[j-nsp] SNMP interface index change after upgrade to 9.2

[Chris Adams]

Once upon a time, Tore Anderson <tore.anderson at redpill-linpro.com> said:
> It has been pointed out that not even MRTG, which I assume is the most
> used traffic graphing suite, handles ifIndex changing (by default).
> Keying off something else, like ifName, adds unnecessary overhead on the
> polling station.

I switched from MRTG to Cricket almost 10 years ago, in part because of
this.  Cricket can map on basically anything; it is very useful for all
kinds of monitoring (not just router interface names).

It keeps the overhead down by storing the index with the data.  When it
requests new data, it asks for the data and map field (e.g. ifDescr),
and only if the map field no longer matches does it walk the whole tree
to update the map (and when it walks the tree, it remembers it for
mapping other interfaces).

> I found that RFC 2863 says the following about ifIndex:
> 
>     The value for each interface must remain constant at least from one
>     re-initialization of the entity's network management system to the
>     next re-initialization.
> 
> I'm not exactly sure what «re-initialization of the entity's network
> management» means, but I guess a JUNOS upgrade probably qualifies.
> Anyway, the «at least» part implies to me that it's even better if it's
> constant across those re-initializations too.

Any reboot (or even a daemon restart) is a re-initialization of the
network management system.

This was an issue when the first modular routers came out; it was years
before router vendors offered any option to keep (or at least try to
keep) static ifIndex values.  Software has had a very long time to
adapt, but I guess it hasn't all caught up yet.

What if your router/switch fails, and you replace with a spare?  The
ifIndex values will all change then as well, since the stored ifIndex
values are not in the regular config.  It is much better to have
software that handles that for you than to depend on static ifIndex
values.

-- 
Chris Adams <cmadams at hiwaay.net>
Systems and Network Administrator - HiWAAY Internet Services
I don't speak for anybody but myself - that's enough trouble.