clarification on parents and dependencies

[Matt Pounsett]

On Sun, 4 Apr 2004, Ivan Fetch wrote:

>    1. In order for the Nagios box to ping a particular Sun workstation, it
> has to go through two switches.  I can specify one switch using the
> parent directive in the host definition for the Sun workstation, but where
> should I specify the relationship of the switch which the Nagios box is
> plugged into?  This seems a bit clugy, but I could specify the switch

Seems like this one is fairly straightforward, if I'm understanding you
correctly.  By "has to go through two switches" do you mean something like
this?

Nagios -> Switch A -> Switch B -> Sun Workstation

Or this?
        Switch A
      /         \
Nagios           Sun Workstation
      \         /
        Switch B

If it's the first one, then Switch B is the parent of the Workstation, and
Switch A is the parent of Switch B.   If it's the second one, then Switches A
and B are parents of the Sun Workstation.

>    2. Our switches are in a different sub net (apparently in order to be
> trunked), so if something is going wrong with our router, we can not ping
> the switches.  Would folks out there recommend specifying the router as a
> parent to
> each switch's host definition?  I see a kind of loop potential in that

This one is more compex.. it sounds like you've got some VLANs on the switch,
and the packets pass through the switch a couple of times on their way to
their destination, right?

it's easier if you think of the parent/child relationship in Nagios as a
dependancy tree, rather than a network map.. 'cause it really is just the
former.  

In those less-clear situations, I find it helps if I consider how a failure is
going to affect other servers, and eventually it becomes clear which device
should be closer ot the top in the tree.  Consider this:

Packets enter a rack via an uplink cable and enter Switch A (the only switch)
on VLAN1.  A cable leaves VLAN1 for the firewall, which splits the network
into three zones.  Cables connect the three back-end ports on the firewall to
VLAN2, 3 and 4 on the switch, and all the servers are connected to some port
in one of those VLANs.

We've got a loop here, if you consider the logical network diagram.  Packets
pass through the switch twice on their way to any given host.  However, from a
dependancy point of view, you need to work out which potential failure will
take out the most devices.

If we start working down the network from Nagios' point of view, the first
device it hits is the switch, then the firewall (so the switch is the
firewall's parent), and then the switch again.  We can't list the switch twice
in the tree, so we skip it and go directly to the hosts, which means that the
firewall is the parent of all the hosts, the switch the parent of the
firewall, and whatever's upstream of our example rack becomes the parent of
the switch.

HTH



> technically one has to go through a switch in order to ping a router, but
> pinging a switch requires going through the router (otherwise I"d be able
> to define a more typical host -> switch1 -> primary switch -> router kind
> of relationship).
> 
> 
>    Of course what I'm going for here is a downed or overloaded switch
> should result in unreachable messages, but not host down messages.
> 
> Thanks in advance for your input,
> Ivan.
> 
> 
> 
> 
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-- 
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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