David Hubbard dhubbard at dino.hostasaurus.com
Sat Oct 13 21:43:35 UTC 2018

I do that too, but I’m referring to XR when you use different speed optics in a multi-speed port; if you have a SFP+ port and 10gig SFP, you’ll get one ifindex.  New use case requires swapping to a gigE SFP and you’ll get a new ifindex.  Take the port out of service, remove the GigE SFP and the related config, yet both ifindexes remain; until the device is reloaded.  At that the gigE ifindex goes away leaving just the native-speed ifindex.

It’s a pain for management because we’re forced to make exclusions in our NMS for ifindex’s that may disappear at some point, because they show as down with no way to make that not the case.  Worse, if that port is put to use again at the non-native speed, and has such an exclusion in place, we don’t auto learn the new usage because of the exclusion.

I tried to argue with TAC that if the gigE SFP has been removed from the SFP+ port, and its config has been deleted, the corresponding ifindex and related counters should be gone; it no longer exists in any form.  If you reload, it will disappear, but that’s the only way.

From: Mel Beckman <mel at beckman.org>
Date: Saturday, October 13, 2018 at 4:46 PM
To: David Hubbard <dhubbard at dino.hostasaurus.com>
Cc: "nanog at nanog.org" <nanog at nanog.org>
Subject: Re: ifIndex


All you have to do is turn on IFindex persistence:


We do this on our XRs and it works perfectly.

-mel via cell

On Oct 13, 2018, at 9:20 AM, David Hubbard <dhubbard at dino.hostasaurus.com<mailto:dhubbard at dino.hostasaurus.com>> wrote:
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<mailto:nanog-bounces at nanog.org> on behalf of mel at beckman.org<mailto:mel at beckman.org>> wrote:


   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<mailto:saku at ytti.fi>> wrote:

On Fri, 12 Oct 2018 at 21:40, Chris Adams <cma at cmadams.net<mailto: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?


-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.nanog.org/pipermail/nanog/attachments/20181013/a2e443fa/attachment.html>

More information about the NANOG mailing list