How is SNMP ifIndex assigned to a interface?

This article provides describes the process of how a SNMP ifIndex is assigned to a interface and why the same interface may have a different ifIndex in two Junos devices.

How a SNMP ifIndex is assigned to a interface and why the same interface may have a different ifIndex in two Junos devices.

The SNMP ifIndex information stored in a text format file – dcd.snmp_ix, which is located in the /var/db folder.

When a new interface gets created in the system, mib2d checks for an entry of the interface in the file /var/db/dcd.snmp_ix. If the entry for this index is present, the system then picks the SNMP interface index value from this file, or else it assigns a new interface index value and writes it to this file.

Since the file is maintained across reboots, the next time somebody deletes and re-creates the same interface, the SNMP ifIndex does not change.

The assignment of the interface index primarily depends on the order in which it is created and if any old value of SNMP exists in the index of /var/db/dcd.snmp_ix.

So, there is no guarantee of the snmpindex values being the same on two different machines.

The /var/db/dcd.snmp_ix file stores SNMP indexes for the interfaces, using the following syntax:

 <snmpindex> <i/f name> <subunit> <logical(1/0)> <xx-xx>

For example, the SNMP ifIndex of ge-0/0/8 can be checked either in the /var/db/dcd.snmp_ix file or through the CLI command – show interface ge-0/0/8.

{master:0}
root@s10-test> file show /var/db/dcd.snmp_ix | match ge-0/0/8 
    513 "ge-0/0/8" 0 0 1;
    552 "ge-0/0/8" 0 1 1;

{master:0}
root@s10-test> show interfaces ge-0/0/8                          
Physical interface: ge-0/0/8, Enabled, Physical link is Down
  Interface index: 140, SNMP ifIndex: 513   Link-level type: Ethernet, MTU: 1514, Speed: Auto, Duplex: Auto,
  BPDU Error: None, MAC-REWRITE Error: None, Loopback: Disabled,
  Source filtering: Disabled, Flow control: Enabled, Auto-negotiation: Enabled,
  Remote fault: Online
  Device flags   : Present Running Down
  Interface flags: Hardware-Down SNMP-Traps Internal: 0x0
  Link flags     : None
  CoS queues     : 8 supported, 8 maximum usable queues
  Current address: 00:19:e2:51:3f:48, Hardware address: 00:19:e2:51:3f:48
  Last flapped   : 2011-11-14 06:56:52 UTC (3d 21:35 ago)
  Input rate     : 0 bps (0 pps)
  Output rate    : 0 bps (0 pps)
  Active alarms  : LINK
  Active defects : LINK
  Interface transmit statistics: Disabled

  Logical interface ge-0/0/8.0 (Index 89) (SNMP ifIndex 552) 
    Flags: Device-Down SNMP-Traps Encapsulation: ENET2
    Input packets : 76333 
    Output packets: 1960364
    Protocol eth-switch
      Flags: None

Related