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Community Member

Trunking question

Switch A is trunked to switch B and switch B is trunked to switch C (all 802.1Q).

Switch A has VLAN50 configured on it.

Switch B does not have VLAN50 configured on it.

Switch C has VLAN50 configured on it.

All switches are in VTP transparent mode.

Will VLAN50 traffic on switch A get to switch C?


Re: Trunking question

Hi Peter

I don't think that traffic from VLAN50 will get from Switch A to Switch C.

With the switches in transparent mode, each switch only knows about its own VLAN's. And even though you have a VLAN50 on Switch A and a VLAN50 on Switch B, these are not the same VLAN.

What you would need to do is to have all your switches as VTP Servers or 1 switch as a VTP server and the other 2 switches as clients. In this configuration all three switches would know about VLAN50 and even if switch B had no hosts in VLAN50 attached to it, it would still pass traffic for VLAN50 from switch A over the trunk to switch B.

Best Regards,


Hall of Fame Super Bronze

Re: Trunking question

> Will VLAN50 traffic on switch A get to switch C?

No, unless you dot1q-tunnel that Vlan, but that's another exercise :)



Community Member

Re: Trunking question

Transparent mode will not process VTP info across trunk BUT will send all packets.

With 3 2950's, I have setup trunks with the above config and a show interfaces trunk displays vlans 1-4096 allowed on trunks of all switches.

I also setup 2 pc's on switch A and C on Vlan 50 and was able to ping from one to the other.


Re: Trunking question

That's very interesting.

I would be interested to see switch B show spanning-tree vlan 50. If it has a spanning-tree for that VLAN, we must ask ourselves why. If it has no spanning tree for that VLAN, then that would imply that a transparent switch could cause a loop.

Did you try show vlan on switch B, just to be 100% sure there was no VLAN 50 there?

Kevin Dorrell


Community Member

Re: Trunking question

You are right. Vlan 50 is configured on B. I was testing concept of transparent switches sending vlan 50 without server/client mode.

I did not notice "no vlan 50 on B" in original post.


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