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Selective Packet Discard on Cisco Router


What are the adv/disadv of this feature.

We are try to implement the same for ibgp/ebgp secnario

But, from Cisco doc, I came to know , It will not help us for 7200 Platform

Will it helpful for 2821/3845 router, same is facing towards my ISP with Ebgp running


  • LAN Switching and Routing

Selective Packet Discard on Cisco Router

Why will it not help for 7200? Did you read this document?

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.

Daniel Dib
CCIE #37149

Please rate helpful posts.

Daniel Dib CCIE #37149 Please rate helpful posts.
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Re: Selective Packet Discard on Cisco Router


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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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