Selective Packet Discard
When the input queue of an interface fills up with traffic destined to the router, there's a good chance that the next packet dropped will be important to the state of the network itself—a routing protocol update or a link keepalive, for instance. Selective Packet Discard (SPD) allows IOS to prioritize traffic that is important to the network's stability when an interface's input queue is congested.
Essentially, any packet recognized by IOS as being important to the stability of the network is given preferential treatment during the process of queuing it on the input queue. SPD is available on all Cisco routers.
The command show ip spd shows SPD details, as demonstrated in Example 8-16.
Example 8-16. show ip spd Displays ...
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