How does class based queuing work with interface queueing?

Simple query: What queues are involved in the output of a packet from a router?

Specifically: When applying a service policy for output packets one can choose such as CBWFQ or PQ or CQ etc. Once the policy has done its queuing/queueing what happens with the packet? Does it immediately get lined up for the transmit ring or does it then also go through the queueing for the interface and only then go to the transmit ring?

Or does it depend on the architecture of the specific router model or family? I'm confused as to the details and cannot find an answer on Cisco's site.

The reason for asking is that some recent IOS versions don't allow the class-default to have fair-queueing applied. We have fair queueing on the interface itself but from customer reports traffic in the default class seems to NOT get fair queue treatment. I can't find out if this is right or not so any pointers would be appreciated.

-- TIA, James

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Agreed that queueing only happens when there is congestion - i.e. when there is no space in the TX ring. When that happens, though, given that there is congestion? After the policy queueing does the router obey the interface queueing subsequently - or does it override it so the packet goes direct from the CB queue into the TX ring?

I suspect it overrides or ignores the queueing configured on the interface. The problem, then, if that is correct, is why more recent versions of IOS don't allow the default class to be fair-queued. (e.g

12.2(16) on a 2600 supports flow based fair queueing in the default class but 12.2(20)S6 on a 7300 does not.) Weird!
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