Internal VLANs are used for services like MPLS, FCoE, Multicast over GRE, enhancement to SPAN, etc. Some Features have special requirements like which VLAN can be reserved for them. Example of such service are Multicast VLAN which can only start with VLAN id’s that is multiple of 64.
Prior to release 5.2(1) the reserved VLAN range was 3968 to 4048, and 4094, and it was not configurable. After the upgrade user-defined VLANs might fall within the new reserved range which now range from 3968 to 4095 and is configurable. If that occurs, switch fallback to old range but the features that need the additional reserved VLANs won’t work propely. What you have to do is change range of reserved VLANs using command:
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.
Now the disadvantage if this is you have to reboot whole chassis. Simple switchover between supervisors on Nexus 7000 won’t be enough. Also, if you are using vPC if one switch is using new VLAN range and other one is still configured to use old range switches will not forward those VLANs on vPC peer-link. Hence, those VLANs will get suspended on vPC port-channel. But this should not affect any other VLANs in vPC.