Maximum VLAN tag per 802.1Q trunk

[intertouch]

Hi,

Can I know the maximum number of VLANs that allowable to be transmit per 802.1Q trunk?

[pkhatri]

Hi,

The VLAN ID within the 802.1q frame format is 12bits long, which implies a theoretical maximum of 2^12 VLAN IDs, or 4096 tags.

By using q-in-q, you can support a larger number of tags though...

Hope that helps.. pls rate the post if it does.

Paresh.

[intertouch]

I agree with you that technically it should be 4096 VLANs. However, I believe Cisco tweaking it and made it only allow 66 VLANs per trunk for lower end switches. I am not sure the main reason for it.

[m.mcconnell]

Actually, the "Cisco" limitation you speak of is 64 VLANs and is not a limitation of the trunk port so to speak. This limitation is a memory limitation on small 1U switches (3500XL, 2950, etc). These switches just don't have enough memory to run STP for more than 64 VLANs. On chassis based switches (4500, 6500) this limitation does not exist.

-Mark

[pkhatri]

Just to add to Mark's post, there is a difference in theoretical and practical limitations and it may come down to the hardware/software in use. A similar situation exists for ATM switches where you could have (theoretically) up to 255*65535 (16711425) PVCs but there aren't too many switches around that can handle that many !!

Paresh.