Using nagios to monitor port status on ethernet switches

First, we need to identify the id of the ports we want to monitor. On the console of the machine you are running nagios, execute the following commands

snmpwalk -v 1 -c MySNMPCommunity my.switch.ip.address ifDescr

you will get something like this:

IF-MIB::ifDescr.509 = STRING: pfh-0/0/0
IF-MIB::ifDescr.510 = STRING: pfe-0/0/0
IF-MIB::ifDescr.511 = STRING: pfh-0/0/0.16383
IF-MIB::ifDescr.512 = STRING: pfh-0/0/0.16384
IF-MIB::ifDescr.513 = STRING: pfe-0/0/0.16383
IF-MIB::ifDescr.514 = STRING: ge-0/0/0
IF-MIB::ifDescr.515 = STRING: ge-0/0/1
IF-MIB::ifDescr.516 = STRING: ge-0/0/2
IF-MIB::ifDescr.517 = STRING: ge-0/0/3
...(more stuff cut off for brevity)

two notes –
1. the interfaces are not always in order!!!
2. make note of the number after ifDescr. This references the interface(s) you are intersted in

For each port you want to monitor – create a service

define service {
    host_name My-Switch
    service_description ge-0-Switch-link
    use generic-service
    check_command check_snmp!-C MySNMPCommunity -m RFC1213-MIB -s "down(2)" -o ifOperStatus.514
    register 1
}

If the switch port is supposed to be up – replace “down(2)” with “up(1)”

Replace MySNMPCommunity with your community name

In my situation – I am monitoring to see if someone plugs in unauthorized equpment – so this switch port is supposed to remain down.

Replace the 514 with the approriate id for the port you want to monitor based on the snmpwalk you ran at the start of this exersize.