2950 vlan limitation

[barryfowles]

Is there any way around the limit of 64 vlans on a 2950? We have a customer with a vtp enabled network with a mix of older Cisco switches and we have exceeded the 64 vlan limit supported by some of these switches. The only solution seems to be to manually add the new vlans to the switches. Can vtp advertisements be blocked for specified vlans?

[Georg Pauwen]

Hello,

you could either enable VTP pruning on the VTP server (vtp pruning global command), or you could specify which VLANs to allow on the trunk to the 2950:

switchport trunk allowed vlan

This way, you could get around the maximum VLAN limit. This, obviously, only works when you do not need 64 VLANs on your 2950...

HTH,

GP

[barryfowles]

The customer doesn't require many of the total number of vlans on each switch. I did think that vtp pruning only blocked broadcast traffic from the trunk to increase bandwidth. The problem I am told is that vtp advertises all of the vlans created on the vtp server to all switches and that once the 64 vlan limit is breached the switches require manual addition of vlans. So even if the vlans were not required on a particular switch vtp would still advertise all of them to the switch even if they were cleared from the trunk using the above command.

[ankurbhasin]

Hi Bfowles,

You can block complete VTP advertisement which I do not think will be a right decision but you cannot block VTP for specified vlans.

Regards,

Ankur

[sundar.palaniappan]

I don't think, you can tell VTP which VLANs you want to prune. It does it dynamically.

However, just like the previous poster mentioned you can manually block/allow certain VLANs on your trunk.

Since you are manually creating VLANs in the switches that doesn't support more than 64 vlans, an eloquent solution would be disabling VTP (transparent mode) in those switches and keep it all static VLANs. These switches will still forward the VTP updates to other switches running VTP but they will not be using them. This way , you would have better control and simplicity of configuration.

[Georg Pauwen]

Hello,

just to avoid confusion: setting a switch to transparent will stop all VTP advertisements and make the switch effectively a standalone switch, with all the additional administrative overhead. I would not recommend it, unless you really need more than 64 VLANs on your 2950, which is actually not likely.

HTH,

GP

[glen.grant]

Is it standard image or EI? I believe EI supports up to 250 with standard supporting 64.

[barryfowles]

They are 2950G switches which are EI. I thought the 64 vlan limitation was a hardware one not software. The software versions may be quite old, i.e., 18 months or more. If newer software supports more vlans they would upgrade but I didn't think it did.

[glen.grant]

By reading the below my interpretation is that the EI will support 250 vlans but will only support 64 individual instances of spanning tree pvst+ . Maybe I'm wrong but that is what it looks like .

Catalyst 2950 switches that run the standard software image (SI) support 64 VLANs; Catalyst 2950 and Catalyst 2955 switches that run the enhanced software image (EI) support 250 VLANs. Refer to the release notes for the list of switches that support each image. VLANs are identified with a number from 1 to 4094 when the EI is installed and 1 to 1005 when the SI is installed. VLAN IDs 1002 through 1005 are reserved for Token Ring and FDDI VLANs. VTP only learns normal-range VLANs, with VLAN IDs 1 to 1005; VLAN IDs greater than 1005 are extended-range VLANs and are not stored in the VLAN database. The switch must be in VTP transparent mode when you create VLAN IDs from 1006 to 4094.

The switch supports per-VLAN spanning-tree plus (PVST+) and rapid PVST+ with a maximum of 64 spanning-tree instances. One spanning-tree instance is allowed per VLAN. See the "Normal-Range VLAN Configuration Guidelines" section for more information about the number of spanning-tree instances and the number of VLANs. The switch supports IEEE 802.1Q trunking for sending VLAN traffic over Ethernet ports.

The switch supports 64 spanning-tree instances. If a switch has more active VLANs than supported spanning-tree instances, spanning tree can be enabled on 64 VLANs and is disabled on the remaining VLANs. If you have already used all available spanning-tree instances on a switch, adding another VLAN anywhere in the VTP domain creates a VLAN on that switch that is not running spanning-tree. If you have the default allowed list on the trunk ports of that switch (which is to allow all VLANs), the new VLAN is carried on all trunk ports. Depending on the topology of the network, this could create a loop in the new VLAN that would not be broken, particularly if there are several adjacent switches that all have run out of spanning-tree instances. You can prevent this possibility by setting allowed lists on the trunk ports of switches that have used up their allocation of spanning-tree instances.

If the number of VLANs on the switch exceeds the number of supported spanning tree instances, we recommend that you configure the IEEE 802.1S Multiple STP (MSTP) on your switch to map multiple VLANs to a single STP instance. For more information about MSTP, see Chapter 15, "Configuring MSTP."