VLAN Tagging

Unanswered Question
Feb 18th, 2010

I have a 3845 router with a 16 port switch module. The 16 port switch module has multiple vlans, and is running in transparent mode. The router has multiple sub-interfaces all running dot1q encap.

When the sub-interface on the router receives a frame from vlan 12, and it needs to route that packet out its Gig0/0 interface, will the router carry over the vlan tagging?

I have this problem too.
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xcz504d1114 Thu, 02/18/2010 - 08:04

No, it will not carry over the VLAN 12 tag after it traverses a

layer 3 boundary. If you need it to remain in VLAN 12 as it goes out Gig 0/0 interface, you either need to configure it to do so on gig 0/0 via subinterfaces / dot1q commands. Alternatively (and less scalable) you can leave the gig 0/0 interface as it is currently configured, and have the next device (switch I'm assuming) be configured to place it in vlan 12 (for example, just a normal access port in VLAN 12).

However, you mention the 16 port switch module, which introduces many different configurations, without knowing your exact configuration, source, and destination, it is hard to answer your question accurately. But if the packet is traversing a layer 3 boundary, and not being tunneled, it will not keep its VLAN tag, the moment the Layer 2 no longer needs to pass the tag, it will strip it off (example: sending information to your PC is a non-trunk port, your PC doesn't recieve tagged frames, same concept in a router with a switch module, the layer 2 engine will strip the tag before sending out its interface to the layer 3 engine).

HTH,

Craig

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