Technical Tip: FortiOS SNMP and RMON indexes and their relations

An SNMP index is unique inside a given device and is used as the SNMP interface identifier when displaying information from the MIB.

The SNMP index inside FortiOS:

Each configured interface inside Configuration -> System -> Interface statement has an SNMP index. This index will identify the interface when using the SNMP protocol.
For instance, consider port1 and port3 configured as:

config system interface

edit "port1"

set vdom "root"
...
set snmp-index 11

next
edit "port3"

set vdom "root"
...
set snmp-index 333

next

The RMON interface index.

Starting from 7.6.0, the Ethernet statistic group of the RMON mib is also supported.

It is enabled and configured per interface in the following statement:

config system snmp rmon-stat

edit 1

set source "port3"
set owner "Port3Owner"

next
edit 2

set source "port1"
set owner "Port1Owner"

next

end

The link between the RMON interface index and the SNMP interface index is done by getting the following OIDs of the RMON MIB:

1.3.6.1.2.1.16.1.1.1.1 (etherStatsIndex)
1.3.6.1.2.1.16.1.1.1.2 (etherStatsDataSource) ==> this will return the Mib-2 IfIndex OID

For instance, the above configuration will return for port3 and port1:

iso.3.6.1.2.1.16.1.1.1.1.1 = INTEGER: 1 index of port3 in config>system>snmp>rmon-stat>entry
iso.3.6.1.2.1.16.1.1.1.1.2 = INTEGER: 2
iso.3.6.1.2.1.16.1.1.1.2.1 = OID: iso.3.6.1.2.1.2.2.1.1.333 result of snmp-index configuration from config>system>interface
iso.3.6.1.2.1.16.1.1.1.2.2 = OID: iso.3.6.1.2.1.2.2.1.1.11

About the kernel interface index:
The kernel index is the one displayed for instance by the command:

diagnose netlink interface list
if=port1 family=00 type=1 index=3 mtu=1500 link=0 master=0
ref=28 state=start present fw_flags=0 flags=up broadcast run multicast
if=port3 family=00 type=1 index=5 mtu=1500 link=0 master=0
ref=7 state=start present no_carrier fw_flags=0 flags=up broadcast multicast

This index is the one displayed in some FortiOS troubleshooting command (diagnose commands). It has no relation with the above SNMP indexes.