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

[William Chu]

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).

Any inputs are appreciated.

Thanks,

William