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

[Doug McIntyre]

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.