>sh ver

Cisco IOS Software, 7200 Software (C7200-A3JK9S-M), Version 12.4(8b), RELEASE SOFTWARE (fc2)

I don't see ietf configured, just:

snmp-server enable traps snmp authentication linkdown linkup coldstart warmstart

This was changed recently so that the link traps will be sent from 1.3.6.1.6.3.1.1.5. However, there should still be an additional varbind at the end of the trap, locIfReason, which gives the reason the interface state changed.

I just confirmed your code is recent enough to have these changes. Basically, all linkUp traps will have the OID snmpTraps.4, and all linkDown traps, snmpTraps.3. Depending on whether or not you enable IETF link traps, the varbinds will change. The Cisco (i.e. non-IETF varbinds) are:

ifIndex

ifDescr

ifType

locIfReason

The IETF varbinds are:

ifIndex

ifAdminStatus

ifOperStatus

ifDescr

ifType

locIfReason

If this interface has a cciDescr (i.e. this is a circuit interface) the traps will contain the following varbinds. Cisco:

ifIndex

ifDescr

ifType

cciDescr

locIfReason

IETF:

ifIndex

ifAdminStatus

ifOperStatus

ifDescr

ifType

cciDescr

locIfReason

So, enabling the IETF link traps will give you more information in the body of the trap.

Does enabling the IETF option always make the linkDown trap more informative across all platforms (7200 vs 6500, etc.)?

Yes, provided the IOS supports enabling this option. The "snmp-server trap link ietf" command was added in 12.1(2)T.