I don't have much experience with SPD but I believe it would be useful when the interface is congested or near congestion to make room for the control plane packets first before the regular input queue.
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The advantage of this feature, if an ingress queue is about to overflow, SPD tries to first drop "less important" packets.
It's unusual to see ingress queues fill, so before worrying about SPD, you might try to determine why the ingress queue is filling.
Also, for some transient ingress queue bursts, an alternative is to just increase the queue size so the queued packets aren't as likely to be dropped. (Often default ingress queues sizes are relatively "shallow" because it's unusual, again, for this queue to fill.)
BTW, for BGP, are the peers taking advantage of full sized packets? (I.e. what's the MTU being used by BGP?)
Large BGP updates, like the initial load of the Internet's routes, will often queue up in ingress queues, as the control plane tries to process them. Larger, and fewer, BGP packets, slightly decrease the packet processing load for these.
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