circular parent relationships

[Paul Dugas]

On Thu, 2008-08-21 at 23:43 -0400, Thomas Guyot-Sionnest wrote:
> On 21/08/08 09:34 PM, Paul Dugas wrote:
> > I have a need to configure parent relationships for switches that are
> > connected in a ring like so:
> > 
> >         NAGIOS     HostA      HostB      HostC
> >            |         |          |          |
> >         Router -- Switch1 -- Switch2 -- Switch3
> >                      |                     |
> >                       ---------------------
> > 
> > Pseudo config like so:
> > 
> >         host Router  { }
> >         host Switch1 { parents Router,Switch2 }
> >         host Switch2 { parents Switch1,Switch3 }
> >         host Switch3 { parents Switch2,Switch1 }
> >         host HostA   { parents Switch1 }
> >         host HostB   { parents Switch2 }
> >         host HostC   { parents Switch3 }
> > 
> > Stock Nagios generates a "circular parent/child path" error.  I've
> > commented out the Switch1/Switch3 parent entries to make it mostly work
> > but a fault with Switch2 will incorrectly put Switch3 in UNREACHABLE.
> > This seems like something that should be supported but after some
> > searching, I found one patch that addresses it but it's not been
> > incorporated into the released version of the software. 
> > 
> > Is there another way to handle this?
> 
> You can create a clustered host with check_cluster that represent your
> stack of switches, but unless you're checking devices connected to
> multiple switches with STP this is probably not what you want.
> 
> Since you depend on switch1 anyways for connecting to the  other
> switches, I would rather do this:
> 
>         NAGIOS     HostA      HostB      HostC
>            |         |          |          |
>         Router -- Switch1 -- Switch2    Switch3
>                      |                     |
>                       ---------------------
> 
> - - switch1 failure will effectively render switch2 and switch3 unreachable.
> - - switch2 or switch3 failure will not incorrectly mark some other switch
> unreachable
> 
> Even if you have more switches in the loop you could make them depend on
> switch1, because you know that without multiple switch failure they will
> always be reachable if switch1 is up.

I left something out in my attempt to simplify the scenario....

  NAGIOS -- ROUTER ---------------------------------------
               |                                          |
            ROUTER -- Switch1 -- Switch2 -- Switch3 -- ROUTER
                         |                     |
                          ---------------------

These switches are in small enclosures scattered around.  We use STP to
keep layer-2 up in the event of a switch failure or connection break.
The switches and attached edge devices are all on a layer-3 subnet and
we have two routers attached at different points.  The routers use VRRP
to decide which one of them will be the gateway off the subnet.  On the
back end, we have use OSPF to manage the routes back to our central
systems.  

I think I really want to be able to use the circular parent/child
relationships. I guess I'm more curious about why this is not supported
in Nagios.  What's the difficulty?

Paul
-- 
Paul Dugas <paul at dugas.cc>
Dugas Enterprises, LLC
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