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[EX Switch] Prior to Junos 9.4 the SNMP ifIndex may change after a Junos upgrade or downgrade resulting in incorrect SNMP trap for wrong ifIndex

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Article ID: KB20637 KB Last Updated: 05 Mar 2017Version: 2.0
Summary:
Junos Software prior to 9.4 will overwrite the "dcd.snmp_ix" file during an upgrade or downgrade resulting in new SNMP ifIndex's being generated.  The purpose of this article is to understand the consequences of this action and how the issue is resolved in Junos 9.4 and above.



Symptoms:
When the EX Switch is upgraded or downgraded the "dcd.snmp_ix" file is overwritten and the SNMP ifIndex's are changed.
The result of this is that the SNMP manager may now have the old ifIndex and will not recognize the new index or trap.

Cause:

Solution:
When the EX Switch is upgraded, downgraded, the "dcd.snmp_ix" file is overwritten and the SNMP ifIndex's are changed. This has been corrected by preserving the file "dcd.snmp_ix" prior to upgrading or downgrading the software.

"dcd.snmp_ix" is the file on the EX Switch RE master which maintains the snmp ifindex of the new interfaces.  For example, if ge-0/0/0 comes up for the first time then a new index is assigned to this interface and the dcd.snmp_ix file is updated. The same index is transferred to dcd.snmp_ix on backup as well.

Directory: /var/db/dcd.snmp_ix

Note: SNMP ifindex should not change during switch reboot or mastership switchover

Importance of SNMP ifindex:
  • SNMP ifindex of the interfaces needs to pre defined (fixed) in the SNMP tools like CACTI or wherever needed.
  • In this case the SNMP manager listens to the Switch for any Critical Traps such as link down and up.
  • If the SNMP ifindex is changed on the EX Switch then the SNMP manager will receive a Trap for wrong ifindex.
  • The SNMP manager will ignore or show wrong Trap information because of wrong index value.
  • To fix this issue the SNMP manager should be reconfigured with new snmp if index.
  • Based on the SNMP ifindex, which is predefined in the SNMP manager, the OID received will be read by the SNMP manager.

For the above reasons the SNMP ifindex values should be maintained, and are with Junos 9.4 and all later releases.
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