Selective Packet Discard

It applies a form of random packet drop to interface FIFO input queue.
It's a hidden command from IOS parser.

ip spd enable

ip spd mode aggressive --> bad packets are dropped as soon as the minimum threshold for hold-queue
is reached

spd headroom <x> --> BGP, IGP, L2 Keepalives

spd extended-headroom <x> --> IGP, L2 Keepalives

ip spd queue max-threshold <x>

ip spd queue min-threshold <x>

show ip spd

http://www.cisco.com/en/US/products/hw/routers/ps167/products_tech_note09186a008012fb87.shtml