Default reserved VLAN numbers on Cisco IOS and NX-OS

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To show the reserved VLAN numbers on both IOS and NX-OS, the common command is:

show vlan internal usage

IOS

Basically, VLAN numbers from 1002 to 1005 in Cisco IOS are reserved. On Cisco's web sites more information are listed below:

Catalyst 6500 Release 12.2SX Software Configuration Guide - VLANs [Cisco Catalyst 6500 Series Switches] - Cisco

Table 23-1 VLAN Ranges

VLANs	Range	Usage	Propagated by VTP
0, 4095	Reserved	For system use only. You cannot see or use these VLANs.	—
1	Normal	Cisco default. You can use this VLAN but you cannot delete it.	Yes
2-1001	Normal	For Ethernet VLANs; you can create, use, and delete these VLANs.	Yes
1002-1005	Normal	Cisco defaults for FDDI and Token Ring. You cannot delete VLANs 1002-1005.	Yes
1006-4094	Extended	For Ethernet VLANs only.	No

NX-OS

Basically, VLAN numbers from 3968 to 4094 in NX-OS are reserved. More details are listed on Cisco's web site below:

Configuring VLANs - Cisco

Table 1 VLAN Ranges

VLANs Numbers	Range	Usage
1	Normal	Cisco default. You can use this VLAN, but you cannot modify or delete it.
2—1005	Normal	You can create, use, modify, and delete these VLANs.
1006—3967 and 4048—4093	Extended	You can create, name, and use these VLANs. You cannot change the following parameters: The state is always active. The VLAN is always enabled. You cannot shut down these VLANs.
3968-4047 and 4094	Internally allocated	These 80 VLANs and VLAN 4094 are allocated for internal device use. You cannot create, delete, or modify any VLANs within the block reserved for internal use.
3968-4094	Internally allocated	Beginning with Cisco release 5.2(1) for Cisco Nexus 7000 Series devices, VLANs 3968 to 4094 are reserved for internal use in each VDC by default. You can change the reserved VLANs to any other 128 contiguous VLAN range. When you reserve such a range, it frees up the range of VLANs that were allocated for internal use by default, and all of those VLANs are available for user configuration except for VLAN 4094. All VDCs inherit the new reserved range of VLANs.

In NX-OS, we can even change these "internal used" VLAN numbers by the command: system vlan X reserve

When you change the range of reserved VLANs, the existing configurations for the new range of VLANs get deleted. A warning note is displayed as in the following example:

switch(config)# system vlan 2000 reserve
This will delete all configs on vlans 2000-2127. Continue anyway? (y/n) [no] y
Note: After switch reload, VLANs 2000-2127 will be reserved for internal use.
      This requires copy running-config to startup-config before
      switch reload. Creating VLANs within this range is not allowed.
switch(config)#

Why do we need to change the reserved range? In case we insist to use VLAN numbers from 3968 to 4094 in our production network, we will need this command.

We only have to decide the first new reserved VLAN number, and then all the contiguous 128 numbers would be reserved.

We also have to save the configuration and reboot to take effect.