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New Member

question on VLAN's

Folks,

What really is the desicion maker to find out what number of VLAN's will be supported on a Cisco switch?

I think during the CCNA course I read that enabling vtp mode transparent supports upto 4096 VLAN's. Some place I read that MST can support more VLAN's.

How can we, on the switch, find out the number of VLAN's we can configure on a particular platform?

Regards,

Nik

5 REPLIES

question on VLAN's

Hi Nik,

Enter global config mode and type 'vlan ?' :

(config)#vlan ?

  WORD        ISL VLAN IDs 1-4094

  access-map  Create vlan access-map or enter vlan access-map command mode

  dot1q       dot1q parameters

  filter      Apply a VLAN Map

  group       Create a vlan group

  internal    internal VLAN

cheers,

Seb.

Cisco Employee

question on VLAN's

Hello Seb,

I respectfully disagree.

The allowed range of VLANs, and the maximum number of concurrently created VLANs are idependent. All common Catalyst platforms support full range of VLAN IDs, i.e. 1 to 4094 inclusive. However, the number of concurrently existing VLANs differs based on the platform.

Fortunately, there is a simple way of finding out the maximum number of supported VLANs - using the show vtp status command.

On an 2950T:

Sw2950# show vtp status

VTP Version                     : 2

Configuration Revision          : 81

Maximum VLANs supported locally : 250

Number of existing VLANs        : 36

On an 2960:

Sw2960# show vtp status

VTP Version                     : running VTP2

Configuration Revision          : 81

Maximum VLANs supported locally : 255

Number of existing VLANs        : 36

On an 3560:

Sw3560# show vtp status

VTP Version                     : running VTP2

Configuration Revision          : 81

Maximum VLANs supported locally : 1005

Number of existing VLANs        : 36

The relation of the maximum number of locally supported VLANs to the STP version is a different story. With Cisco's Per-VLAN (Rapid) STP, there is a separate instance of (R)STP running for each created VLAN. There is a software and a hardware limit on how many STP instances can be run on a single device. Usually, this limit is 128 STP instances (older 2950 switches supported around 64 STP instances). This means in turn that if you have more than 128 VLANs, only the first 128 VLANs will be running STP. All remaining VLANs will not be protected by STP, so if there is a physical loop in the network, there will also be a switching loop in all the remaining VLANs. So this effectively limits you in the number of VLANs you can use (not how many you can create).

Because MST runs in instances that are independent of VLANs, and the VLANs are simply mapped onto these instances, you can have 4094 VLANs mapped onto just a handful of instances - perhaps 3, 4... tens at most. Here, MST is not the "bottleneck" because the number of MST instances you run is independent of the number of VLANs, and can be orders of magnitudes lower - you can have thousands of VLANs, yet just a pair of MST instances. That is why using MST allows you to use more VLANs.

Best regards,

Peter

New Member

question on VLAN's

Thanks Peter,

So vtp mode(client, transparent) does not affect the number of VLAN's we can configure. I read something like that in the CCNA books.

Can any command display number of STP instances that the hardware supports?

Even in MST, I can simply club all the VLAN's into a single instance, correct? This would be a bad design but this is permitted.

Regards,

Nikhil Kulkarni.

Cisco Employee

question on VLAN's

Hello Nikhil,

So vtp mode(client, transparent) does not affect the number of VLAN's we  can configure. I read something like that in the CCNA books.

Well, to be honest, VTP mode and version pose an additional limit on the range of VLAN IDs you can create (and indirectly, on the number of concurrently existing VLANs as well, logically). If you are running VTPv1 or VTPv2 in server or client mode, these VTP versions support only normal-range VLANs 1-1005. If you want to use extended-range VLANs 1006-4094, you either have to go to VTP transparent mode, or use VTPv3 that supports the full range of VLAN IDs.

Can any command display number of STP instances that the hardware supports?

Honestly, I do not recall seeing such command.

Even in MST, I can simply club all the VLAN's into a single instance, correct? This would be a bad design but this is permitted.

Yes, it is perfectly permitted. In fact, if you do not change VLAN-to-instance mappings, all VLANs are in the instance 0. Whether that is a bad design is a matter of debate - if you do not need multiple spanning-trees, why run multiple instances?

Best regards,

Peter

Super Bronze

Re: question on VLAN's

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Posting

Even in MST, I can simply club all the VLAN's into a single instance, correct? This would be a bad design but this is permitted.

As also noted by Peter, having all your VLANs contained within a single MST instance isn't necessarily a bad design.  Multiple MST instances are often used to allow L2 load balancing, of multiple VLANs, across multiple L2 physical paths, without having to manage each VLAN's L2 topology individually.

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