The MPLS WG Archive

[Keith McCloghrie <kzm@cisco.com>]

FYI.  In response to my message below, I received a private email
saying:

     It is more likely than you think. I once worked on a device which
     did not reuse ifIndexes across reboots until it had to. It
     basically kept a maxIfIndex var in nvram. It continued up until it
     hit 32bits worth of numbers, and only then would it roll-over to 1.

and I believe that's an unusual but compliant implementation.

Keith.
 
> Yakov,
> 
> I don't know about OSPF, the protocol, but the OSPF MIB (RFC 1850)
> uses different MIB objects to hold an IP address and an ifIndex value.
> For example,
> 
>     ospfIfIpAddress OBJECT-TYPE
>         SYNTAX   IpAddress
>         MAX-ACCESS   read-only
>         STATUS   current
>         DESCRIPTION
>            "The IP address of this OSPF interface."
>        ::= { ospfIfEntry 1 }
> 
>     ospfAddressLessIf OBJECT-TYPE
>         SYNTAX   Integer32
>         MAX-ACCESS   read-only
>         STATUS   current
>         DESCRIPTION
>            "For the purpose of easing  the  instancing  of
>            addressed   and  addressless  interfaces;  This
>            variable takes the value 0 on  interfaces  with
>            IP  Addresses,  and  the corresponding value of
>            ifIndex for interfaces having no IP Address."
>        ::= { ospfIfEntry 2 }
> 
> ifIndex was originally defined (in RFC 1066) as INTEGER, which was
> later refined (in RFC 1563) as:
> 
>    InterfaceIndex ::= TEXTUAL-CONVENTION
>        DISPLAY-HINT "d"
>        STATUS       current
>        DESCRIPTION
>                "A unique value, greater than zero, for each interface
>                or interface sub-layer in the managed system.  It is
>                recommended that values are assigned contiguously
>                starting from 1.  The value for each interface sub-
>                layer must remain constant at least from one re-
>                initialization of the entity's network management
>                system to the next re-initialization."
>        SYNTAX       Integer32
> 
> and Integer32 is defined as:
> 
>    Integer32 ::=
>            INTEGER (-2147483648..2147483647)
> 
> So, it's unlikely, but increasingly possible, that ifIndex values can
> be as large as 2147483647.
> 
> Keith. 
> 
> 
> > Keith,
> > 
> > > An SNMP ifIndex will not fit inside 3 bytes.
> > 
> > Taking this point of view, an SNMP ifIndex wouldn't fit in anything
> > less than 4 bytes. Yet, in OSPF with unnumbered links ifIndex is
> > carried in the same field as a plain IP address So, when this field
> > carries the value 33620225, is that an IP address (2.1.1.1) or an
> > ifIndex (33620225) ?
> > 
> > Yakov.
> > 
> 
>