vlan with trunks question

marcusaureliusbrutus

Hi,

Sorry am a bit confused. If i have 3 switches named A, B, and C, where A is connected to B and B is connected to C and C is connected to A. vlan2 is configured on SW A but not on sw B so no stp blocking appears for vlan2. SW C also has vlan 2. When a station on vlan 2 broadcasts packets, will it still go to sw B and will switch B forward it to sw C? By the way these switches are L3 switches.

Thanks.

yuriz43

When a station on vlan 2 broadcasts a frame it will go to Switch B, but switch B will do nothing with the frame. The only way to prevent this is with VTP pruning, or by manually removing that vlan from the trunk links to switch B.

So on your trunk links to switch B you would do:

switchport trunk allowed vlan remove 2

OR enable VTP, and VTP pruning on all of your switches.

marcusaureliusbrutus

Hi yuriz43,

Thanks for the reply.

If i use:

switchport trunk allowed vlan remove 2

Does this mean that all packets destined to vlan 2 will be dropped on the trunk port of SWA connected to SWB?

Also, will this filtering affect the CPU of SWA?

Thanks again.

yuriz43

marcusaureliusbrutus wrote: »

Does this mean that all packets destined to vlan 2 will be dropped on the trunk port of SWA connected to SWB?

Correct.

marcusaureliusbrutus wrote: »

Also, will this filtering affect the CPU of SWA?

It' reduces unneeded traffic on the link to switch B. No effect on the CPU.