Layer 2 keepalives and IGP packets are allowed to be enqueued in any of the 3 queues in the following order: Extended Headroom, Headroom, Input Queue.
Other packets with IP Precedence 6, like BGP are allowed to be enqueed, in this order, only in the Headroom and the Input Queue.
All other packets are only allowed in the Input Queue.
When they are serviced, the Extended Headroom takes Priority over the Headroom and over the Input Queue