Tuesday 12 March 2019

Checking switch port bandwidth utilisation with SNMP / Nagios

In order to monitor port bandwidth utilization on Cisco switches via SNMP we'll firstly need to install a plugin from the Nagios Exchange called 'iftraffic2':

Download and install the plugin:

cd /usr/local/nagios/libexec
curl https://exchange.nagios.org/components/com_mtree/attachment.php?link_id=1720&cf_id=24 -O check_iftraffic
chmod +x check_iftraffic

The usage for the plugin is as follows:

./check_iftraffic -H <hostname> -C <community-string> -r -i <interface-name> -b <interface-capacity> -u <interface-unit> -w <warning-limit-percentage> -c <critical-limit-percentage>

We'll need to obtain the interface name of the interface we wish to poll - we can use snmpwalk to do this for us:

yum -y install net-snmp-utils
snmpwalk -v 2c -c <community-string> <hostname>

> IF-MIB::ifDescr.67 = STRING: Port-channel1
> IF-MIB::ifDescr.68 = STRING: Port-channel2
> IF-MIB::ifDescr.69 = STRING: Port-channel3
> IF-MIB::ifDescr.70 = STRING: Port-channel4

Note: '' is the OID for interface descriptions - more information can be found here.

We'll also need the interface capacity:

snmpwalk -v 2c -c <community-string> <hostname>

> IF-MIB::ifSpeed.67 = Gauge32: 2000000000
> IF-MIB::ifSpeed.68 = Gauge32: 2000000000
> IF-MIB::ifSpeed.69 = Gauge32: 2000000000
> IF-MIB::ifSpeed.70 = Gauge32: 2000000000

Now note that the OID '' returns the interface capacity in bits per second - so we'll need to convert this to gigabits per second (as the plugin doesn't support bits per second) - so we do:

2000000000 / 1000000000 = 2 (Gigabits)

For this example we'll use 'Port-channel1' - so the plugin would be executed as follows:

./check_iftraffic -H <hostname> -C <community-string> -r -i Port-channel1 -b 2 -u g -w 70 -c 85

The -b switch specifies our 2 Gbps and the -u switch instructs the plugin that we are giving the measurements in Gigabits.

If successfull you should have something like the following:

> Total RX Bytes: 1000.00 MB, Total TX Bytes: 2000.00 MB<br>Average Traffic: 5.75 MB/s (2.2%) in, 1.48 MB/s (0.6%) out| inUsage=2.2,50,70 outUsage=0.6,50,70 inAbsolut=10000000 outAbsolut=20000000

Now we simply need to setup the respective command and service definitions in nagios e.g.:

### commands.conf

# check switch port bandwidth
define command{
    command_name check_bandwidth
    command_line /usr/local/nagios/libexec/check_iftraffic -H $HOSTADDRESS$ -C $ARG1$ -r -i $ARG2$ -b $ARG3$ -u g -w 50 -c 70

### switches.conf

define service{
    use generic-service
    host_name SWITCH-STACK
    service_description HHSVRSTK Uplink: Bandwidth Utilization
    check_command check_bandwidth!<community-string>!Port-channel1!2
    normal_check_interval 1
    retry_check_interval 1


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