Vlan allowed statements on switchport interfaces

[lerdalt]

Need some help clarifying the function of the following command:

switchport trunk allowed vlan 1,200-207,217,280,1002-1005

I understand that this is only allowing these vlans to communicate across the link. And if I'm understanding it correctly, the main purpose is to prevent broadcasts from other devices on the same switch from crossing that link...for instance if a vlan 4 existed on the same switch.

The only thing that a co-worker and I are not clear on is if it is IP only broadcasts or if IPX is included. I think it is, but am having difficulty tracking down an answer.

Thanks

 

[helpdeskdan]

Hi Lerdalt!

The allowed Vlans specifies just that - allowed vlans. IPX broadcasts use the same layer 2 broadcast address (FF:FF:FF:FF:FF:FF) as IP so at the vlan level they would act exactly the same.

When you disallow a vlan on a trunk, no traffic from that vlan will go across that trunk. We do it to contain broadcasts but it's accomplished by disallowing ALL vlan traffic not just broadcasts.

 

[lerdalt]

thanks for clarifying...I thought I had it right in my head.