clarification on parents and dependencies

[Ivan Fetch]

Hello,
   Thanks again for your reply -


> >    Ok, will switch a need to be the parent of all other switches
> > because the Nagios box is connected to switch a, even if both switch a and
> > switch b are connected by fiber to the router (vs. fiber connecting the
> > switches directly)?
>
> I'm not sure I follow this.  In my diagram above, the workstation would not
> list switch A as a parent, only switch B.  Switch A is, by implication, a
> grandparent of the workstation.   If switch A were to go down, both switch B
> and the workstation would be marked as "unreachable" by Nagios.

   Ok, so the workstation has a parent of switch b, switch b has a parent
of switch a (switch a is where the nagios box is connected).  IF switch a
goes down, nagios will not be able to reach the workstation - it'll try
reaching switch b, which will also fail.  As switch a is a parent of
switch b, nagios will continue walking the tree and end up marking switch
a as down, and switch b, as well as the workstation, as unreachable.  I
believe this is what you're saying as well?


> > Yes, the switches are trunked and getting their vlans from the router, so
> > even for local trafic on one switch to get where it's going, it looks like
> > communication with the router is necessary.  IF the router is experiencing
> > high load or some other problem, local trafic on a switch is still
> > effected (although none of that traffic should have to go anywhere except
> > from one fast ethernet port to another).  This makes it difficult for
> > Nagios to know whether the problem is the switch, the router, or a
> > host/service.  Since I can not ping the switch in this kind of a situation
> > (because the switch is in another subnet and has to go through the router)
> > perhaps incorparating the switches into my monitoring will not be
> > possible, or will not be very reliable?
>
> If problems with the router will cause the switch to be unreachable, then the
> router should be the switch's parent.  Remember, the parent/child
> relationships are more about dependancy than they are about the network
> physical or logical layout.  You have a bit of a unique situation where your
> switch may be rendered useless by the router, but the router is only reachable
> through the switch... which means they are really co-dependant, and Nagios
> doesn't have a way to diagram that.  I'd say it's a judgement call which
> device you make the parent.   Just make sure that anyone who may be
> potentially paged about those two devices know about the configuration, and
> that it might be either device which is affected.

   Ok, let me give that another try - what I've done for now is to specify
the relationship of the switches
in terms of how they are connected (in other words, secondary switches
have the primary switch as a parent).  The primary switch (ps0) has a
parent of ps1 (another switch which is connected directly to the router by
fiver, this is where the nagios box is connected), and ps1 has a parent of
the router.  SO, if a host on a secondary switch can't be reached, my
logic is that nagios will check the secondary switch, then the primary
switch (ps0), then the switch which nagios itself is connected to (ps1),
and then the router.
I've got my host alias for the router saying assentially "could be the
router or could be a switch" for notification purposes.  Now it might be
nice to schedule a bit of time and down some switch ports to test things.
:)

Thanks - Ivan.



>
> --
> Matt Pounsett                 CIRA - Canadian Internet Registration Authority
> Technical Support Programmer                    350 Sparks Street, Suite 1110
> matt.pounsett at cira.ca                                 Ottawa, Ontario, Canada
> 613.237.5335 ext. 231                                      http://www.cira.ca
>
>


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