Subject: [Isis-wg] isisCircLevelIDOctet in the WG MIB

From: Jonathan Harrison <[email-masked]>
Date: Tue, 9 Nov 2004 16:51:16 -0000

Jeff,

I've got a concern over the usability of the isisCircLevelIDOctet object
defined in the isisCircLevelTable.

    isisCircLevelIDOctet OBJECT-TYPE
        SYNTAX Integer32(0..255)
        MAX-ACCESS read-create
        STATUS current
        DESCRIPTION
            "A one byte identifier that can be used in protocol packets
             to identify a circuit.  Values of isisCircLevelIDOctet
             do not need to be unique.  They are only required to
             differ on LANs where the Intermediate System is the
             Designated Intermediate System."
    ::= { isisCircLevelEntry 5 }

There are the following issues with this.

 - The properties of the object are different for point-to-point circuits
and for LAN circuits.  For a point-to-point circuit, the value 0 is
acceptable, and could be used as a sensible default.  On the other hand, for
a broadcast circuit, the value 0 cannot be used if the IS-IS instance could
become DIS on the LAN, and there is no sensible default value (due to the
uniqueness requirement).

 - As a general point, it would be good for all read-create objects in the
isisCircLevel table to have MIB defined default values.  Currently, the
IDOctet is the only non-defaultable value, and since the IDOctet is used in
IIH PDUs, a value must be chosen for this object before a circuit can be
activated via the MIB.  Circuit configuration would be much simpler if it
only required the creation of a row in the isisCircuitTable, without the
modification of any objects in the isisCircLevel table.

 - The reason for making the value level specific, was to allow support for
the "Maintaining more than 255 circuits in IS-IS" draft
(draft-ietf-isis-wg-255adj).  However, if this draft is supported, then a
unique value for the IDOctet object is chosen by the IS-IS instance when it
becomes DIS on a LAN, and so it makes more sense for the MIB object to be
read-only.

These points suggest that the usability of the MIB would be improved by
splitting this object into two separate objects.  

 - The object for point-to-point circuits is read-create, with the default
value of zero.  Although this object could move to the isisCircTable, it
probably makes more sense to leave it in the isisCircLevel table (since it
should be kept with isisCircLevelID).

 - The object for broadcast circuits is read-only, and reflects the value
chosen by the IS-IS instance.  Since it is level specific, it must remain in
the isisCircLevelTable.

What do you think?  If you agree, I'm happy to suggest some text.

Thanks,
Jon

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From: "Parker, Jeff" <[email-masked]>
Date: Tue, 09 Nov 2004 12:28:40 -0500

Jonathan -
    I see your point, but wonder if the importance of a default value
outweighs the confusion of having two names for this byte.

- jeff parker