Information Technology Reference
In-Depth Information
The SPD process divides the queue to the route processor into a general packet queue
(GPQ) and a priority queue. The packets in the GPQ are subject to the preemptive discard
mechanism in SPD. The GPQ is for IP packets only, and it is a global queue, not a per-
interface queue. The packets in the priority queue are not subject to this discard process.
The SPD random discard process is done through the SPD state check on the GPQ. Two
thresholds determine the GPQ's SPD state: the minimum threshold and the maximum
threshold. There are three SPD queue states:
•
Normal state:
GPQ depth <= minimum threshold
•
Random Drop state:
minimum threshold < GPQ depth <= maximum threshold
•
Full Drop state:
maximum threshold < GPQ depth
The SPD state check uses the queue's state to determine the action to take on the packet.
When the queue is in a Normal state, packets are not discarded. If the queue depth crosses
the minimum threshold, the SPD process is in Random Drop state. At this point, SPD begins
discarding normal-priority packets randomly. If the queue depth crosses the maximum
threshold, the SPD process is in Full Drop state. All normal-priority packets are discarded
until the queue depth drops below the maximum threshold.
In Random Drop state, SPD can operate in two modes: normal and aggressive. When
operating in aggressive mode, SPD drops malformed IP packets. In normal mode, SPD
does not pay attention to whether packets are malformed.
The minimum and maximum thresholds are determined by the smallest hold queue on the
router. The minimum threshold is 2 less than the size of the queue. The maximum threshold
is 1 less than the size of the queue. This is to ensure that no interface will be throttled, which
is what occurs when the input queue is completely full.
In addition to providing queue management, the SPD process is also used to protect high-
priority traffic. Although Random Drop state and Full Drop state drop only normal-priority
traffic, two additional extensions to the input queue are available to high-priority traffic:
SPD Headroom and SPD Extended Headroom.
SPD Headroom allows the input queue to exceed the configured input hold queue. If the
input hold queue is 75 and the SPD Headroom is 100, which are the default values, the input
hold queue can hold 175 packets. As soon as the input hold queue reaches its maximum
