ifIndex

Sat Oct 13 16:19:33 UTC 2018

Cisco tries very hard to make such useless data occur in XR.  If you have a gigE SFP in an SFP+ port, a new ifindex will appear for the resulting GigabitEthernetX port, then it remains even if both the config and SFP have been removed.  Automated systems will keep querying it as if it were a downed port, but wait, reboot, and suddenly it vanishes.  I went back and forth with TAC for weeks explaining that SNMP interfaces should not disappear as a result of a reboot, I should either be able to remove it, or it's stuck there forever, but a reboot should not cause a change.  They didn't care; it is 'by design'. 

On 10/13/18, 8:47 AM, "NANOG on behalf of Mel Beckman" <nanog-bounces at nanog.org on behalf of mel at beckman.org> wrote:

    Saku,

    The issue isn't that ifindexes change during operation. That would truly make SNMP useless. The issue is that they change across reboots. That's where features such as Cisco's Interface Index Persistence helps out. 

    -mel via cell

    > On Oct 13, 2018, at 2:59 AM, Saku Ytti <saku at ytti.fi> wrote:
    > 
    >> On Fri, 12 Oct 2018 at 21:40, Chris Adams <cma at cmadams.net> wrote:
    >> 
    >> Is there any good excuse that SNMP client software can't handle a basic
    >> design of SNMP - indexed tables?  ifIndex is far from the only index in
    >> SNMP, and many of them still change today at various times.
    >> 
    >> It isn't that hard to fetch the indexed field in a bulk get, rewalking
    >> the table if you don't get what you expected.  Cricket did this in 1999.
    > 
    > It's never going to be provably correct, depending on what stability means.
    > 
    > You fetch relation at t0, then at t1 you fetch data. Was the relation
    > same at t0 and t1? You can gain some confidence by fetching relation
    > again at t2 and disregard data if t0 != t2. But this becomes polling
    > expensive quite fast, and still not provably correct. This may be
    > nitpicking, but I've always felt uneasy about the lack of guarantee.
    > 
    > I wonder if those who have stable indeces, have them for all cases,
    > all logical interfaces and virtual interfaces?
    > 
    > -- 
    >  ++ytti