circular parent relationships

[Thomas Guyot-Sionnest]

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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.

- --
Thomas
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