Vlan Internal Usage

[kris-andrews]

Hi All,

Can someone please clarify the "show vlan internal usage" command on a catalyst 6500.

I have read some docs on the web but i'm still a little unclear as to what vlans are clasified as internal usage?

Can someone give me a explanation around this?

Many Thanks

Kris

[Peter Paluch · Accepted solution]

Hi,

Internal usage VLANs are created for a specific purpose in a switch. Creating a routed port (no switchport) results in the switch silently creating a specific internal VLAN and assigning the routed port into that VLAN. I am not certain if there are also another situations where a switch creates an internal VLAN but the routed port is the most common reason.

From the 3750 Configuration Guide at

http://www.cisco.com/en/US/docs/switches/lan/catalyst3560/software/release/12.2_55_se/configuration/guide/swvlan.html

• Each routed port on the  switch creates an internal VLAN for its use. These internal VLANs use  extended-range VLAN numbers, and the internal VLAN ID cannot be used for  an extended-range VLAN. If you try to create an extended-range VLAN  with a VLAN ID that is already allocated as an internal VLAN, an error  message is generated, and the command is rejected

  • Because  internal VLAN IDs are in the lower part of the extended range, we  recommend that you create extended-range VLANs beginning from the  highest number (4094) and moving to the lowest (1006) to reduce the  possibility of using an internal VLAN ID.

  • Before configuring extended-range VLANs, enter the show vlan internal usage privileged EXEC command to see which VLANs have been allocated as internal VLANs.

Best regards,

Peter