skipped SNMP ethernet index

Hi, I’m using RB 493 as a switch and I’m using fw 4.17. I’m using SNMP get gather statistics informations from this device and got this:

IF-MIB::ifNumber.0 = INTEGER: 10
IF-MIB::ifIndex.1 = INTEGER: 1
IF-MIB::ifIndex.3 = INTEGER: 3
IF-MIB::ifIndex.4 = INTEGER: 4
IF-MIB::ifIndex.5 = INTEGER: 5
IF-MIB::ifIndex.6 = INTEGER: 6
IF-MIB::ifIndex.7 = INTEGER: 7
IF-MIB::ifIndex.8 = INTEGER: 8
IF-MIB::ifIndex.9 = INTEGER: 9
IF-MIB::ifIndex.10 = INTEGER: 10
IF-MIB::ifIndex.11 = INTEGER: 11
IF-MIB::ifDescr.1 = STRING: ether1
IF-MIB::ifDescr.3 = STRING: ether2
IF-MIB::ifDescr.4 = STRING: ether3
IF-MIB::ifDescr.5 = STRING: ether4
IF-MIB::ifDescr.6 = STRING: ether5
IF-MIB::ifDescr.7 = STRING: ether6
IF-MIB::ifDescr.8 = STRING: ether7
IF-MIB::ifDescr.9 = STRING: ether8
IF-MIB::ifDescr.10 = STRING: ether9
IF-MIB::ifDescr.11 = STRING: bridge1

Why is the index = 2 skipped in SNMP? It’s some bug or feature? If it’s feature - why this?