[Isis-wg] ISIS MIB question

[Jeff Parker]

>   I'm trying to figure out why it's necessary to use
> isisCircIndex and isisISAdjIndex as indices for the
> isisCircTable and the isisISAdjTable.
> 
>   Neither object has any relationship to any protocol
> value.
> 
>   It seems to me that there are already enough objects
> available (isisCircIfIndex, isisCircIfSubIndex, and
> isisISAdjNeighSNPAAddress) to uniquely identify rows
> in both tables.
> 
>   What is the reason that isisCircIndex and 
> isisISAdjIndex are required?
> 
>  Brad

Let's start with the easy one.

You might have multiple adjacencies on a LAN interface,
such as Ethernet.  In this case, you will need to 
distinguish adjacencies which share the same circuit
attributes.  The AdjIndex serves to mark such different
adjacencies: clearly no circuit index can do that job.  

While there seem to be too many circuit indicies, they
play different roles.  The CircIndex may match the
IfIndex, but it need not.  Your implementation is
free to make these the same value: the overhead of
supporting the two names is minimal, and allows 
implementations that make the CircIndex match a
physical interface (port 3 on card 2) which the
IfIndex is not well suited for.  Since most SNMP
port management depends on the IfIndex, I found it
helpful to include both.  

We felt that the subInterface was useful to support
circuits on interfaces such as ATM and Frame Relay
which split one physical interface into multiple
portions.  This allows you to quickly find the
fiber via the IfIndex and then the VCI via the
SubInterface.  

         isisCircIndex OBJECT-TYPE
            SYNTAX Integer32
            MAX-ACCESS not-accessible
            STATUS current
            DESCRIPTION
                "The identifier of this circuit, unique within the
                instance of the protocol. This object follows the index
                behaviour.  This is for SNMP Indexing purposes only
                and has no relation to any protocol value."
        ::= { isisCircEntry 2 }

        isisCircIfIndex OBJECT-TYPE
            SYNTAX Integer32
            MAX-ACCESS read-only
            STATUS current
            DESCRIPTION
                "The value of ifIndex for the interface to which this
                circuit corresponds.   This object cannot be modified
                after creation"
        ::= { isisCircEntry 3 }

        isisCircIfSubIndex OBJECT-TYPE
             SYNTAX Integer32
             MAX-ACCESS read-only
             STATUS current
             DESCRIPTION
                 "A specifier for the part of the interface ifIndex to which
                 this circuit corresponds, such as a DLCI or VPI/VCI.
                 This object cannot be modified after creation"
         ::=    { isisCircEntry 4 }

- jparker@nexabit.com