[j-nsp] Rationale behind "set chassis aggregated-devices ethernet device-count"

[Kevin R. Cullimore]

On 8/23/2013 1:04 PM, Doug McIntyre wrote:
> On Fri, Aug 23, 2013 at 05:38:34PM +0100, Phil Mayers wrote:
>> On 23/08/13 17:14, Michael Loftis wrote:
>>> Part of it probably has to do with SNMP.  Pre-allocating the count
>>> keeps the SNMP index ID's from changing when devices are
>>> added/removed.  ae0 is always index blah.  A lot of tools are very
>>> dependent upon the SNMP index ID.
>> I don't think so TBH. Having just snmpwalk'ed a JunOS box, the aeX
>> interfaces and sub-ints ifindex values appear allocated sequentially:
>>
>> IF-MIB::ifDescr.521 = STRING: ae0
>> IF-MIB::ifDescr.522 = STRING: ae0.32767
>> IF-MIB::ifDescr.524 = STRING: ae1
>> IF-MIB::ifDescr.529 = STRING: ae0.1
>> IF-MIB::ifDescr.530 = STRING: xe-1/0/0.1
>> IF-MIB::ifDescr.531 = STRING: ae0.11
>> IF-MIB::ifDescr.532 = STRING: ae0.10
>> IF-MIB::ifDescr.533 = STRING: ae0.9
>> IF-MIB::ifDescr.534 = STRING: ae0.8
>> IF-MIB::ifDescr.535 = STRING: ae0.7
>> IF-MIB::ifDescr.536 = STRING: ae0.6
>> IF-MIB::ifDescr.537 = STRING: ae0.5
>> IF-MIB::ifDescr.538 = STRING: ae0.4
>> IF-MIB::ifDescr.539 = STRING: ae0.3
>> IF-MIB::ifDescr.540 = STRING: ae0.2
>> IF-MIB::ifDescr.541 = STRING: xe-1/0/0.11
>> IF-MIB::ifDescr.542 = STRING: xe-1/0/0.10
>>
>> So, unless I'm misunderstanding what you mean, it's not doing anything
>> interesting here.
>
> But imagine the case, where you start with needing 4 LAGs, so that is
> what you set. Then you add in 10 SFPs for various things, and then you
> add in 4 more LAGs, then you remove 4 SFPs for some other project.
>
> What do your SNMP indexes look like now? And hopefully your
> /var/db/dcd.snmp_ix file doesn't get corrupt and the indexes have
> to be recomputed at that point..
>
> Back to the original question, there probably is some resources taken
> up by having the LAGs there. And if you just do max # right away, you do
> have all those new interfaces to gather stats on if your SNMP queries
> just do a bulk walk of all interfaces.
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>
I had assumed It had to do with statically-allocated resources assigned 
during the boot sequence as well. I'll note for the record that 
baby/non-ISP SRXs take many, MANY more minutes to boot when you max out 
the device-count parameter.