Nagios/SNMP - devices alerting when ppp/tun connections cycle

Sorry if this is a repeat. I searched, but with no luck. I'm using SNMPd on an openwrt/wr host with some ppp and tun connections. These connections get IDs in the if table, and will actually get a new ID whenever the tunnels reconnect. Nagios (check_mk), when that happens, complains that an interface went down; oh, and a different one with the same name came up right afterward. In the meantime, it's iterating over so many interfaces that the reports are of 'interface 4933 down'; and an snmpwalk shows close to 4932 datapoints before it. How are the helpful folks here handling a monitoring situation like that?