Understanding Selective Packet Discard (SPD)
SPD State Check
The IP process queue on the RP is divided into two parts: a general packet queue and a priority queue. Packets put in the general packet queue are subject to the SPD state check, and those that are put in the priority queue are not. Packets that qualify for the priority packet queue are high priority packets such as those of IP precedence 6 or 7 and should never be dropped. The non-qualifiers, however, can be dropped here depending on the length of the general packet queue depending on the SPD state. The general packet queue can be in three states and, as such, the low priority packets may be serviced differently:
- NORMAL: queue size <= min
- RANDOM DROP: min <= queue size <= max
- FULL DROP: max <= queue size
In the NORMAL state, we never drop well-formed and malformed packets.
In the RANDOM DROP state, we randomly drop well-formed packets. If aggressive mode is configured, we drop all malformed packets; otherwise, we treat them as well-formed packets.
In FULL DROP state, we drop all well-formed and malformed packets. These minimum (default 73) and maximum (default 74) values are derived from the smallest hold-queue on the chassis, but can be overridden with the global commands ip spd queue min-threshold and ip spd queue max-threshold.
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