Re: Cisco ASR9902 SNMP polling ... is interesting

[Nick Hilliard via NANOG <nanog () lists nanog org>]

Drew Weaver via NANOG wrote on 08/08/2025 14:31:
> It couldn't be bothered to simply set it to 50000 if you set it to the configured maximum of 4294967295
> It couldn't be bothered to simply say: "Hey we know the max for this platform is50000 so we set it to 50000  but you probably 
> shouldn't be using 50000 for this value anyway"
> It could be bothered to do absolutely nothing and silently reject the command which made me laugh for about 5 minutes 
> this morning.

Some years ago I was fighting with a low level pps rate limiter for a 
telemetry service on a long obsolete platform. The default limit caused 
packets to be dropped, and we finally settled on an updated figure based 
on the usual compromise of performance vs consequence. But: if we 
increased the limiter above what we had measured to be reasonable, this 
fairly quickly caused a performance cliff which affected other services, 
e.g. snmp / lacp timeouts, etc, so production impact. Although this was 
in the days of in-house NOS schedulers, I'd be fairly cautious in this 
area - particular on RTOS platforms like XR.

If Cisco have implemented a pps limiter of 50k/s, that's a lot of snmp 
pps. Is this a realistic amount of requests to be properly serviced per 
second? SNMP packet encapsulation / general handling is one thing, but 
stats collection / intermediation can be more heavyweight. Bear in mind 
that the failure modes in this sort of situation are often non-linear.

For sure it's a bit annoying that they don't warn that this is the 
maximum (possibly a platform / LC limit? i.e. possible that this is not 
a generic limit across all SPs on all types of unit), but at least the 
box won't fall over in production just because someone tweaked a 
parameter beyond what the hardware was likely capable of handling.

Nick
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