Maximum VLANs in transparent mode

[jamesl0112]

On a low-end switch like a 2960 the maximum VLANs is 255, as shown in the output of VTP status:

Maximum VLANs supported locally   : 255

Number of existing VLANs          : 245

When the VTP mode is changed to transparent, VLANs from the extended range can be added without increasing the number of existing VLANs - e.g. if I create VLANs 3000 - 4000 the number of existing VLANs is still listed as 245.

If MST is used to cut down on the number of spanning-tree instances - and assuming propagation of VLAN configuration via VTP is not required - is there any downside to using transparent-mode VTP to increase the number of available VLANs? It does feel like I'm cheating the maximum listed in the datasheet.

This is just a lab, I'm aware that normally if you wanted a large number of VLANs you might use a more high-end switch.

[Giuseppe Larosa]

Hello James,

in low end switches there is a limit on the max number of Vlans that can exist on the device at the same time you can get some advantage in VTP transparent mode but you cannot get over the platform limit.

the advantage is the access to the full range of IEEE Vlan-ids as you have noted.

MST can provide some benefits because the limit of STP instances in PVST+ can even be lower ( it happened on C2950 in some models) and likely it should support 16 instances.

The limitation is in the hardware implementation that probably uses a single octet table for the vlan pointers and not a 12 bit field.

You should carefully consider to purge unused vlans and to use an explicit list of permitted vlans on L2 trunk ports of the device.

Also routed ports are implemented by consuming internally a vlan number ( like an SVI with a single L2 port) for re-using the logic of standard SVIs so also routed ports count against the total number of vlans.

Hope to help

Giuseppe

[jamesl0112]

Hi,

However, surely I have gone over the platform limit - if I had 245 standard VLANs, and have now created 100 new VLANs from 3000 - 3100 - I should have received an error but the configuration was accepted. show vtp status still has 245 as the current number of VLANs.

[Giuseppe Larosa]

Hello James,

well C2960 may be brighter then older models so I may have underestimated its capabilities, the real question is if the additional Vlans can be used for example allowed in a trunk port.

Edit:

http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps6406/product_data_sheet0900aecd80322c0c.html

data sheet says max 255 Vlans in table 4. Documentation may be wrong. also for C2960-S.

Best Regards

Giuseppe

[jamesl0112]

Hi,

Yes, the additional high-number VLANs (that aren't being counted towards the 255 limit) do work across trunks - I have trunked them to another switch and then onto a server's NIC as a test - the server could ping VLAN interface IPs that I temporarily set up on both switches.