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QID:BCM21
#1
To Admin / Skilled other,

One of the answers to this question is that 802.1Q trunk can only be configured on a layer 2 port.

I disagree, it can be configured on a router layer 3 port (sub interfaces etc), hence router on a stick.

Please advise.



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#2
Subinterfaces on a router are not trunk ports.  Trunk ports transmit all vlans whereas a router subinterface recognizes only the vlan for which it is configured.  That's why you need multiple subinterfaces to connect to one trunk line coming from a switch.  Each subinterface needs to have an explicit command to encapsulate the ip packets in a dot1q frame of its own particular vlan.  I'd need to research more to give a better explanation.   

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#3
Thanks Graham,

I don't know, the question states that trunk links can only be formed on layer 2 interfaces, given  a router-on-a-stick deployment, whereby a trunk is formed from the layer 2 interface of the switch to the layer 3 interface of the router, I believe this question to be wrong.

I think it's one of those question that Cisco throws in to catch you out.

Granted there may be other factors involved such as encapsulation and other mechanism, but the fact is that the trunk is still formed on the router layer 3 interface, which is routing for a particular subnet.

I can also form a trunk on a routed port on my firewall, which is also a layer 3 interface.

Hopefully someone can provide a solid source / reference to support this or state otherwise.
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