Quick Tip: SPD
One of my friends told me about a feature called SPD and whether i knew about it. I didnt so i decided to read a bit more about it.
SPD stands for Selective Packet Discard. As the name implies it has something to do with discarding packets. Something that is never good.
The documentation i read about it can be found here: http://www.cisco.com/en/US/products/hw/routers/ps167/products_tech_note09186a008012fb87.shtml
It is basically a feature to protect critical data from being dropped due to congestion. The emphasis is “critical” data. SPD considers this to be of IP Precedence 6 or 7, but now also L2 information is considered critical.
This means that stuff like your IGP and BGP which goes to the route processor will get priority and have the least chance of getting dropped.
Its worth pointing out, that only data destined for the router undergoes the SPD, and not data that the router forwards further on as this will often use CEF or fast-switching.
What i dont get about this whole thing is that alot of those commands are hidden:
R1#sh ip s? sap sdee sla snat source-track ssh R1#sh ip spd Current mode: normal. Queue min/max thresholds: 73/74, Headroom: 100, Extended Headroom: 75 IP normal queue: 0, priority queue: 0. SPD special drop mode: none
The same goes for configuring some options for the feature:
R1(config)#ip sp? % Unrecognized command R1(config)#ip spd mode aggressive R1(config)#do sh ip spd Current mode: normal. Queue min/max thresholds: 73/74, Headroom: 100, Extended Headroom: 75 IP normal queue: 0, priority queue: 0. SPD special drop mode: aggressively drop bad packets
Weird behavior but a nice feature.
I would very much like to know, just how this is implemented on specific router models. Especially since the explanation hints as using some sort of queues to differentiate traffic. This will have to wait for a later investigation though.
Thats a quick tip for now.