Administrative private-vlan trunk encapsulation: dot1q
Administrative private-vlan trunk normal VLANs: none
Administrative private-vlan trunk private VLANs: none
Operational private-vlan: none
Trunking VLANs Enabled: ALL
Pruning VLANs Enabled: 2-1001
Capture Mode Disabled
Capture VLANs Allowed: ALL
Protected: false
Appliance trust: none

The reason I'm pointing that out is that with the basic show interface command, you'll see the phrase "administratively down" - and you know from your CCNA studies that this phrase really means "you forgot to open the interface."  

SW1#show interface fast 0/2
FastEthernet0/2 is administratively down, line protocol is down (disabled)

When you run show interface switchport, you're not going to see "administratively down", but just "down" - which may lead you to look for a more complex solution.  It certainly did that to me once!  Just remember to always check the interface's open/shut status first, no matter what the router or switch is telling you.  : )

Here's what the output looks like when a trunk port is specified.  Note that you can also see what VLANs are allowed across the trunk and which VLANs are being pruned.

SW1#show interface fast 0/11 switchport
Name: Fa0/11
Switchport: Enabled
Administrative Mode: dynamic desirable
Operational Mode: trunk
Administrative Trunking Encapsulation: dot1q
Operational Trunking Encapsulation: dot1q
Negotiation of Trunking: On
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 1 (default)
Voice VLAN: none
Administrative private-vlan host-association: none
Administrative private-vlan mapping: none
Administrative private-vlan trunk native VLAN: none
Administrative private-vlan trunk encapsulation: dot1q
Administrative private-vlan trunk normal VLANs: none
Administrative private-vlan trunk private VLANs: none
Operational private-vlan: none
Trunking VLANs Enabled: ALL
Pruning VLANs Enabled: 2-1001
Capture Mode Disabled
Capture VLANs Allowed: ALL
Protected: false
Appliance trust: none

show interface switchport is a valuable command to know for both your CCNP BCMSN exam and for use in the real world - be familiar with its output for the exam room and the network room!

To your success,

Chris Bryant

CCIE #12933

chris@thebryantadvantage.com