Need help for Switch VLAN

Unanswered Question
Jan 12th, 2010

Hi, I have one Layer3 switch name as SW-SUP having vlans- IT, MKT, FNC & OPS. It has layer 2 connectivity with L2 switch A & B. All are in VTP Transparent mode. I have configured Vlan 999 on all switches for default gateway connectivity.

On Switch - A, I have created all four Vlans & on B, I have created only IT, MKT & FNC Vlan. My question is, if I ping OPS vlan workstations IP from Switch B, will it reply or drop on switch-B itself.

I have this problem too.
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Peter Paluch Tue, 01/12/2010 - 00:48

Hi Rupesh,

A short answer is that if you have a switched network then the VLANs should be created on all switches. A switch that does not know about a particular VLAN will drop the frames for that VLAN.

Consider three switches in a row, Sw1, Sw2 and Sw3, connected by trunks. On Sw1 and Sw3, the VLAN 100 is created but on Sw2, this VLAN is missing. The stations in VLAN 100 on Sw1 will not be able to communicate with stations in VLAN 100 on Sw3 until the VLAN 100 is also created on Sw2.

Your question asks whether you can successfully ping the OPS workstation from the Switch B. As the OPS VLAN is probably not the VLAN 999, the Switch B will send its frames towards the default gateway in VLAN 999. The success of the ping now depends on whether the gateway can reach the OPS VLAN. It can be that your Switch B can reach the default gateway but it will re-tag the frame to the OPS VLAN which is not allowed to traverse your switched network as explained above.

I am sorry that this is probably a confusing and unclear answer.

Best regards,


Rupesh Kashyap Tue, 01/12/2010 - 02:18

Thanks Peter. You mean, even we have perfect reachability on VLan 999 on all switches, we can not ping OPS vlan workstations as they can be dropped on returning.

Peter Paluch Tue, 01/12/2010 - 02:46

Hello Rupesh,

The frames can be dropped either on their way from the router to the OPS station, or on their way back from the OPS station to the router. This depends on where exactly is the router connected that routes between the VLAN 999 and the OPS VLAN and what switches must the frames in either direction traverse.

Best regards,



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