I am currently monitoring several routers that have 2 Internet connections. Since those are remote routers, I have entered both interfaces in the "SNMP interfaces" configuration fields.

There are 2 problem in Zabbix with multiple snmp interfaces :

1/ It happens that the first internet connection goes down, at this point the router automatically switch to the other internet connection and the customer doesn't see anything, but Zabbix doesn't switch to the secondary interface at all, resulting in the snmp icon being red with the message "Timeout while connecting to [[a.b.c.d]:161]" where a.b.c.d is the first interface configured on the host
2/ Even when I manually change the "default" snmp interface to the second one, Zabbix still doesn't use it and continue to try to use the first one, again resulting in the message "Timeout while connecting to [[a.b.c.d]:161]" where a.b.c.d is the first interface configured on the host

The first point may be longer to fix, but I think that the second one is critical because Zabbix doesn't respect the "default" interface selection. I didn't test to change other "default" interface type beside snmp so maybe this bug is global to all interface types.