[c-nsp] Cisco Selective Packet Discard and ISR 3825 Router running IOS 12.4 mainline

[William Chu]

Thanks, Bruce.

Yes, I have read it before but I would like to know how SPD performs random 
drops. This doc talks about dropping low priority packets, but exactly what 
are low priority packets the document didn't really say it.  For example, 
does SPD look at ip precedence and perform drops like WRED? Or does it look 
at IP packet types (like snmp is higher priority than icmp so it drops icmp 
first)?

William


>From: Bruce Pinsky <bep at whack.org>
>Reply-To: bep at whack.org
>To: William Chu <william_w_chu at hotmail.com>
>CC: cisco-nsp at puck.nether.net
>Subject: Re: [c-nsp] Cisco Selective Packet Discard and ISR 3825 Router 
>running IOS 12.4 mainline
>Date: Tue, 10 Oct 2006 14:25:16 -0700
>
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>
>William Chu wrote:
> > Hi:
> >
> > Has anyone have any experience on Selective Packet Discard (SPD) and how 
>to
> > tune the queues?
> >
> > I am seeing flushes on the Ge0/0 interface, but no drops on the input 
>queue.
> > When I issued the show ip spd command on the router I saw that the SPD 
>queue
> > depth was set at 73/74 for min and max, respectively.  The IP SPD 
>commands
> > are hidden on the router, but yet they are enabled by  default.
> >
> > The reason I am asking is because I have a Cisco 3825 on our network and 
>we
> > send out a series of ping packets through this router to ping up to a
> > thousand devices over the WAN. Due to network addressing issue we must 
>PAT
> > our ping server's IP before it hits the WAN. Our server sends out a 
>series
> > of ping on batches, but each batch consisted of a few hundred ping 
>requests
> > at the same time so it kind of overrun the SPD queue and thus causing
> > premature ping failures randomly.
> >
> > Does anyone know how SPD work?  Because ICMP is kind of a low priority
> > packet type so I wonder if SPD drops ICMP packets first before it drops
> > other (such as SNMP).
> >
>
>Although written primarily from the perspective of the 7500 and 12K, this
>is fairly good explanation of SPD:
>
>http://www.cisco.com/en/US/products/hw/routers/ps167/products_tech_note09186a008012fb87.shtml
>
>- --
>=========
>bep
>
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